Stefan A. Maier

The further integration of optical devices will require the fabrication of waveguides for electromagnetic energy below the diffraction limit of light. We investigate the possibility of using arrays of closely spaced metal nanoparticles for this purpose. Coupling between adjacent particles sets up coupled plasmon modes that give rise to coherent propagation of energy along the array. A point dipole analysis predicts group velocities of energy transport that exceed 0.1c along straight arrays and shows that energy transmission and switching through chain networks such as corners (see Figure) and tee structures is possible at high efficiencies. Radiation losses into the far field are expected to be negligible due to the near-field nature of the coupling, and resistive heating leads to transmission losses of about 6 dB/μm for gold and silver particles. We analyze macroscopic analogues operating in the microwave regime consisting of closely spaced metal rods by experiments and full field electrodynamic simulations. The guiding structures show a high confinement of the electromagnetic energy and allow for highly variable geometries and switching. Also, we have fabricated gold nanoparticle arrays using electron beam lithography and atomic force microscopy manipulation. These plasmon waveguides and switches could be the smallest devices with optical functionality.

In emerging optoelectronic applications, such as water photolysis, exciton fission and novel photovoltaics involving low-dimensional nanomaterials, hot-carrier relaxation and extraction mechanisms play an indispensable and intriguing role in their photo-electron conversion processes. Two-dimensional transition metal dichalcogenides have attracted much attention in above fields recently; however, insight into the relaxation mechanism of hot electron-hole pairs in the band nesting region denoted as C-excitons, remains elusive. Using MoS2 monolayers as a model two-dimensional transition metal dichalcogenide system, here we report a slower hot-carrier cooling for C-excitons, in comparison with band-edge excitons. We deduce that this effect arises from the favourable band alignment and transient excited-state Coulomb environment, rather than solely on quantum confinement in two-dimension systems. We identify the screening-sensitive bandgap renormalization for MoS2 monolayer/graphene heterostructures, and confirm the initial hot-carrier extraction for the C-exciton state with an unprecedented efficiency of 80%, accompanied by a twofold reduction in the exciton binding energy.

Double-slit experiments—where a wave is transmitted through a thin double aperture in space—have confirmed the wave–particle duality of quantum objects, such as single photons, electrons, neutrons, atoms and large molecules. Yet, the temporal counterpart of Young’s double-slit experiment—a wave interacting with a double temporal modulation of an interface—remains elusive. Here we report such a time-domain version of the classic Young’s double-slit experiment: a beam of light twice gated in time produces an interference in the frequency spectrum. The ‘time slits’, narrow enough to produce diffraction at optical frequencies, are generated from the optical excitation of a thin film of indium tin oxide near its epsilon-near-zero point. The separation between time slits determines the period of oscillations in the frequency spectrum, whereas the decay of fringe visibility in frequency reveals the shape of the time slits. Surprisingly, many more oscillations are visible than expected from existing theory, implying a rise time that approaches an optical cycle. This result enables the further exploration of time-varying physics, towards the spectral synthesis of waves and applications such as signal processing and neuromorphic computation. A temporal version of Young’s double-slit experiment shows characteristic interference in the frequency domain when light interacts with time slits produced by ultrafast changes in the refractive index of an epsilon-near-zero material.

Abstract Structuring light emission from single-photon emitters (SPEs) in multiple degrees of freedom is of great importance for quantum information processing towards higher dimensions. However, traditional control of emission from quantum light sources relies on the use of multiple bulky optical elements or nanostructured resonators with limited functionalities, constraining the potential of multi-dimensional tailoring. Here we introduce the use of an ultrathin polarisation-beam-splitting metalens for the arbitrary structuring of quantum emission at room temperature. Owing to the complete and independent polarisation and phase control at the single meta-atom level, the designed metalens enables simultaneous mapping of quantum emission from ultra-bright defects in hexagonal boron nitride and imprinting of an arbitrary wavefront onto orthogonal polarisation states of the sources. The hybrid quantum metalens enables simultaneous manipulation of multiple degrees of freedom of a quantum light source, including directionality, polarisation, and orbital angular momentum. This could unleash the full potential of solid-state SPEs for their use as high-dimensional quantum sources for advanced quantum photonic applications.

This paper reviews the fundamentals of surface plasmon polariton (SPP) excitations sustained by interfaces between metallic and insulating media, with a focus on applications in waveguiding of electromagnetic waves at visible and near-infrared frequencies. The large wavevectors accessible via SPP oscillations allow for significantly reduced wavelengths and thus increased confinement of the propagating modes, promising a subwavelength photonic infrastructure suitable for integration on Si-based photonic chips. Various geometries such as single interfaces, multilayer structures, and nanoparticle ensembles are discussed and their properties assessed in terms of light confinement and energy attenuation of the guided modes. Metal/insulator/metal (MIM) and certain forms of nanoparticle waveguides promise novel avenues for light confinement, guiding, and coupling, which could provide the basis for subwavelength photonic devices

Abstract The chemical bond is one of the most powerful, yet much debated concepts in chemistry, explaining property trends in solids. Recently, a novel type of chemical bonding was identified in several higher chalcogenides, characterized by a unique property portfolio, unconventional bond breaking, and sharing of about one electron between adjacent atoms. This metavalent bond is a fundamental type of bonding in solids, besides covalent, ionic, and metallic bonding, raising the pertinent question as to whether there is a well‐defined transition between metavalent and covalent bonds. Here, three different pseudo‐binary lines, namely, GeTe 1− x Se x , Sb 2 Te 3(1− x ) Se 3 x , and Bi 2−2 x Sb 2 x Se 3 , are studied, and a sudden change in several properties, including optical absorption ε 2 (ω), optical dielectric constant ε ∞ , Born effective charge Z *, electrical conductivity, as well as bond breaking behavior for a critical Se or Sb concentration, is evidenced. These findings provide a blueprint to experimentally explore the influence of metavalent bonding on attractive properties of phase‐change materials and thermoelectrics. Particularly important is its impact on optical properties, which can be tailored by the amount of electrons shared between adjacent atoms. This correlation can be used to design optoelectronic materials and to explore systematic changes in chemical bonding with stoichiometry and atomic arrangement.

Abstract Understanding the nature of chemical bonding in solids is crucial to comprehend the physical and chemical properties of a given compound. To explore changes in chemical bonding in lead chalcogenides (PbX, where X = Te, Se, S, O), a combination of property‐, bond‐breaking‐, and quantum‐mechanical bonding descriptors are applied. The outcome of the explorations reveals an electron‐transfer‐driven transition from metavalent bonding in PbX (X = Te, Se, S) to iono‐covalent bonding in β‐PbO. Metavalent bonding is characterized by adjacent atoms being held together by sharing about a single electron (ES ≈ 1) and small electron transfer (ET). The transition from metavalent to iono‐covalent bonding manifests itself in clear changes in these quantum‐mechanical descriptors (ES and ET), as well as in property‐based descriptors (i.e., Born effective charge ( Z *), dielectric function ε(ω), effective coordination number (ECoN), and mode‐specific Grüneisen parameter (γ TO )), and in bond‐breaking descriptors. Metavalent bonding collapses if significant charge localization occurs at the ion cores (ET) and/or in the interatomic region (ES). Predominantly changing the degree of electron transfer opens possibilities to tailor material properties such as the chemical bond ( Z *) and electronic (ε ∞ ) polarizability, optical bandgap, and optical interband transitions characterized by ε 2 (ω). Hence, the insights gained from this study highlight the technological relevance of the concept of metavalent bonding and its potential for materials design.

Dielectric optical antennas have emerged as a promising nanophotonic architecture for manipulating the propagation and localization of light. However, the optically induced Mie resonances in an isolated nanoantenna are normally with broad spectra and poor Q-factors, limiting their performances in sensing, lasing, and nonlinear optics. Here, we dramatically enhance the Q-factors of Mie resonances in silicon (Si) nanoparticles across the optical band by arranging the nanoparticles in a periodic lattice. We select monocrystalline Si with negligible material losses and develop a unique method to fabricate nanoparticle arrays on a quartz substrate. By extinction dispersion measurements and electromagnetic analysis, we can identify three types of collective Mie resonances with Q-factors ∼ 500 in the same nanocylinder arrays, including surface lattice resonances, bound states in the continuum, and quasi-guided modes. Our work paves the way for fundamental research in strong light-matter interactions and the design of highly efficient light-emitting metasurfaces.

All-dielectric optical metasurfaces with high quality (Q) factors have been hampered by the lack of simultaneously lossless and high-refractive-index materials over the full visible spectrum. In fact, the use of low-refractive-index materials is unavoidable for extending the spectral coverage due to the inverse correlation between the bandgap energy (and therefore the optical losses) and the refractive index (n). However, for Mie resonant photonics, smaller refractive indices are associated with reduced Q factors and low mode volume confinement. Here, symmetry-broken quasi bound states in the continuum (qBICs) are leveraged to efficiently suppress radiation losses from the low-index (n ≈ 2) van der Waals material hexagonal boron nitride (hBN), realizing metasurfaces with high-Q resonances over the complete visible spectrum. The rational use of low- and high-refractive-index materials as resonator components is analyzed and the insights are harnessed to experimentally demonstrate sharp qBIC resonances with Q factors above 300, spanning wavelengths between 400 and 1000 nm from a single hBN flake. Moreover, the enhanced electric near fields are utilized to demonstrate second-harmonic generation with enhancement factors above 102 . These results provide a theoretical and experimental framework for the implementation of low-refractive-index materials as photonic media for metaoptics.

This paper reviews recent progress toward the creation of a nanophotonic framework for confining and guiding electromagnetic energy at visible and near-infrared frequencies using surface plasmon excitations sustained by metallic nanostructures. Prominent geometries such as metal/insulator/metal-gap waveguides are assessed in terms of light confinement and the energy attenuation of the guided modes, as well as metal-nanoparticle waveguides that guide light via near-field coupling of localized particle or gap plasmon modes for deep subwavelength confinement. The effective mode volume concept of dielectric optics is then applied to plasmonic nanoresonators, which allows a comparison with established dielectric microcavities and demonstrates the deep subwavelength confinement achievable in metallic nanocavities. Lastly, a solution to the coupling problem of surface plasmon-polariton modes to the outside world is presented in the form of a fiber-accessible metal-nanoparticle plasmon waveguide with experimentally demonstrated power transfer up to 75% at lambda=1.5 mum

A Rietveld analysis of neutron powder diffraction patterns obtained in situ during temperature scans shows that Fe2VAl crystallizes at room temperature in the fully ordered L21 structure and transforms at 1080 °C and at 1190 °C into partially disordered B2 and fully disordered A2 variants respectively. The low temperature stability of the L21 structure as well as the two high temperature L21 → B2 → A2 transitions are theoretically predicted by a combination of ab-initio (electronic structure and phonons) and thermodynamic calculations performed on special quasi-random structures. Pronounced cold work effects and the pre-transitional antisite defects which are observed respectively at 25 °C under mechanical stress and at high temperature (above 800 °C), appear to be generic effects in the family of Heusler alloys.

Since Purcell's seminal report 75 years ago, electromagnetic resonators have been used to control light-matter interactions to make brighter radiation sources and unleash unprecedented control over quantum states of light and matter. Indeed, optical resonators such as microcavities and plasmonic antennas offer excellent control but only over a limited spectral range. Strategies to mutually tune and match emission and resonator frequency are often required, which is intricate and precludes the possibility of enhancing multiple transitions simultaneously. In this letter, we report a strong radiative emission rate enhancement of Er3+-ions across the telecommunications C-band in a single plasmonic waveguide based on the Purcell effect. Our gap waveguide uses a reverse nanofocusing approach to efficiently enhance, extract and guide emission from the nanoscale to a photonic waveguide while keeping plasmonic losses at a minimum. Remarkably, the large and broadband Purcell enhancement allows us to resolve Stark-split electric dipole transitions, which are typically only observed under cryogenic conditions. Simultaneous radiative emission enhancement of multiple quantum states is of great interest for photonic quantum networks and on-chip data communications.

Ultrafine and nanocrystalline states of equiatomic face‐centered cubic (fcc) high‐entropy alloy (HEA) CoCrFeNiMn (“Cantor” alloy) are achieved by high‐pressure torsion (HPT) at 300 K (room temperature, RT) and 77 K (cryo). Although the hardness after RT‐HPT reaches exceptionally high values, those from cryo‐HPT are distinctly lower, at least when the torsional strain lies beyond γ = 25. The values are stable even during long‐time storage at ambient temperature. A similar paradoxal result is reflected by torque data measured in situ during HPT processing. The reasons for this paradox are attributed to the enhanced hydrostatic pressure, cryogenic temperature, and especially large shear strains achieved by the cryo‐HPT. At these conditions, selected area electron diffraction (SAD) patterns indicate that a partial local phase change from fcc to hexagonal close‐packed (hcp) structure occurs, which results in a highly heterogeneous structure. This heterogeneity is accompanied by both an increase in average grain size and especially a strong decrease in average dislocation density, which is estimated to mainly cause the paradox low strength.

Thermoelectric materials could play an important role in global sustainable energy. However, improving thermoelectric efficiency has proved difficult, largely due to the complex interdependence of electronic properties of solids. Early work by Ioffe has developed into the standard thermoelectric optimization paradigm of tuning the electronic carrier concentration in semiconductors. Although the localization theory of electrons by Anderson and Mott has developed in parallel, its potential for thermoelectrics optimization has not been explored. Here, we show that structural-disorder-induced electron localization also provides an effective optimization strategy for thermoelectric materials. By using a transport model that includes the relevant physics of localization, it is shown that the maximum thermoelectric figure of merit can be increased ∼20% by tuning both carrier concentration and disorder. The benefit of slight disorder is confirmed in two model Ge-Sb-Te material systems. Particularly for highly degenerate semiconductors, this bidimensional optimization strategy provides a new methodology to attain high thermoelectric performance.

Advanced MaterialsVolume 15, Issue 7-8 p. 562-562 Correction Plasmonics—A Route to Nanoscale Optical Devices (Advanced Materials, 2001, 13, 1501) S.A. Maier, S.A. MaierSearch for more papers by this authorM.L. Brongersma, M.L. BrongersmaSearch for more papers by this authorP.G. Kik, P.G. KikSearch for more papers by this authorS. Meltzer, S. MeltzerSearch for more papers by this authorA.A.G. Requicha, A.A.G. RequichaSearch for more papers by this authorB.E. Koel, B.E. KoelSearch for more papers by this authorH.A. Atwater, H.A. AtwaterSearch for more papers by this author S.A. Maier, S.A. MaierSearch for more papers by this authorM.L. Brongersma, M.L. BrongersmaSearch for more papers by this authorP.G. Kik, P.G. KikSearch for more papers by this authorS. Meltzer, S. MeltzerSearch for more papers by this authorA.A.G. Requicha, A.A.G. RequichaSearch for more papers by this authorB.E. Koel, B.E. KoelSearch for more papers by this authorH.A. Atwater, H.A. AtwaterSearch for more papers by this author First published: 09 April 2003 https://doi.org/10.1002/adma.200390134Citations: 42AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article.Citing Literature Volume15, Issue7-8April, 2003Pages 562-562 RelatedInformation

Abstract Tailoring critical light‐matter coupling is a fundamental challenge of nanophotonics, impacting fields from higher harmonic generation and energy conversion to surface‐enhanced spectroscopy. Plasmonic perfect absorbers (PAs), where resonant antennas couple to their mirror images in adjacent metal films, excel at obtaining different coupling regimes by tuning the antenna‐film gap size. However, practical PA applications require constant gap size, making it impossible to maintain critical coupling beyond singular wavelengths. Here, a new approach for plasmonic PAs is introduced by combining mirror‐coupled resonances with the unique loss engineering capabilities of plasmonic quasi‐bound states in the continuum. This novel combination allows to tailor the light–matter interaction within the under‐coupling, over‐coupling, and critical coupling regimes using flexible tuning knobs including asymmetry parameter, dielectric gap, and geometrical scaling factor. The study demonstrates a pixelated PA metasurface with optimal absorption over a broad range of mid‐infrared wavenumbers (950–2000 cm −1 ) using only a single gap size and applies it for multispectral surface‐enhanced molecular spectroscopy. Moreover, the asymmetry parameter enables convenient adjustment of the quality factor and resonance amplitude. This concept expands the capabilities and flexibility of traditional gap‐tuned PAs, opening new perspectives for miniaturized sensing platforms towards on‐chip and in situ detection.

SmallVolume 14, Issue 39 1870179 FrontispieceFree Access Raman Scattering Mapping: Sensitive and Reproducible Immunoassay of Multiple Mycotoxins Using Surface-Enhanced Raman Scattering Mapping on 3D Plasmonic Nanopillar Arrays (Small 39/2018) Xiaokun Wang, Xiaokun Wang Department of Bionano Engineering, Hanyang University, Ansan, 15588 South KoreaSearch for more papers by this authorSung-Gyu Park, Sung-Gyu Park Advanced Nano-Surface Department, Korea Institute of Materials Science (KIMS), Changwon, 51508 South KoreaSearch for more papers by this authorJuhui Ko, Juhui Ko Department of Bionano Engineering, Hanyang University, Ansan, 15588 South KoreaSearch for more papers by this authorXiaofei Xiao, Xiaofei Xiao The Blackett Laboratory, Department of Physics, Imperial College London, London, SW7 2AZ UKSearch for more papers by this authorVincenzo Giannini, Vincenzo Giannini The Blackett Laboratory, Department of Physics, Imperial College London, London, SW7 2AZ UK Instituto de Estructura de la Materia (IEM-CSIC), Consejo Superior de Investigaciones Científicas, Madrid, 28006 SpainSearch for more papers by this authorStefan A. Maier, Stefan A. Maier The Blackett Laboratory, Department of Physics, Imperial College London, London, SW7 2AZ UK Chair in Hybrid Nanosystems, Nanoinstitut München, Fakultät für Physik, Ludwig-Maximilians-Universität München, München, 80539 GermanySearch for more papers by this authorDong-Ho Kim, Dong-Ho Kim Advanced Nano-Surface Department, Korea Institute of Materials Science (KIMS), Changwon, 51508 South KoreaSearch for more papers by this authorJaebum Choo, Jaebum Choo Department of Bionano Engineering, Hanyang University, Ansan, 15588 South KoreaSearch for more papers by this author Xiaokun Wang, Xiaokun Wang Department of Bionano Engineering, Hanyang University, Ansan, 15588 South KoreaSearch for more papers by this authorSung-Gyu Park, Sung-Gyu Park Advanced Nano-Surface Department, Korea Institute of Materials Science (KIMS), Changwon, 51508 South KoreaSearch for more papers by this authorJuhui Ko, Juhui Ko Department of Bionano Engineering, Hanyang University, Ansan, 15588 South KoreaSearch for more papers by this authorXiaofei Xiao, Xiaofei Xiao The Blackett Laboratory, Department of Physics, Imperial College London, London, SW7 2AZ UKSearch for more papers by this authorVincenzo Giannini, Vincenzo Giannini The Blackett Laboratory, Department of Physics, Imperial College London, London, SW7 2AZ UK Instituto de Estructura de la Materia (IEM-CSIC), Consejo Superior de Investigaciones Científicas, Madrid, 28006 SpainSearch for more papers by this authorStefan A. Maier, Stefan A. Maier The Blackett Laboratory, Department of Physics, Imperial College London, London, SW7 2AZ UK Chair in Hybrid Nanosystems, Nanoinstitut München, Fakultät für Physik, Ludwig-Maximilians-Universität München, München, 80539 GermanySearch for more papers by this authorDong-Ho Kim, Dong-Ho Kim Advanced Nano-Surface Department, Korea Institute of Materials Science (KIMS), Changwon, 51508 South KoreaSearch for more papers by this authorJaebum Choo, Jaebum Choo Department of Bionano Engineering, Hanyang University, Ansan, 15588 South KoreaSearch for more papers by this author First published: 26 September 2018 https://doi.org/10.1002/smll.201870179Citations: 14AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Graphical Abstract In article number 1801623, Dong-Ho Kim, Jaebum Choo, and co-workers develop a conceptually new surface-enhanced Raman scattering (SERS)-based immunoassay platform using a 3D gold nanopillar substrate. In this substrate, decreased gap distances between nanopillars as well as multiple hot spots between SERS nanotags and nanopillars greatly enhance the coupling of local plasmonic fields, and this makes it possible to perform highly sensitive detection of multiple mycotoxins. Citing Literature Volume14, Issue39September 27, 20181870179 RelatedInformation

Metamaterialien sind künstliche Materialien, die neue Funktionalitäten wie höchstauflösende Bildgebung und optische Tarnung bieten. Präsentiert werden Berechnungen der photonischen Eigenschaften von dreidimensional isotropen Metamaterialien mit kubisch-doppeltgyroidalen und alternierenden gyroidalen Morphologien, die aus der Selbstorganisation von Blockcopolymeren hervorgehen. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Here, we unlock the properties of the recently introduced on-chip hollow-core microgap waveguide in the context of optofluidics which allows for intense light-water interaction over long lengths with fast response times. The nanoprinted waveguide operates by the anti-resonance effect in the visible and near-infrared domain and includes a hollow core with defined gaps every 176 µm. The spectroscopic capabilities are demonstrated by various absorption-related experiments, showing that the Beer-Lambert law can be applied without any modification. In addition to revealing key performance parameters, time-resolved experiments showed a decisive improvement in diffusion times resulting from the lateral access provided by the microgaps. Overall, the microgap waveguide represents a pathway for on-chip spectroscopy in aqueous environments.

Abstract Gallium phosphide (GaP) is a promising material for nanophotonics, given its large refractive index and a transparency over most of the visible spectrum. However, since easy phase‐matching is not possible with bulk GaP, a comprehensive study of its nonlinear optical properties for harmonic generation, especially when grown as thin films, is still missing. Here, second harmonic generation is studied from epitaxially grown GaP thin films, demonstrating that the absolute conversion efficiencies are comparable to a bulk wafer over the pump wavelength range from 1060 to 1370 nm. Furthermore, the results are compared to nonlinear simulations, and the second order nonlinear susceptibility is extracted, showing a similar dispersion and magnitude to that of the bulk material. Furthermore, the third order nonlinear susceptibility of amorphous GaP thin films is extracted from third harmonic generation to be more than one order of magnitude larger than that of the crystalline material, and generation of up to the fifth harmonic is reported. The results show the potential of crystalline and amorphous thin films for nonlinear optics with nanoantennas and metasurfaces, particularly in the visible to near infrared part of the spectrum.

Abstract High‐index Mie‐resonant dielectric nanostructures provide a new framework to manipulate light at the nanoscale. In particular their local field confinement together with their inherently low losses at frequencies below their bandgap energy allows to efficiently boost and control linear and nonlinear optical processes. Here, nanoantennas composed of a thin indium‐tin oxide (ITO) layer in the center of a dielectric gallium phosphide (GaP) nanodisc are investigated. While the linear response is similar to that of a pure GaP nanodisc, it is shown that second harmonic generation is enhanced across a broadband wavelength range. On the other hand, third harmonic generation is only marginally enhanced around the epsilon‐near‐zero wavelength of ITO. Linear and nonlinear finite‐difference time‐domain simulations show that despite the high refractive index contrast leading to strong field confinement inside the antenna's ITO layer, the nanogap enhancement effect is mitigated by the low nonlinear volume of the nanogap layer and the antenna's behavior at the harmonic wavelength. Measurement of ITO and GaP nonlinear susceptibilities additionally show a comparative advantage for harmonic generation in GaP. These investigations deliver insights on the mechanisms at play in nonlinear nanogap antennas and their potential applications as nanoscale devices.

The development of nanoscale nonlinear elements in photonic integrated circuits is hindered by the physical limits to the nonlinear optical response of dielectrics, which requires that the interacting waves propagate in transparent volumes for distances much longer than their wavelength. Here we present experimental evidence that optical nonlinearities in doped semiconductors are due to free-electron and their efficiency could exceed by several orders of magnitude that of conventional dielectric nonlinearities. Our experimental findings are supported by comprehensive computational results based on the hydrodynamic modeling, which naturally includes nonlocal effects, of the free-electron dynamics in heavily doped semiconductors. By studying third-harmonic generation from plasmonic nanoantenna arrays made out of heavily n-doped InGaAs with increasing levels of free-carrier density, we discriminate between hydrodynamic and dielectric nonlinearities. As a result, the value of maximum nonlinear efficiency as well as its spectral location can now be controlled by tuning the doping level. Having employed the common material platform InGaAs/InP that supports integrated waveguides, our findings pave the way for future exploitation of plasmonic nonlinearities in all-semiconductor photonic integrated circuits.

Resonances are usually associated with finite systems - the vibrations of clamped strings in a guitar or the optical modes in a cavity defined by mirrors. In optics, resonances may be induced in infinite continuous media via periodic modulations of their optical properties. Here we demonstrate that periodic modulations of the permittivity of a featureless thin film can also act as a symmetry breaking mechanism, allowing the excitation of photonic $\textit{quasi}$-bound states in the continuum ($\textit{q}$BICs). By interfering two ultrashort laser pulses in the unbounded film, transient resonances can be tailored through different parameters of the pump beams. We show that the system offers resonances tunable in wavelength and quality-factor, and spectrally selective enhancement of third harmonic generation. Due to a fast decay of the permittivity asymmetry, we observe ultrafast dynamics, enabling time-selective near-field enhancement with picosecond precision. Optically-induced permittivity asymmetries may be exploited in on-demand weak to ultrastrong light-matter interaction regimes and light manipulation at dynamically chosen wavelengths in lithography-free metasurfaces.

Plasmonic nanoantennas have proven to be efficient transducers of electromagnetic to mechanical energy and vice versa. The sudden thermal expansion of these structures after an ultrafast optical pulsed excitation leads to the emission of hypersonic acoustic waves to the supporting substrate, which can be detected by another antenna that acts as a high-sensitive mechanical probe due to the strong modulation of its optical response. Sophisticated fabrication techniques, together with the implementation of numerical simulations, have allowed the engineering of nanostructures for the controlled directional generation and detection of high-frequency acoustic phonons at the nanoscale, with many potential applications in telecommunications, sensing, mechanical switching, and energy transport. Here, we propose and experimentally demonstrate a nanoscale acoustic lens comprised of 11 gold nanodisks whose collective oscillation gives rise to an interference pattern that results in a diffraction-limited surface acoustic beam of about 340 nm width, with an amplitude contrast of 60%. Via spatially decoupled pump-probe experiments, we were able to map the radiated acoustic energy in the proximity of the focal area, obtaining a very good agreement with the continuum elastic theory.

Globally, motorcycles attract vast and varied users. However, since the rate of severe injury and fatality in motorcycle accidents far exceeds passenger car accidents, efforts have been directed toward increasing passive safety systems. Impact simulations show that the risk of severe injury or death in the event of a motorcycle-to-car impact can be greatly reduced if the motorcycle is equipped with passive safety measures such as airbags and seat belts. For the passive safety systems to be activated, a collision must be detected within milliseconds for a wide variety of impact configurations, but under no circumstances may it be falsely triggered. For the challenge of reliably detecting impending collisions, this paper presents an investigation towards the applicability of machine learning algorithms. First, a series of simulations of accidents and driving operation is introduced to collect data to train machine learning classification models. Their performance is henceforth assessed and compared via multiple representative and application-oriented criteria.

Abstract Optical imaging systems have greatly extended human visual capabilities, enabling the observation and understanding of diverse phenomena. Imaging technologies span a broad spectrum of wavelengths from X-ray to radio frequencies and impact research activities and our daily lives. Traditional glass lenses are fabricated through a series of complex processes, while polymers offer versatility and ease of production. However, modern applications often require complex lens assemblies, driving the need for miniaturization and advanced designs with micro- and nanoscale features to surpass the capabilities of traditional fabrication methods. Three-dimensional (3D) printing, or additive manufacturing, presents a solution to these challenges with benefits of rapid prototyping, customized geometries, and efficient production, particularly suited for miniaturized optical imaging devices. Various 3D printing methods have demonstrated advantages over traditional counterparts, yet challenges remain in achieving nanoscale resolutions. Two-photon polymerization lithography (TPL), a nanoscale 3D printing technique, enables the fabrication of intricate structures beyond the optical diffraction limit via the nonlinear process of two-photon absorption within liquid resin. It offers unprecedented abilities, e.g., alignment-free fabrication, micro- and nanoscale capabilities, and rapid prototyping of almost arbitrary complex 3D nanostructures. In this review, we emphasize the importance of the criteria for optical performance evaluation of imaging devices, discuss material properties relevant to TPL, fabrication techniques, and highlight the application of TPL in optical imaging. As the first panoramic review on this topic, it will equip researchers with foundational knowledge and recent advancements of TPL for imaging optics, promoting a deeper understanding of the field. By leveraging on its high-resolution capability, extensive material range, and true 3D processing, alongside advances in materials, fabrication, and design, we envisage disruptive solutions to current challenges and a promising incorporation of TPL in future optical imaging applications.

Abstract The rapid development of mixed‐halide perovskites has established a versatile optoelectronic platform owing to their extraordinary physical properties, but there remain challenges toward achieving highly reliable synthesis and performance, in addition, post‐synthesis approaches for tuning their photoluminescence properties after device fabrication remain limited. In this work, an effective approach is reported to leveraging hot electrons generated from plasmonic nanostructures to regulate the optical properties of perovskites. A plasmonic metasurface composed of Au nanoparticles can effectively tailor both photoluminescence and location‐specific phase segregation of mixed‐halide CsPbI 2 Br thin films. The ultrafast transient absorption spectroscopy measurements reveal hot electron injection on the timescale of hundreds of femtoseconds. Photocurrent measurements confirm the hot‐electron‐enhanced photon‐carrier conversion, and in addition, gate‐voltage tuning of phase segregation is observed because of correlated carrier injection and halide migration in the perovskite films. Finally, the characteristics of the gate‐modulated light emission are found to conform to a rectified linear unit function, serving as nonlinear electrical‐to‐optical converters in artificial neural networks. Overall, the hot electron engineering approach demonstrated in this work provides effective location‐specific control of the phase and optical properties of halide perovskites, underscoring the potential of plasmonic metasurfaces for advancing perovskite technologies.

In the rapidly evolving field of nanophotonics, achieving precise spectral and temporal light manipulation at the nanoscale remains a critical challenge. While photonic bound states in the continuum (BICs) have emerged as a powerful means of controlling light, their common reliance on geometrical symmetry breaking for obtaining tailored resonances makes them highly susceptible to fabrication imperfections and fundamentally limits their maximum resonance quality factor. Here, we introduce the concept of environmental symmetry breaking by embedding identical resonators into a surrounding medium with carefully placed regions of contrasting refractive indexes, activating permittivity-driven quasi-BIC resonances without any alterations of the underlying resonator geometry and unlocking an additional degree of freedom for light manipulation through actively tuning the surrounding refractive index contrast. We demonstrate this concept by integrating polyaniline (PANI), an electro-optically active polymer, to achieve electrically reconfigurable qBICs. This integration not only demonstrates rapid switching speeds, and exceptional durability but also significantly boosts the system's optical response to environmental perturbations. Our strategy significantly expands the capabilities of resonant light manipulation through permittivity modulation, opening avenues for on-chip optical devices, advanced sensing, and beyond.

Canavan disease is a devastating neurometabolic disorder caused by accumulation of N-acetylaspartate in brain and body fluids due to genetic defects in the aspartoacylase gene (ASPA). New gene therapies are on the horizon but will require early presymptomatic diagnosis to be fully effective. We therefore developed a fast and highly sensitive liquid chromatography mass spectrometry (LC-MS/MS)-based method for quantification of N-acetylaspartate in dried blood spots and established reference ranges for neonates and older controls. With this test, we investigated 45 samples of 25 Canavan patients including 8 with a neonatal sample. Measuring N-acetylaspartate concentration in dried blood with this novel test, all Canavan patients (with variable severity) were well separated from the control group (median; range: 5.7; 1.6–13.6 μmol/L [n = 45] vs 0.44; 0.24–0.99 μmol/L [n = 59] (p < 0.05)). There was also no overlap when comparing neonatal samples of Canavan patients (7.3; 5.1–9.9 μmol/L [n = 8]) and neonatal controls (0.93; 0.4–1.8 μmol/L [n = 784]) (p < 0.05). We have developed a new LC-MS/MS-based screening test for early postnatal diagnosis of Canavan disease that should be further evaluated in a population-based study once a promising treatment becomes available. The method meets the general requirements of newborn screening and should be appropriate for multiplexing with other screening approaches that combine chromatographic and mass spectrometry techniques.

Photonic metasurfaces offer exceptional control over light at the nanoscale, facilitating applications spanning from biosensing, and nonlinear optics to photocatalysis. Many metasurfaces, especially resonant ones, rely on periodicity for the collective mode to form, which makes them subject to the influences of finite size effects, defects, and edge effects, all of which have considerable negative impact at the application level. These aspects are especially important for quasi-bound state in the continuum (BIC) metasurfaces, for which the collective mode is highly sensitive to perturbations due to high quality factors and strong near-field enhancement. Here, we quantitatively investigate the mode formation in quasi-BIC metasurfaces on the individual resonator level using scattering scanning near-field optical microscopy (s-SNOM) in combination with a new image processing technique. We find that the quasi-BIC mode is formed at a minimum size of 10 x 10-unit cells much smaller than expected from far-field measurements. Furthermore, we show that the coupling direction of the resonators, defects and edge states have pronounced influence on the quasi-BIC mode. This study serves as a link between the far-field and near-field responses of metasurfaces, offering crucial insights for optimizing spatial footprint and active area, holding promise for augmenting applications such as catalysis and biospectroscopy.

Fe2VAl exhibits promising properties for thermoelectric applications. Here, we investigated the microstructure of melt spun Fe2VAl using electron microscopy, atom probe tomography and field ion microscopy. We observe platelet-shaped VCxNy precipitates in the vicinity of antiphase boundaries (APB) oriented along the {100}-plane. The mean distance between these precipitates is (140 ± 40) nm. This distance is shorter than the mean free phonon path at room temperature in Fe2VAl. Thus, these VCxNy precipitates, combined with the APB may efficiently lower the thermal conductivity of the alloy.

The unique crystal structure of BaBiTe3 containing Te···Te resonant bonds and its narrow band gap motivated the systematic study of the thermoelectric transport properties of BaBiTe3–xSex (x = 0, 0.05, and 0.1) presented here. This study gives insight in the chemical bonding and thermoelectric transport properties of BaBiTe3. The study shows that the presence of Te···Te resonant bonds in BaBiTe3 is best described as a linear combination of interdigitating (Te1–)2 side groups and infinite Ten chains. Rietveld X-ray structure refinements and extrinsic defect calculations reveal that the substitution of Te by Se occurs preferentially on the Te4 and Te5 sites, which are not involved in Te···Te bonding. This work strongly suggests that both multiband effects and native defects play an important role in the transport properties of BaBiTe3–xSex (x = 0, 0.05, and 0.1). The carrier concentration of BaBiTe3 can be tuned via Se substitution (BaBiTe3–xSex with x = 0, 0.05, and 0.1) to values near those needed to optimize the thermoelectric performance. The thermal conductivity of BaBiTe3–xSex (x = 0, 0.05, and 0.1) is found to be remarkably low (ca. 0.4 Wm–1K–1 at 600 K), reaching values close to the glass limit of BaBiSe3 (0.34 W m–1 K–1) and BaBiTe3 (0.28 W m–1 K–1). Calculations of the defect formation energies in BaBiTe3 suggest the presence of native BiBa+1 and TeBi+1 antisite defects, which are low in energy and likely responsible for the native n-type conduction and the high carrier concentration (ca. 1020 cm–3) found for all samples. The analyses of the electronic structure of BaBiTe3 and of the optical absorption spectra of BaBiTe3–xSex (x = 0, 0.05, 0.1, and 3) strongly suggest the presence of multiple electron pockets in the conduction band (CB) in all samples. These analyses also provide a possible explanation for the two optical transitions observed for BaBiTe3. High-temperature optical absorption measurements and thermoelectric transport analyses indicate that bands higher in the conduction band converge with the conduction band minimum (CBM) with increasing temperature and contribute to the thermoelectric transport properties of BaBiTe3 and BaBiTe2.95Se0.05. This multiband contribution can account for the ∼50% higher zTmax of BaBiTe3 and BaBiTe2.95Se0.05 (∼0.4 at 617 K) compared to BaBiTe2.9Se0.1 (∼0.2 at 617 K), for which no such contribution was found. The increase in the band offset between the CBM and bands higher in the conduction band with respect to the selenium content is one possible explanation for the absence of multiband effects in the thermoelectric transport properties of BaBiTe2.9Se0.1.

Abstract The bonding nature between chalcogenides and rare‐earth‐elements is typically described as ionic in the spirit of the Zintl‐Klemm formalism; yet, recent efforts showed that lanthanides also act as d ‐metals in transition‐metal‐post‐transition‐metal‐element bonding. Hence, how can we describe the bonding nature between chalcogen and europium atoms, which have frequently acted as electron‐donors like group‐I/II‐elements? To answer this question, we prototypically explored the electronic structure of Eu 2 CuSe 3 , which was obtained in considerable yields from solid‐state reactions of the pure elements at 600 °C. The crystal structure of Eu 2 CuSe 3 was determined based on X‐ray diffraction experiments and it is composed of diverse types of linear chains of selenium polyhedra enclosing the copper and europium atoms. These chains are condensed into [EuCuSe 3 ] layers, which are separated by additional europium atoms. From analyses of the crystal structure and electronic structure of Eu 2 CuSe 3 , it is clear that there are two different europium valence states, whose nature controls if europium acts as an electron‐donor like a group‐I/II‐element or as a d ‐metal.

We have studied the nonlinear optical properties of single β-barium borate nanocrystals, with potential applications as probes in nonlinear sensing and imaging schemes. Our work demonstrates their ability to generate second, third, fourth, and fifth harmonics. The particles’ polarization response is studied and compared with simulations based on the bulk nonlinear tensors, with good agreement. Furthermore, the nonlinear susceptibilities of different orders are estimated.

Because of their bosonic nature, exciton-polaritons can condense into a single macroscopic coherent state under the right conditions. This process is uniquely characterized by its out-of-equilibrium nature, which gives rise to some distinctions when compared to conventional Bose-Einstein condensation. This Chapter begins by describing condensation in general and then describes room-temperature demonstrations of polariton condensation in organic microcavities along with phenomena that have recently been observed in these systems such as long-range spatial coherence, spontaneous vortex formation and dynamic instabilities.

Strongly coupled phonon polaritons in patterned polar dielectric nano-resonators give rise to the formation of hybridized energy states with intriguing properties. However, direct observation of mode coupling in these periodic nanostructures is still challenging for momentum-matching-required far-field spectroscopies. Here, we explore the near-field response of strong coupling between propagating and localized polariton modes sustained in SiC phonon polaritonic crystals (PhPCs) to reveal the evolution of Rabi splittings with the change of lattice constant in the near-field perspective. The near-field nano-spectra of PhPCs show distinct Rabi splitting near the forbidden bands of ∼16 cm−1 in the band structures. In particular, an exotic three-polariton-coupling effect is observed with three splitting peaks in the nano-spectra induced by the interaction between local monopolar modes in nano-pillars and zone-folded phonon polaritons. Furthermore, sharp dips indicating weak near-field scatterings appear in nano-spectra at the intrinsic frequencies of the monopolar modes with strong local-field enhancement, which are estimated to be bright scattering peaks intuitively. These results would inspire the dispersion engineering and characterization of coupled phononic nano-resonators for diverse nanophotonic applications.

Dielectric metasurfaces supporting quasi-bound states in the continuum (quasi-BICs) exhibit very high quality factor resonances and electric field confinement. However, accessing the high-Q end of the quasi-BIC regime usually requires marginally distorting the metasurface design from a BIC condition, pushing the needed nanoscale fabrication precision to the limit. This work introduces a novel concept for generating high-Q quasi-BICs, which strongly relaxes this requirement by incorporating a relatively large perturbative element close to high-symmetry points of an undistorted BIC metasurface, acting as a coupler to the radiation continuum. We validate this approach by adding a $\sim$100 nm diameter cylinder between two reflection-symmetry points separated by a 300 nm gap in an elliptical disk metasurface unit cell, using gallium phosphide as the dielectric. We find that high-Q resonances emerge when the cylindrical coupler is placed at any position between such symmetry points. We further explore this metasurface's second harmonic generation capability in the optical range. Displacing the coupler as much as a full diameter from a BIC condition produces record-breaking normalized conversion efficiencies >10$^{2}$ W$^{-1}$. The strategy of enclosing a disruptive element between multiple high-symmetry points in a BIC metasurface could be applied to construct robust high-Q quasi-BICs in many geometrical designs.

Metamaterials are extremely important in advanced technologies, but usually, they rely on the resonant behavior of their constituent blocks. This strongly limits the application of metamaterials to particular frequency band ranges. However, metamaterials with broadband behaviors are highly desirable and are essential for many applications. Herein, recently discovered metamaterials that are composed of densely packed metallic nanoparticles but behave as effective dielectrics are explored. Such metamaterials are extremely transparent for all wavelengths within or exceeding the near infrared and their performance is constant across an ultra‐broadband range of frequencies, which is vital to many devices that operate across the same frequency range. The ability to tune the refractive index of these metamaterials to unnaturally high values while maintaining transparency opens new avenues, such as creating flat, thin metalenses in the terahertz region where only bulk lenses are currently available. To highlight those features, several new possible infrared and terahertz applications of these metamaterials which push the boundary of existing technology in THz photonics are shown.

The realization of lossless metasurfaces with true chirality crucially requires the fabrication of three-dimensional structures, constraining their feasibility for experiments and hampering practical implementations. Even though the three-dimensional assembly of metallic nanostructures has been demonstrated previously, the resulting plasmonic resonances suffer from high intrinsic and radiative losses. The concept of photonic bound states in the continuum (BICs) is instrumental for tailoring radiative losses in diverse geometries, especially when implemented using lossless dielectrics, but applications have so far been limited to planar and intrinsically achiral structures. Here, we introduce a novel nanofabrication approach to unlock the height of generally flat all-dielectric metasurfaces as an accessible parameter for efficient resonance and functionality control. In particular, we realize out-of-plane symmetry breaking in quasi-BIC metasurfaces and leverage this design degree of freedom to demonstrate, for the first time, an optical all-dielectric quasi-BIC metasurface with maximum intrinsic chirality that responds selectively to light of a particular circular polarization depending on the structural handedness. Our experimental results not only open a new paradigm for all-dielectric BICs and chiral nanophotonics but also promise advances in the realization of efficient generation of optical angular momentum, holographic metasurfaces, and parity-time symmetry-broken optical systems.

Abstract The Large Hadron Collider (LHC) at CERN will be upgraded to the High-Luminosity LHC (HL-LHC) by 2029. In order to fully exploit the physics potential of the high luminosity era the experiments must undergo major upgrades. In the context of the upgrade of the Compact Muon Solenoid (CMS) experiment the silicon tracker will be fully replaced. The outer part of the new tracker (Outer Tracker) will be equipped with about 13,000 double-layer silicon sensor modules with two different flavors: PS modules consisting of a macro-pixel and a strip sensor and 2S modules using two strip sensors. These modules can discriminate between trajectories of charged particles with low and high transverse momentum. The different curvature of the trajectories in the CMS magnetic field leads to different hit signatures in the two sensor layers. By reading out both sensors, matching hits in the seed and correlation layer “stubs” are identified. This stub information is generated at the LHC bunch crossing frequency of 40 MHz and serves as input for the first stage of the CMS trigger. In order to quantify the hit and stub detection efficiency, beam tests have been performed. This article comprises selected studies from measurements gathered during two beam tests at the DESY test beam facility with 2S prototype modules assembled in 2021, featuring the Low Power Gigabit Transceiver (lpGBT). In order to compare the module performance at the beginning and end of the CMS runtime, a module with irradiated components has been built and intensively tested.

In a temporal version of a single slit and Young’s double slit experiments, newly generated optical frequencies form a diffraction pattern. The spectral extent of these frequencies is beyond the expected bandwidth of the modulation.

Gold films, made suitably thin, exhibit high transparency and high conductivity at the same time. A. Kossoy, K. Leosson, and co-workers from the University of Iceland and Imperial College London have fabricated continuous gold films as thin as approximately 5 nm on glass substrates, using conventional deposition techniques. Experiments on page 71 show that transmission of light through such films is consistently lower than the theoretically expected value, an effect attributed to atomic-scale interface roughness.

The ultimate miniaturization of optical devices requires structures that guide electromagnetic energy with a lateral confinement below the diffraction limit of light. In this thesis, the possibility of employing plasmon-polariton excitations in plasmon waveguides consisting of closely spaced metal nanoclusters for this purpose is examined. The feasibility of energy transport with mode sizes below the diffraction limit of visible light over distances of several hundred nanometers is demonstrated. As a macroscopic analogue to plasmon waveguides, the transport of electromagnetic energy in the microwave regime along closely spaced centimeter-scale metal rods is examined. Full-field electrodynamic simulations show that information transport occurs at a group velocity of 0.65c for fabricated structures consisting of copper rods excited at 8 GHz. A variety of passive routing structures and an all-optical modulator are demonstrated. The possibility of guiding electromagnetic energy at visible frequencies with mode sizes below the diffraction limit using plasmon waveguides is analyzed using a point-dipole model and finite-difference time-domain simulations. It is shown that energy transport occurs via near-field coupling between metal nanoparticles, which leads to coherent propagation of energy. For spherical gold particles in air, group velocities up to 0.06c are demonstrated, and a change in particle shape to spheroidal particles shows up to a threefold increase in group velocity. Pulses with transverse polarization are shown to propagate with negative phase velocities antiparallel to the energy flow. Plasmon waveguides consisting of gold and silver nanoparticles were fabricated using electron beam lithography. The key parameters that govern the energy transport are determined for various interparticle spacings and particle chain lengths using far-field measurements of the collective plasmon modes. Spherical gold nanoparticles with a diameter of 50 nm and an interparticle spacing of 75 nm show an energy attenuation of 6 dB/30 nm. This loss can be reduced by one order of magnitude by a geometry change to spheroidal particles. Using the tip of a near-field optical microscope as a local excitation source and fluorescent nanospheres as detectors, experimental evidence for energy transport over a distance of 0.5 micron is presented for plasmon waveguides consisting of silver rods with a 3:1 aspect ratio.

Quantum plasmonics is an exciting subbranch of nanoplasmonics where the laws of quantum theory are used to describe light–matter interactions on the nanoscale. Plasmonic materials allow extreme subdiffraction confinement of (quantum or classical) light to regions so small that the quantization of both light and matter may be necessary for an accurate description. State-of-the-art experiments now allow us to probe these regimes and push existing theories to the limits which opens up the possibilities of exploring the nature of many-body collective oscillations as well as developing new plasmonic devices, which use the particle quality of light and the wave quality of matter, and have a wealth of potential applications in sensing, lasing, and quantum computing. This merging of fundamental condensed matter theory with application-rich electromagnetism (and a splash of quantum optics thrown in) gives rise to a fascinating area of modern physics that is still very much in its infancy. In this review, we discuss and compare the key models and experiments used to explore how the quantum nature of electrons impacts plasmonics in the context of quantum size corrections of localized plasmons and quantum tunneling between nanoparticle dimers. We also look at some of the remarkable experiments that are revealing the quantum nature of surface plasmon polaritons.

The on-chip detection of fluorescent light is essential for many bioanalytical and life-science related applications. Here, the optofluidic light cage consisting of a sparse array of micrometer encircling a hollow core represents an innovative concept, particularly for on-chip waveguide-based spectroscopy. In the present work, we demonstrate the potential of the optofluidic light cage concept in the context of integrated on-chip fluorescence spectroscopy. Specifically, we show that fluorescent light from a dye-doped aqueous solution generated in the core of a nanoprinted dual-ring light cage can be efficiently captured and guided to the waveguide ports. Notably, the fluorescence collection occurs predominantly in the fundamental mode, a property that distinguishes it from evanescent field-based waveguide detection schemes that favor collection in higher-order modes. Through exploiting the flexibility of waveguide design and 3D nanoprinting, both excitation and emission have been localized in the high transmission domains of the fundamental core mode. Fast diffusion, detection limits comparable to bulk measurements, and the potential of this approach in terms of device integration were demonstrated. Together with previous results on absorption spectroscopy, the achievements presented here suggest that the optofluidic light cage concept defines a novel photonic platform for integrated on-chip spectroscopic devices and real-time sensors compatible with both the fiber circuitry and microfluidics. Applications in areas such as bioanalytics and environmental sciences are conceivable, while more sophisticated applications such as nanoparticle tracking analysis and integrated Raman spectroscopy could be envisioned.

All-dielectric optical metasurfaces with high quality (Q) factors have so far been hampered by the lack of simultaneously lossless and high refractive index (RI) materials over the full visible spectrum. To achieve broad spectral coverage, the use of low-index materials is, in fact, unavoidable due to the inverse correlation between the band-gap energy (and therefore the optical losses) and the RI. However, for Mie resonant photonics, smaller RIs are associated with reduced Q factors and mode volume confinement. In this work, we leverage symmetry-broken bound states in the continuum (BICs) to efficiently suppress radiation losses from the low-index (n~2) van der Waals material hexagonal boron nitride (hBN), realizing metasurfaces with high-Q resonances over the complete visible spectrum. In particular, we analyze the rational use of low and high RI materials as resonator components and harness our insights to experimentally demonstrate sharp BIC resonances with Q factors above 300, spanning wavelengths between 400 nm and 1000 nm from a single hBN flake. Moreover, we utilize the enhanced electric near-fields to demonstrate second harmonic generation (SHG) with enhancement factors above 102. Our results provide a theoretical and experimental framework for the implementation of low RI materials as photonic media for metaoptics.

Abstract The CMS Outer Tracker phase-2 upgrade silicon modules are required to reach noise levels close to the ones expected from the analogue front-end attached to an ideal pixel/strip. Module prototypes, featuring the latest and final prototype hybrids before the production, showed noise that was higher than the expected which could pose a problem in terms of achieving the hit efficiency target. Investigations, reveal the failure modes which are modelled in order to guide mitigation tweaks for the production designs. Knowledge acquired from the investigations along with the noise mitigation design changes implemented on the production hybrids are presented.

This paper describes a novel procedure for the purification of raw atelocollagen solutions, based on a multi-stage selective precipitation of the protein fractions using polyethylene glycols (PEGs) as crowding agents. The precipitation selectivity was provided by the simultaneous effect of pH and crowding agents. Two adjuvants were added to improve the precipitation selectivity: ethanol and trimethylamine-N-oxide (TMAO). An optimal composition of the crowding mixture, consisting of 23 % PEG 400, 61 % PEG 6000 and 16 % PEG 20000, induced a „screening” effect within the isoelectric range of atelocollagen. The optimal recipe for the screening of an atelocollagen solution obtained at plant scale was: 80 g/L crowding mixture, 0.2 molar TMAO, 0.3 molar ethanol, pH = 3.8, processing time: 4-6 hr at 5 ÷ 10 °C. The proposed procedure is useful in the manufacture of colloidal collagen solutions, in order to replace or to facilitate expensive and denaturing operations, like ultrafiltration.ResumenEste trabajo describe un procedimiento novedoso para la purificacion de soluciones crudas de atelocolageno [extracto acuoso de colageno por uso de proteasas], basado en precipitaciones selectivas en varias etapas de fracciones proteinicas por medio de glicoles polietilenicos (PEGs) como agentes de hacinamiento [a la solubilidad]. La selectividad de precipitacion fue efectuada por el uso simultaneo de agentes de hacinamiento y pH. Dos adyuvantes fueron anadidos para incrementar la selectividad de la precipitacion: etanol y oxido-N-trimetil amina (TMAO). La composicion optima de la mezcla de hacinamiento constituida por 23% PEG 400, 61% PEG 6000 y 16% de PEG 2000, indujeron un effecto de apantallamiento en el rango isoelectrico de la solucion de atelocolageno. La optima composicion para el apantallamiento de una solucion de atelocolageno a escala de planta fue: 80 g/L de las mezcla de hacinamiento, 0,2 molar TMAO, 0,3 molar etanol, pH=3,8, y tiempo de procesamiento: 4-6 horas.entre 5-10°C. El procedimiento propuesto es util en la preparacion de soluciones colagenicas coloidales, en vista de reemplazar o facilitar costosas operaciones con riesgo de denaturacion, como lo es el de la ultrafiltracion.

In article number 1904257, Sung-Gyu Park, Hyungsoon Im, Dong-Ho Kim, and co-workers develop a direct formation and selective self-assembly of spherical plasmonic nanoparticles on slippery 3D Au nanopillars through a simple vacuum deposition process by enhancing the surface diffusion of adsorbed plasmonic atoms. The 3D plasmonic chips show very high performance in both surface-enhanced Raman spectroscopy and plasmon-enhanced fluorescence for avian influenza detection.

Surface oxygen vacancy defects are some of the most reactive sites but are notoriously difficult to detect, often healing upon exposure to air. In article number 1901841, Ivan P. Parkin, Stefan A. Maier, and co-workers show how photo-induced enhanced Raman spectroscopy (PIERS) can be used to indirectly track vacancy formation and healing of photo-induced vacancies under ambient conditions on metal oxide semiconductors.

Nanostructuring boron-rich materials should significantly impact their thermal and electrical transport properties.Nonetheless, nanostructured monoliths of such materials could not be achieved in the 10 nm range so far, because of the large temperatures required to synthesize and produce boron-rich compounds.Such a nanostructuration may have important consequences for achieving a trade-off between enhanced electrical and low thermal conductivity in boron-rich materials, which are among the few materials enabling thermoelectric power generation above 1000 K thanks to their thermal stability, high positive Seebeck coefficients, and low thermal conductivity.In this study, we use a one-pot synthesis in inorganic molten salts to yield a nanocomposite consisting of metallic HfB 2 nanocrystals dispersed in an insulating amorphous boron-rich matrix with a controlled volume fraction of nanocrystals from 16 to 56 vol %.We show that this controlled liquid-phase synthesis can be coupled to spark plasma sintering for densification preserving the nanostructure.The relationships between the reagent ratio in the liquid-phase synthesis, sintering conditions, and transport properties of the densified nanocomposites are then highlighted.We then design materials exhibiting metallic electrical conductivity related to the HfB 2 nanocrystals, together with enhanced thermal dissipation attributed to the nanostructured amorphous boron matrix.Combined with the versatility offered by in-solution routes toward boride-based nanocomposites, this work opens a new avenue for tuning transport properties in boron-rich nanomaterials.

We report optically pumped hybrid photonic – plasmonic ZnO nanowire lasers operating near the surface plasmon frequency. Here, we use the non-linearity of the laser process itself to reveal the internal ~1 ps dynamics of these plasmonic lasers.

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

Electrocatalysis plays a crucial role in realizing the transition towards green energy, driving research directions from hydrogen generation to carbon dioxide reduction. Understanding electrochemical reactions is crucial to improve their efficiency and to bridge the gap toward a sustainable zero-carbon future. Surface-enhanced infrared absorption spectroscopy (SEIRAS) is a suitable method for investigating these processes because it can monitor with chemical specificity the mechanisms of the reactions. However, it remains difficult to detect many relevant aspects of electrochemical reactions such as short-lived intermediates. Here, we develop and experimentally realize an integrated nanophotonic-electrochemical SEIRAS platform for the in situ investigation of molecular signal traces emerging during electrochemical experiments. Specifically, we implement a platinum nano-slot metasurface featuring strongly enhanced electromagnetic near fields and spectrally target it at the weak vibrational bending mode of adsorbed CO at ~2033 cm-1. Crucially, our platinum nano-slot metasurface provides high molecular sensitivity. The resonances can be tuned over a broad range in the mid-infrared spectrum. Compared to conventional unstructured platinum layers, our nanophotonic-electrochemical platform delivers a substantial improvement of the experimentally detected characteristic absorption signals by a factor of 27, enabling the detection of new species with weak signals, fast conversions, or low surface concentrations. By providing a deeper understanding of catalytic reactions, we anticipate our nanophotonic-electrochemical platform to open exciting perspectives for electrochemical SEIRAS, surface-enhanced Raman spectroscopy, and the study of reactions in other fields of chemistry such as photoelectrocatalysis.

Structured light has proven useful for numerous photonic applications. However, the current use of structured light in optical fiber science and technology is severely limited by mode mixing or by the lack of optical elements that can be integrated onto fiber end-faces for complex wavefront control, and hence generation of structured light is still handled outside the fiber via bulky optics in free space. We report a metafiber platform capable of creating arbitrarily structured light on the hybrid-order Poincar\'e sphere. Polymeric metasurfaces, with unleashed height degree of freedom and a greatly expanded 3D meta-atom library, were laser nanoprinted and interfaced with polarization-maintaining single-mode fibers. Multiple metasurfaces were interfaced on the fiber end-faces, transforming the fiber output into different structured-light fields, including cylindrical vector beams, circularly polarized vortex beams, and an arbitrary vector field. Our work provides a new paradigm for advancing optical fiber science and technology towards fiber-integrated light shaping, which may find important applications in fiber communications, fiber lasers and sensors, endoscopic imaging, fiber lithography, and lab-on-fiber technology.

Selective control of light is essential for optical science and technology with numerous applications. Nanophotonic waveguides and integrated couplers have been developed to achieve selective coupling and spatial control of an optical beam according to its multiple degrees of freedom. However, previous coupling devices remain passive with an inherently linear response to the power of incident light limiting their maximal optical selectivity. Here, we demonstrate nonlinear optical selectivity through selective excitation of individual single-mode nanolasers based on the spin and orbital angular momentum of light. Our designed nanolaser circuits consist of plasmonic metasurfaces and individual perovskite nanowires, enabling subwavelength focusing of angular-momentum-distinctive plasmonic fields and further selective excitation of single transverse laser modes in nanowires. The optically selected nanolaser with nonlinear increase of light emission greatly enhances the baseline optical selectivity offered by the metasurface from about 0.4 up to near unity. Our demonstrated nonlinear optical selectivity may find important applications in all-optical logic gates and nanowire networks, ultrafast optical switches, nanophotonic detectors, and on-chip optical and quantum information processing.

Two-dimensional (2D) semiconductors possess strongly bound excitons, opening novel opportunities for engineering light-matter interaction at the nanoscale. However, their in-plane confinement leads to large non-radiative exciton-exciton annihilation (EEA) processes, setting a fundamental limit for their photonic applications. In this work, we demonstrate suppression of EEA via enhancement of light-matter interaction in hybrid 2D semiconductor-dielectric nanophotonic platforms, by coupling excitons in WS$ _2 $ monolayers with optical Mie resonances in dielectric nanoantennas. The hybrid system reaches an intermediate light-matter coupling regime, with photoluminescence enhancement factors up to 10$ ^2 $. Probing the exciton ultrafast dynamics reveal suppressed EEA for coupled excitons, even under high exciton densities $>$ 10$^{12}$ cm$^{-2} $. We extract EEA coefficients in the order of 10$^{-3} $, compared to 10$^{-2} $ for uncoupled monolayers, as well as absorption enhancement of 3.9 and a Purcell factor of 4.5. Our results highlight engineering the photonic environment as a route to achieve higher quantum efficiencies for low-power hybrid devices, and larger exciton densities, towards strongly correlated excitonic phases in 2D semiconductors.

Active metasurfaces provide unique advantages for on-demand light manipulation at a subwavelength scale for emerging applications of 3D displays, augmented/virtual reality (AR/VR) glasses, holographic projectors and light detection and ranging (LiDAR). These applications put stringent requirements on switching speed, cycling duration, controllability over intermediate states, modulation contrast, optical efficiency and operation voltages. However, previous demonstrations focus only on particular subsets of these key performance requirements for device implementation, while the other performance metrics have remained too low for any practical use. Here, we demonstrate an active Huygens' metasurface based on in-situ grown conductive polymer with holistic switching performance, including switching speed of 60 frames per second (fps), switching duration of more than 2000 switching cycles without noticeable degradation, hysteresis-free controllability over intermediate states, modulation contrast of over 1400%, optical efficiency of 28% and operation voltage range within 1 V. Our active metasurface design meets all foundational requirements for display applications and can be readily incorporated into other metasurface concepts to deliver high-reliability electrical control over its optical response, paving the way for compact and robust electro-optic metadevices.

Van der Waals (vdW) materials, including hexagonal boron nitride (hBN), are layered crystalline solids with appealing properties for investigating light-matter interactions at the nanoscale. hBN has emerged as a versatile building block for nanophotonic structures, and the recent identification of native optically addressable spin defects has opened up exciting possibilities in quantum technologies. However, these defects exhibit relatively low quantum efficiencies and a broad emission spectrum, limiting potential applications. Optical metasurfaces present a novel approach to boost light emission efficiency, offering remarkable control over light-matter coupling at the sub-wavelength regime. Here, we propose and realise a monolithic scalable integration between intrinsic spin defects in hBN metasurfaces and high quality (Q) factor resonances leveraging quasi-bound states in the continuum (qBICs). Coupling between spin defect ensembles and qBIC resonances delivers a 25-fold increase in photoluminescence intensity, accompanied by spectral narrowing to below 4 nm linewidth facilitated by Q factors exceeding $10^2$. Our findings demonstrate a new class of spin based metasurfaces and pave the way towards vdW-based nanophotonic devices with enhanced efficiency and sensitivity for quantum applications in imaging, sensing, and light emission.

Surface-enhanced spectroscopy techniques are the method-of-choice to characterize adsorbed intermediates occurring during electrochemical reactions, which are crucial in realizing a green sustainable future. Characterizing species with low coverages or short lifetimes have so far been limited by low signal enhancement. Recently, metasurface-driven surface-enhanced infrared absorption spectroscopy (SEIRAS) has been pioneered as a promising narrowband technology to study single vibrational modes of electrochemical interfaces during CO oxidation. However, many reactions involve several species or configurations of adsorption that need to be monitored simultaneously requiring reproducible and broadband sensing platforms to provide a clear understanding of the underlying electrochemical processes. Here, we experimentally realize multi-band metasurface-driven SEIRAS for the in-situ study of electrochemical CO2 reduction on a Pt surface. We develop an easily reproducible and spectrally-tunable platinum nano-slot metasurface. Two CO adsorption configurations at 2030 cm-1 and 1840 cm-1 are locally enhanced as a proof of concept that can be extended to more vibrational bands. Our platform provides a 41-fold enhancement in the detection of characteristic absorption signals compared to conventional broadband electrochemically roughened platinum films. A straightforward methodology is outlined starting by baselining our system in CO saturated environment and clearly detecting both configurations of adsorption, in particular the hitherto hardly detectable CO bridge configuration. Then, thanks to the signal enhancement provided by our platform, we find that the CO bridge configuration on platinum does not play a significant role during CO2 reduction in an alkaline environment. We anticipate that our technology will guide researchers in developing similar sensing platforms.

High-index Mie-resonant dielectric nanostructures provide a new framework to manipulate light at the nanoscale. In particular their local field confinement together with their inherently low losses at frequencies below their band-gap energy allows to efficiently boost and control linear and nonlinear optical processes. Here, we investigate nanoantennas composed of a thin indium-tin oxide layer in the center of a dielectric Gallium Phosphide nanodisk. While the linear response is similar to that of a pure GaP nanodisk, we show that the second and third-harmonic signals of the nanogap antenna are boosted at resonance. Linear and nonlinear finite-difference time-domain simulations show that the high refractive index contrast leads to strong field confinement inside the antenna's ITO layer. Measurement of ITO and GaP nonlinear susceptibilities deliver insight on how to engineer nonlinear nanogap antennas for higher efficiencies for future nanoscale devices.

Nanophotonic devices excel at confining light into intense hot spots of the electromagnetic near fields, creating unprecedented opportunities for light-matter coupling and surface-enhanced sensing. Recently, all-dielectric metasurfaces with ultrasharp resonances enabled by photonic bound states in the continuum have unlocked new functionalities for surface-enhanced biospectroscopy by precisely targeting and reading out molecular absorption signatures of diverse molecular systems. However, BIC-driven molecular spectroscopy has so far focused on endpoint measurements in dry conditions, neglecting the crucial interaction dynamics of biological systems. Here, we combine the advantages of pixelated all-dielectric metasurfaces with deep learning-enabled feature extraction and prediction to realize an integrated optofluidic platform for time-resolved in-situ biospectroscopy. Our approach harnesses high-Q metasurfaces specifically designed for operation in a lossy aqueous environment together with advanced spectral sampling techniques to temporally resolve the dynamic behavior of photoswitchable lipid membranes. Enabled by a software convolutional neural network, we further demonstrate the real-time classification of the characteristic cis and trans membrane conformations with 98% accuracy. Our synergistic sensing platform incorporating metasurfaces, optofluidics, and deep learning opens exciting possibilities for studying multi-molecular biological systems, ranging from the behavior of transmembrane proteins to the dynamic processes associated with cellular communication.

Semiconductor-based surface-enhanced Raman spectroscopy (SERS) substrates, as a new frontier in the field of SERS, are hindered by their poor electromagnetic field confinement, and weak light-matter interaction. Metasurfaces, a class of 2D artificial materials based on the electromagnetic design of nanophotonic resonators, enable strong electromagnetic field enhancement and optical absorption engineering for a wide range of semiconductor materials. However, the engineering of semiconductor substrates into metasurfaces for improving SERS activity remains underexplored. Here, we develop an improved SERS metasurface platform that leverages the combination of titanium oxide (TiO2) and the emerging physical concept of optical bound states in the continuum (BICs) to boost the Raman emission. Moreover, fine-tuning of BIC-assisted resonant absorption offers a pathway for maximizing the photoinduced charge transfer effect (PICT) in SERS. We achieve ultrahigh values of BIC-assisted electric field enhancement (|E/E0|^2 ~ 10^3), challenging the preconception of weak electromagnetic (EM) field enhancement on semiconductor SERS substrates. Our BIC-assisted TiO2 metasurface platform offers a new dimension in spectrally-tunable SERS with earth-abundant and bio-compatible semiconductor materials, beyond the traditional plasmonic ones.

Off-axis twisted waveguides possess unique optical properties such as circular and orbital angular momentum (OAM) birefringence, setting them apart from their straight counterparts. Analyzing mode formation in such helical waveguides relies on the use of specific coordinate frames that follow the twist of the structure, making the waveguide invariant along one of the new coordinates. In this study, the differences between modes forming in high-contrast off-axis twisted waveguides defined in the three most important coordinate systems - the Frenet-Serret, the helicoidal, or the Overfelt frame - are investigated through numerical simulations. We explore modal characteristics up to high twist rates (pitch: 50 $\mu$m) and clarify a transformation allowing to map the modal fields and the effective index back to the laboratory frame. In case the waveguide is single-mode, the fundamental modes of the three types of waveguides show significant differences in terms of birefringence, propagation loss, and polarization. Conversely, the modal characteristics of the investigated waveguides are comparable in the multimode domain. Furthermore, our study examines the impact of twisting on spatial mode properties with the results suggesting a potential influence of the photonic spin Hall and orbital Hall effects. Additionally, modes of single-mode helical waveguides were found to exhibit superchiral fields on their surfaces. Implementation approaches such as 3D-nanoprinting or fiber-preform twisting open the doors to potential applications of such highly twisted waveguides, including chip-integrated devices for broadband spin- and OAM-preserving optical signal transport, as well as applications in chiral spectroscopy or nonlinear frequency conversion.

Enhancing and controlling light-matter interactions is crucial in nanotechnology and material science, propelling research on green energy, laser technology, and quantum cryptography. Central to enhanced light-matter coupling are two parameters: the spectral overlap between an optical cavity mode and the material's spectral features (e.g., excitonic or molecular absorption lines), and the quality factor of the cavity. Controlling both parameters simultaneously is vital, especially in complex systems requiring extensive data to uncover the numerous effects at play. However, so far, photonic approaches have focused solely on sampling a limited set of data points within this 2D parameter space. Here we introduce a nanophotonic approach that can simultaneously and continuously encode the spectral and quality factor parameter space of light-matter interactions within a compact spatial area. Our novel dual-gradient metasurface design is composed of a 2D array of smoothly varying subwavelength nanoresonators, each supporting a unique mode. This results in 27,500 distinct modes within one array and a resonance density approaching the theoretical upper limit for metasurfaces. By applying our dual-gradient to surface-enhanced molecular sensing, we demonstrate the importance of coupling tailoring and unveil an additional coupling-based dimension of spectroscopic data. Our metasurface design paves the way for generalized light-matter coupling metasurfaces, leading to advancements in the field of photocatalysis, chemical sensing, and entangled photon generation.

Source data for the results shown in the main text and extended figures of 'Double-slit time diffraction at optical frequencies'. Methodology and instrumentation described in the Methods section of the main text.

ADVERTISEMENT RETURN TO ISSUEEditorialNEXTAnnouncing the Recipients of the 2012 ACS Nano Lectureship AwardsJillian M. Buriak, Stefan A. Maier, Wolfgang J. Parak, Andrew T. S. Wee, and Paul S. WeissCite this: ACS Nano 2012, 6, 2, 987–989Publication Date (Web):February 28, 2012Publication History Published online28 February 2012Published inissue 28 February 2012https://doi.org/10.1021/nn300584tCopyright © 2012 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views1315Altmetric-Citations2LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (2 MB) Get e-AlertsSUBJECTS:Materials science,Gold,Nanoparticles Get e-Alerts

Sepsis remains a persistent problem on intensive care units all over the world. Understanding the complex mechanisms of sepsis is the precondition for establishing new therapeutic approaches in this field. Therefore, animal models are required that are able to closely mimic the human disease and also sufficiently deal with scientific questions. The Colon Ascendens Stent Peritonitis (CASP) is a highly standardized model for polymicrobial abdominal sepsis in rodents. In this model, a small stent is surgically inserted into the ascending colon of mice or rats leading to a continuous leakage of intestinal bacteria into the peritoneal cavity. The procedure results in peritonitis, systemic bacteraemia, organ infection by gut bacteria, and systemic but also local release of several pro- and anti-inflammatory cytokines. The lethality of CASP can be controlled by the diameter of the inserted stent. A variant of this model, the so-called CASP with intervention (CASPI), raises opportunity to remove the septic focus by a second operation according to common procedures in clinical practice. CASP is an easily learnable and highly reproducible model that closely mimics the clinical course of abdominal sepsis. It leads way to study on questions in several scientific fields e.g. immunology, infectiology, or surgery.

We report on coupled changes in the dielectric permittivity and the magnetic susceptibility in the insulating antiferromagnet Ba2FeSbSe5. The real part of the dielectric permittivity (ε′) and the thermal conductivity (κ) shows pronounced anomalies at the Néel temperature (TN). Our findings show that there is a weak coupling between electric dipoles and magnetic spins, which is mediated by spin-lattice coupling possibly through exchange striction effects.

Understanding the nature of chemical bonding in solids is crucial to comprehend the physical and chemical properties of a given compound. To explore changes in chemical bonding in lead chalcogenides (PbX, where X = Te, Se, S, O), a combination of property-, bond breaking- and quantum-mechanical bonding descriptors have been applied. The outcome of our explorations reveals an electron transfer driven transition from metavalent bonding in PbX (X = Te, Se, S) to iono-covalent bonding in beta-PbO. Metavalent bonding is characterized by adjacent atoms being held together by sharing about a single electron and small electron transfer (ET). The transition from metavalent to iono-covalent bonding manifests itself in clear changes in these quantum-mechanical descriptors (ES and ET), as well as in property-based descriptors (i.e. Born effective charge, dielectric function, effective coordination number (ECON) and mode-specific Grueneisen parameter, and in bond breaking descriptors (PME). Metavalent bonding collapses, if significant charge localization occurs at the ion cores (ET) and/or in the interatomic region (ES). Predominantly changing the degree of electron transfer opens possibilities to tailor materials properties such as the chemical bond and electronic polarizability, optical band gap and optical interband transitions characterized by the imaginary part of the dielectric function. Hence, the insights gained from this study highlight the technological relevance of the concept of metavalent bonding and its potential for materials design.

Metallic nanogaps are fundamental components of nanoscale photonic and electronic devices. However, the lack of reproducible high-yield fabrication methods with nanometric control over the gap-size has hindered practical applications. Here, we report a patterning technique based on molecular self-assembly and physical peeling that allows the gap-width to be tuned over the range 3 – 30 nm and enables the fabrication of massively parallel nanogap arrays containing hundreds of millions of ring-shaped nanogaps (RSNs). The method is used here to prepare molecular diodes across sub-3-nm metallic nanogaps and to fabricate visible-light-active plasmonic substrates based on large-area, gold-based RSN arrays. The substrates are applicable to a broad range of optical applications, and are used here as substrates for surface-enhanced Raman spectroscopy (SERS), providing high enhancement factors of up to 3e8 relative to similar, gap-free thin gold films.

By imaging single-shot realizations of an organic polariton quantum fluid, we observe the long-sought dynamical instability of non-equilibrium condensates. Without any free parameters, we find an excellent agreement between the experimental data and a numerical simulation of the open-dissipative Gross-Pitaevskii equation, which allows us to draw several important conclusions about the physics of the system. We find that the reservoir dynamics are in the strongly nonadiabatic regime, which renders the complex Ginzburg-Landau description invalid. The observed transition from stable to unstable fluid can only be explained by taking into account the specific form of reservoir-mediated instability as well as particle currents induced by the finite extent of the pump spot.

Tailoring critical light-matter coupling is a fundamental challenge of nanophotonics, impacting diverse fields from higher harmonic generation and energy conversion to surface-enhanced spectroscopy. Plasmonic perfect absorbers (PAs), where resonant antennas couple to their mirror images in adjacent metal films, have been instrumental for obtaining different coupling regimes by tuning the antenna-film distance. However, for on-chip uses, the ideal PA gap size can only match one wavelength, and wide range multispectral approaches remain challenging. Here, we introduce a new paradigm for plasmonic PAs by combining mirror-coupled resonances with the unique loss engineering capabilities of plasmonic bound states in the continuum (BICs). Our BIC-driven PA platform leverages the asymmetry of the constituent meta-atoms as an additional degree of freedom for reaching the critical coupling (CC) condition, delivering resonances with unity absorbance and high quality factors approaching 100 in the mid-infrared. Such a platform holds flexible tuning knobs including asymmetry parameter, dielectric gap, and geometrical scaling factor to precisely control the coupling condition, resonance frequency, and selective enhancement of magnetic and electric fields while maintaining CC. We demonstrate a pixelated PA metasurface with optimal absorption over a broad range of mid-infrared frequencies (950 ~ 2000 1/cm) using only a single spacer layer thickness and apply it for multispectral surface-enhanced molecular spectroscopy in tailored coupling regimes. Our concept greatly expands the capabilities and flexibility of traditional gap-tuned PAs, opening new perspectives for miniaturized sensing platforms towards on-chip and in-situ detection.

Photonic bound states in the continuum (BICs) are a standout nanophotonic platform for strong light-matter coupling with transition metal dichalcogenides (TMDCs), but have so far mostly been employed as all-dielectric metasurfaces with adjacent TMDC layers, incurring limitations related to strain, mode overlap, and material integration. In this work, we experimentally demonstrate for the first time asymmetry-dependent BIC resonances in 2D arrays of monolithic metasurfaces composed solely of the nanostructured bulk TMDC WS$_2$ with BIC modes exhibiting sharp and tailored linewidths, ideal for selectively enhancing light-matter interactions. Geometrical variation enables the tuning of the BIC resonances across the exciton resonance in bulk WS$_2$, revealing the strong-coupling regime with an anti-crossing pattern and a Rabi splitting of 116 meV. The precise control over the radiative loss channel provided by the BIC concept is harnessed to tailor the Rabi splitting via a geometrical asymmetry parameter of the metasurface. Crucially, the coupling strength itself can be controlled and is shown to be independent of material-intrinsic losses. Our BIC-driven monolithic metasurface platform can readily incorporate other TMDCs or excitonic materials to deliver previously unavailable fundamental insights and practical device concepts for polaritonic applications.

For enhanced light–matter interactions with 2D materials, ultra-small metallic nanogaps can be utilized. As shown in the image from S.-H. Oh and co-workers, light incident from below is strongly confined within these annular gaps and efficiently funneled through for extraordinarily large transmission. On page 667, by placing monolayer graphene at the exit of these gaps (the right side is covered with graphene), this enhanced transmission is blocked since the graphene absorbs 99% of the strongly confined waves.

We demonstrate an organic polariton condensate that exhibits nonlinear interactions at room-temperature. Upon reaching threshold, we observe a superlinear power dependence, a power-dependent blueshift and the emergence of long-range spatial coherence resulting from polariton interactions.

Invited by Stavroula Foteinopoulou © (2015) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Silicon carbide is well-known as a high quality wide-band gap semiconductor for use in electronic and optoelectronic applications at high powers, temperatures and radiation environments due to its high breakdown field, wide bandgap, radiation hardness and thermal conductivity. Due to its relatively low lattice mismatch with GaN and AlN and its high thermal conductivity SiC also serves as a high quality, semi-insulating substrate for use in III-N growth for high frequency electronics and optoelectronic devices. Over the past 20 years, dramatic progress in materials and device quality have been realized leading to extremely low-defect density substrates and epilayers. During this same period, significant effort has also been focused on the field of metal-based plasmonics and its applications in enhanced spectroscopy, light emitters, waveguides, and absorbers for photodetectors. However, while metal-based plasmonic systems have attracted attention for their ability to achieve sub-diffraction confinement with large concentrations of electromagnetic fields at optical frequencies, many potentially exciting applications are severely limited by the inherent optical losses caused by the high carrier densities within metals. It has been shown previously that an alternative to plasmonic metals in the mid-infrared-THz spectral ranges are polar dielectrics such as silicon carbide, whereby such large, sub-diffraction limited, localized electromagnetic fields can be sustained through the optical stimulation of plasmon-like surface phonon polaritons (SPhPs). Unlike plasmons, these SPhP modes are due to atomic displacements in polar semiconductors, and thus do not require free carriers to interact with the electromagnetic radiation. Because of this, SPhPs exhibit exceptionally low optical losses. Initial investigations into SiC for SPhPs began in the early part of the last decade and proof-of-principle experiments demonstrated that both localized and propagating SPhP modes could be supported. Recently, our group has demonstrated the first fabricated localized SPhP nano-resonators. The nanostructure arrays are composed of SiC nanoresonators having geometries that are 10 to 40 times smaller than the resonant wavelengths, which occur in the 10.3-12.5 um spectral range. The structures used varied in height from 150-1500 nm-tall nanopillars fabricated from semi-insulating 6H- and 4H-SiC and had diameters ranging from 150-1000 nm and interpillar separations between 20-10,000 nm. The low optical losses resulted in exceptionally narrow resonance linewidths of 3-24 cm -1 , corresponding to quality factors in the 40-300 range, which is almost an order of magnitude greater than what is possible with silver (Q&lt;40). Coupled with theoretical simulations of the electromagnetic fields, we have achieved an understanding of the local electromagnetic field distributions and intensities at such extreme confinement as a function of nanostructure size and interpillar separation. This allows us to identify the optical modes involved in the resonances and their optical selection rules. Our work shows that extremely small, high-quality, optical resonators are possible when we use SPhPs instead of surface plasmons in metamaterial designs in the IR. Unlike metals that are limited to the UV-visible, a larger wavelength range is possible with these materials as the phonon modes in polar semiconductors depend on the masses of the constituent atoms and their bond strength.

Perovskite solar cells (PSCs) own rapidly increasing power conversion efficiencies (PCEs), but their concentrated counterparts (i.e., PCSCs) show a much lower performance. A deeper understanding of PCSCs relies on a thorough study of the intensive energy losses of the device along with increasing the illumination intensity. Taking the low band gap Sn-Pb PCSC as an example, we realize a device-level optoelectronic simulation to thoroughly disclose the internal photovoltaic physics and mechanisms by addressing the fundamental electromagnetic and carrier-transport processes within PCSCs under various concentration conditions. We find that the primary factor limiting the performance improvement of PCSCs is the significantly increased bulk recombination under the increased light concentration, which is attributed mostly to the inferior transport/collection ability of holes determined by the hole transport layer (HTL). We perform further electrical manipulation on the perovskite layer and the HTL so that the carrier-transport capability is significantly improved. Under the optoelectronic design, we fabricate low band gap PCSCs, which exhibit particularly high PCEs of up to 22.36% at 4.17 sun.

Plasmon resonances play a pivotal role in enhancing light-matter interactions in nanophotonics, but their low-quality factors have hindered applications demanding high spectral selectivity. Even though symmetry-protected bound states in the continuum with high-quality factors have been realized in dielectric metasurfaces, impinging light is not efficiently coupled to the resonant metasurfaces and is lost in the form of reflection due to low intrinsic losses. Here, we demonstrate a novel design and 3D laser nanoprinting of plasmonic nanofin metasurfaces, which support symmetry-protected bound states in the continuum up to 4th order. By breaking the nanofins out-of-plane symmetry in parameter space, we achieve high-quality factor (up to 180) modes under normal incidence. We reveal that the out-of-plane symmetry breaking can be fine-tuned by the triangle angle of the 3D nanofin meta-atoms, opening a pathway to precisely control the ratio of radiative to intrinsic losses. This enables access to the under-, critical-, and over-coupled regimes, which we exploit for pixelated molecular sensing. Depending on the coupling regime we observe negative, no, or positive modulation induced by the analyte, unveiling the undeniable importance of tailoring light-matter interaction. Our demonstration provides a novel metasurface platform for enhanced light-matter interaction with a wide range of applications in optical sensing, energy conversion, nonlinear photonics, surface-enhanced spectroscopy, and quantum optics.