Publications
707 results found
Berté R, Weber T, de Souza Menezes L, et al., 2023, Permittivity-Asymmetric Quasi-Bound States in the Continuum., Nano Lett
Breaking the in-plane geometric symmetry of dielectric metasurfaces allows us to access a set of electromagnetic states termed symmetry-protected quasi-bound states in the continuum (qBICs). Here we demonstrate that qBICs can also be accessed by a symmetry breaking in the permittivity of the comprising materials. While the physical size of atoms imposes a limit on the lowest achievable geometrical asymmetry, weak permittivity modulations due to carrier doping, and electro-optical Pockels and Kerr effects, usually considered insignificant, open the possibility of infinitesimal permittivity asymmetries for on-demand, dynamically tunable resonances of extremely high quality factors. As a proof-of-principle, we probe the excitation of permittivity-asymmetric qBICs (ε-qBICs) using a prototype Si/TiO2 metasurface, in which the asymmetry in the unit cell is provided by the permittivity contrast of the materials. ε-qBICs are also numerically demonstrated in 1D gratings, where quality-factor enhancement and tailored interference phenomena of qBICs are shown via the interplay of geometrical and permittivity asymmetries.
Kalinic B, Cesca T, Balasa IG, et al., 2023, Quasi-BIC Modes in All-Dielectric Slotted Nanoantennas for Enhanced Er3+Emission, ACS PHOTONICS, ISSN: 2330-4022
Abdelwahab I, Tilmann B, Zhao X, et al., 2023, Highly Efficient Sum-Frequency Generation in Niobium Oxydichloride NbOCl<inf>2</inf> Nanosheets, Advanced Optical Materials
Parametric infrared (IR) upconversion is a process in which low-frequency IR photons are upconverted into high-frequency ultraviolet/visible photons through a nonlinear optical process. It is of paramount importance for a wide range of security, material science, and healthcare applications. However, in general, the efficiencies of upconversion processes are typically extremely low for nanometer-scale materials due to the short penetration depth of the excitation fields. Here, parametric IR upconversion processes, including frequency doubling and sum-frequency generation, are studied in layered van der Waals NbOCl2. An upconversion efficiency of up to 0.004% is attained for the NbOCl2 nanosheets, orders of magnitude higher than previously reported values for nonlinear layered materials. The upconverted signal is sensitive to layer numbers, crystal orientation, excitation wavelength, and temperature, and it can be utilized as an optical cross-correlator for ultrashort pulse characterization.
Kühner L, Sortino L, Tilmann B, et al., 2022, High-Q Nanophotonics over the Full Visible Spectrum Enabled by Hexagonal Boron Nitride Metasurfaces., Adv Mater
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.
Liu C, Zhang S, Maier SA, et al., 2022, Disorder-Induced Topological State Transition in the Optical Skyrmion Family, PHYSICAL REVIEW LETTERS, Vol: 129, ISSN: 0031-9007
Aigner A, Tittl A, Wang J, et al., 2022, Plasmonic bound states in the continuum to tailor light-matter coupling, SCIENCE ADVANCES, Vol: 8, ISSN: 2375-2548
Sapienza R, Pendry J, Maier S, et al., 2022, Saturable time-varying mirror based on an epsilon-near-zero material, Physical Review Applied, Vol: 18, ISSN: 2331-7019
We report a switchable time-varying mirror, composed of an indium-tin-oxide–gold bilayer, displaying a tenfold modulation of reflectivity (ΔR≈0.6), which saturates for a driving-pump intensity Ipump≈100GW/cm2. Upon interacting with the saturated time-varying mirror, the frequency content of a reflected pulse is extended up to 31 THz, well beyond the pump spectral content (2.8 THz). We interpret the spectral broadening as a progressive shortening of the mirror rise time from 110 fs to below 30 fs with increasing pump power, which is confirmed by four-wave-mixing experiments and partially captured by a linear time-varying model of the mirror. A temporal response unbounded by the pump bandwidth enables applications for spectral manipulation from time-varying systems with impact for communication networks, optical switching, and computing.
Mancini A, Nan L, Wendisch FJ, et al., 2022, Near-Field Retrieval of the Surface Phonon Polariton Dispersion in Free-Standing Silicon Carbide Thin Films, ACS PHOTONICS, Vol: 9, Pages: 3696-3704, ISSN: 2330-4022
Vincon I, Wendisch FJ, De Gregorio D, et al., 2022, Strong polarization dependent nonlinear excitation of a perovskite nanocrystal monolayer on a chiral dielectric nanoantenna array, ACS Photonics, Vol: 9, Pages: 3506-3514, ISSN: 2330-4022
With their unique optoelectronic properties, perovskite nanocrystals are highly advantageous semiconductor materials for tailored light applications including an interaction with circularly polarized light. Although chiral perovskite nanocrystals have been obtained by the adsorption of chiral molecules, their chiroptical response is still intrinsically weak. Alternatively, perovskites have been combined with artificial chiral surfaces demonstrating enhanced chiroptical responses. However, bulk perovskite films of considerable thickness were required, mitigating the perovskite’s photoluminescence efficiency and processability. Here we developed a hybrid system of a dielectric chiral nanoantenna array that was coated with a monolayer of cubic all-inorganic lead halide perovskite nanocrystals. By tuning the thickness of the perovskite film down to one monolayer of nanocrystals, we restricted the interactions exclusively to the near-field regime. The chiral surface built of z-shaped Si nanoantennas features pronounced chiral resonances in the visible to IR region. We demonstrate that the two-photon excited photoluminescence emission of the nanocrystals can be enhanced by up to one order of magnitude in this configuration. This emission increase is controllable by the choice of the excitation wavelength and polarization with an asymmetry in emission of up to 25% upon left and right circularly polarized illumination. Altogether, our findings demonstrate a pathway to an all-optical control and modulation of perovskite light emission via strong polarization sensitive light–matter interactions in the near-field, rendering this hybrid system interesting for sensing and display technologies.
Fu M, Mota MPDSP, Xiao X, et al., 2022, Near-unity Raman beta-factor of surface-enhanced Raman scattering in a waveguide, Nature Nanotechnology, Vol: 17, Pages: 1251-1257, ISSN: 1748-3387
The Raman scattering of light by molecular vibrations is a powerful technique to fingerprint molecules through their internal bonds and symmetries. Since Raman scattering is weak1, methods to enhance, direct and harness it are highly desirable, and this has been achieved using optical cavities2, waveguides3,4,5,6 and surface-enhanced Raman scattering (SERS)7,8,9. Although SERS offers dramatic enhancements2,6,10,11 by localizing light within vanishingly small hot-spots in metallic nanostructures, these tiny interaction volumes are only sensitive to a few molecules, yielding weak signals12. Here we show that SERS from 4-aminothiophenol molecules bonded to a plasmonic gap waveguide is directed into a single mode with >99% efficiency. Although sacrificing a confinement dimension, we find a SERS enhancement of ~103 times across a broad spectral range enabled by the waveguide’s larger sensing volume and non-resonant waveguide mode. Remarkably, this waveguide SERS is bright enough to image Raman transport across the waveguides, highlighting the role of nanofocusing13,14,15 and the Purcell effect16. By analogy to the β-factor from laser physics10,17,18,19,20, the near-unity Raman β-factor we observe exposes the SERS technique to alternative routes for controlling Raman scattering. The ability of waveguide SERS to direct Raman scattering is relevant to Raman sensors based on integrated photonics7,8,9 with applications in gas sensing and biosensing.
Genco A, Cruciano C, Corti M, et al., 2022, k-Space Hyperspectral Imaging by a Birefringent Common-Path Interferometer, ACS PHOTONICS, ISSN: 2330-4022
Kim J, Foerster R, Wieduwilt T, et al., 2022, Locally Structured On-Chip Optofluidic Hollow-Core Light Cages for Single Nanoparticle Tracking, ACS SENSORS, ISSN: 2379-3694
Kim J, Jang B, Wieduwilt T, et al., 2022, On-chip fluorescence detection using photonic bandgap guiding optofluidic hollow-core light cage, APL PHOTONICS, Vol: 7, ISSN: 2378-0967
Dinter T, Li C, Kuehner L, et al., 2022, Metasurface measuring twisted light in turbulence, ACS Photonics, Vol: 9, Pages: 3043-3051, ISSN: 2330-4022
Orbital angular momentum (OAM) of light represents an independent degree of freedom using orthogonal helical modes for optical and quantum multiplexing, offering great potential to transform future ultrahigh-bandwidth information systems. Practical OAM communication systems suffer from turbulence-induced phase distortions to the propagating beams, decreasing the orthogonality of OAM modes through introduced modal crosstalk. To date, optical systems used for measuring OAM orthogonality breakdown in different turbulence conditions are too bulky and slow (e.g., one OAM mode at a time) for any practical use. Here, we demonstrate the use of an ultrathin OAM mode-sorting metasurface for characterizing the OAM orthogonality breakdown under different turbulence conditions. Our approach allows the measurement of the whole OAM spectrum at the same time. This metasurface exhibits strong OAM selectivity with an average modal crosstalk below −42.4 dB for OAM modes with topological charges ranging from −15 to +15. Our results suggest that higher-order OAM modes are as robust as lower-order modes in particular turbulence environments, paving the way for future practical free-space OAM communications harnessing high-dimensional OAM multiplexing. We demonstrated that a flat optical device with a small form factor can be integrated with practical communication systems for compact, fast, and efficient generation and detection of twisted light.
Bürger J, Schalles V, Kim J, et al., 2022, 3D-nanoprinted antiresonant hollow-core microgap waveguide: an on-chip platform for integrated photonic devices and sensors., ACS Photonics, Vol: 9, Pages: 3012-3024, ISSN: 2330-4022
Due to their unique capabilities, hollow-core waveguides are playing an increasingly important role, especially in meeting the growing demand for integrated and low-cost photonic devices and sensors. Here, we present the antiresonant hollow-core microgap waveguide as a platform for the on-chip investigation of light-gas interaction over centimeter-long distances. The design consists of hollow-core segments separated by gaps that allow external access to the core region, while samples with lengths up to 5 cm were realized on silicon chips through 3D-nanoprinting using two-photon absorption based direct laser writing. The agreement of mathematical models, numerical simulations and experiments illustrates the importance of the antiresonance effect in that context. Our study shows the modal loss, the effect of gap size and the spectral tuning potential, with highlights including extremely broadband transmission windows (>200 nm), very high contrast resonance (>60 dB), exceptionally high structural openness factor (18%) and spectral control by nanoprinting (control over dimensions with step sizes (i.e., increments) of 60 nm). The application potential was demonstrated in the context of laser scanning absorption spectroscopy of ammonia, showing diffusion speeds comparable to bulk diffusion and a low detection limit. Due to these unique properties, application of this platform can be anticipated in a variety of spectroscopy-related fields, including bioanalytics, environmental sciences, and life sciences.
Abdelwahab I, Tilmann B, Wu Y, et al., 2022, Giant second-harmonic generation in ferroelectric NbOI2, Nature Photonics, Vol: 16, Pages: 644-650, ISSN: 1749-4885
Implementing nonlinear optical components in nanoscale photonic devices is challenged by phase-matching conditions requiring thicknesses in the order of hundreds of wavelengths, and is disadvantaged by the short optical interaction depth of nanometre-scale materials and weak photon–photon interactions. Here we report that ferroelectric NbOI2 nanosheets exhibit giant second-harmonic generation with conversion efficiencies that are orders of magnitude higher than commonly reported nonlinear crystals. The nonlinear response scales with layer thickness and is strain- and electrical-tunable; a record >0.2% absolute SHG conversion efficiency and an effective nonlinear susceptibility χ(2)eff in the order of 10−9 m V−1 are demonstrated at an average pump intensity of 8 kW cm–2. Due to the interplay between anisotropic polarization and excitonic resonance in NbOI2, the spatial profile of the polarized SHG response can be tuned by the excitation wavelength. Our results represent a new paradigm for ultrathin, efficient nonlinear optical components.
Kühner L, Sortino L, Berté R, et al., 2022, Radial bound states in the continuum for polarization-invariant nanophotonics, Nature Communications, Vol: 13, ISSN: 2041-1723
All-dielectric nanophotonics underpinned by the physics of bound states in the continuum (BICs) have demonstrated breakthrough applications in nanoscale light manipulation, frequency conversion and optical sensing. Leading BIC implementations range from isolated nanoantennas with localized electromagnetic fields to symmetry-protected metasurfaces with controllable resonance quality (Q) factors. However, they either require structured light illumination with complex beam-shaping optics or large, fabrication-intense arrays of polarization-sensitive unit cells, hindering tailored nanophotonic applications and on-chip integration. Here, we introduce radial quasi-bound states in the continuum (radial BICs) as a new class of radially distributed electromagnetic modes controlled by structural asymmetry in a ring of dielectric rod pair resonators. The radial BIC platform provides polarization-invariant and tunable high-Q resonances with strongly enhanced near fields in an ultracompact footprint as low as 2 µm2. We demonstrate radial BIC realizations in the visible for sensitive biomolecular detection and enhanced second-harmonic generation from monolayers of transition metal dichalcogenides, opening new perspectives for compact, spectrally selective, and polarization-invariant metadevices for multi-functional light-matter coupling, multiplexed sensing, and high-density on-chip photonics.
Hu H, Weber T, Bienek O, et al., 2022, Catalytic metasurfaces empowered by bound states in the continuum, ACS Nano, Vol: 16, Pages: 13057-13068, ISSN: 1936-0851
Photocatalytic platforms based on ultrathin reactive materials facilitate carrier transport and extraction but are typically restricted to a narrow set of materials and spectral operating ranges due to limited absorption and poor energy-tuning possibilities. Metasurfaces, a class of 2D artificial materials based on the electromagnetic design of nanophotonic resonators, allow optical absorption engineering for a wide range of materials. Moreover, tailored resonances in nanostructured materials enable strong absorption enhancement and thus carrier multiplication. Here, we develop an ultrathin catalytic metasurface platform that leverages the combination of loss-engineered substoichiometric titanium oxide (TiO2–x) and the emerging physical concept of optical bound states in the continuum (BICs) to boost photocatalytic activity and provide broad spectral tunability. We demonstrate that our platform reaches the condition of critical light coupling in a TiO2–x BIC metasurface, thus providing a general framework for maximizing light–matter interactions in diverse photocatalytic materials. This approach can avoid the long-standing drawbacks of many naturally occurring semiconductor-based ultrathin films applied in photocatalysis, such as poor spectral tunability and limited absorption manipulation. Our results are broadly applicable to fields beyond photocatalysis, including photovoltaics and photodetectors.
Yao K, Li J, Wang H, et al., 2022, Mechanistic insights into OC-COH coupling in CO2 electroreduction on fragmented copper, Journal of the American Chemical Society, Vol: 144, Pages: 14005-14011, ISSN: 0002-7863
The carbon–carbon (C–C) bond formation is essential for the electroconversion of CO2 into high-energy-density C2+ products, and the precise coupling pathways remain controversial. Although recent computational investigations have proposed that the OC–COH coupling pathway is more favorable in specific reaction conditions than the well-known CO dimerization pathway, the experimental evidence is still lacking, partly due to the separated catalyst design and mechanistic/spectroscopic exploration. Here, we employ density functional theory calculations to show that on low-coordinated copper sites, the *CO bindings are strengthened, and the adsorbed *CO coupling with their hydrogenation species, *COH, receives precedence over CO dimerization. Experimentally, we construct a fragmented Cu catalyst with abundant low-coordinated sites, exhibiting a 77.8% Faradaic efficiency for C2+ products at 300 mA cm–2. With a suite of in situ spectroscopic studies, we capture an *OCCOH intermediate on the fragmented Cu surfaces, providing direct evidence to support the OC–COH coupling pathway. The mechanistic insights of this research elucidate how to design materials in favor of OC–COH coupling toward efficient C2+ production from CO2 reduction.
Ren H, Jang J, Li C, et al., 2022, An achromatic metafiber for focusing and imaging across the entire telecommunication range (July, 10.1038/s41467-022-31902-3, 2022), NATURE COMMUNICATIONS, Vol: 13
Ren H, Jang J, Li C, et al., 2022, An achromatic metafiber for focusing and imaging across the entire telecommunication range, Nature Communications, Vol: 13, Pages: 1-10, ISSN: 2041-1723
Dispersion engineering is essential to the performance of most modern optical systems including fiber-optic devices. Even though the chromatic dispersion of a meter-scale single-mode fiber used for endoscopic applications is negligible, optical lenses located on the fiber end face for optical focusing and imaging suffer from strong chromatic aberration. Here we present the design and nanoprinting of a 3D achromatic diffractive metalens on the end face of a single-mode fiber, capable of performing achromatic and polarization-insensitive focusing across the entire near-infrared telecommunication wavelength band ranging from 1.25 to 1.65 µm. This represents the whole single-mode domain of commercially used fibers. The unlocked height degree of freedom in a 3D nanopillar meta-atom largely increases the upper bound of the time-bandwidth product of an achromatic metalens up to 21.34, leading to a wide group delay modulation range spanning from −8 to 14 fs. Furthermore, we demonstrate the use of our compact and flexible achromatic metafiber for fiber-optic confocal imaging, capable of creating in-focus sharp images under broadband light illumination. These results may unleash the full potential of fiber meta-optics for widespread applications including hyperspectral endoscopic imaging, femtosecond laser-assisted treatment, deep tissue imaging, wavelength-multiplexing fiber-optic communications, fiber sensing, and fiber lasers.
Ma T, An Y, Li S, et al., 2022, Low Band Gap Perovskite Concentrator Solar Cells: Physics, Device Simulation, and Experiment, ACS APPLIED MATERIALS & INTERFACES, ISSN: 1944-8244
Cortes E, Wendisch FJ, Sortino L, et al., 2022, Optical Metasurfaces for Energy Conversion, CHEMICAL REVIEWS, ISSN: 0009-2665
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- Citations: 8
Rosenberger P, Dagar R, Zhang W, et al., 2022, Imaging elliptically polarized infrared near-fields on nanoparticles by strong-field dissociation of functional surface groups, European Physical Journal D: Atomic, Molecular, Optical and Plasma Physics, Vol: 76, Pages: 1-9, ISSN: 0011-4626
We investigate the strong-field ion emission from the surface of isolated silica nanoparticles aerosolized from an alcoholic solution, and demonstrate the applicability of the recently reported near-field imaging at 720 nm [Rupp et al., Nat. Comm., 10(1):4655, 2019] to longer wavelength (2 μm) and polarizations with arbitrary ellipticity. Based on the experimental observations, we discuss the validity of a previously introduced semi-classical model, which is based on near-field driven charge generation by a Monte-Carlo approach and classical propagation. We furthermore clarify the role of the solvent in the surface composition of the nanoparticles in the interaction region. We find that upon injection of the nanoparticles into the vacuum, the alcoholic solvent evaporates on millisecond time scales, and that the generated ions originate predominantly from covalent bonds with the silica surface rather than from physisorbed solvent molecules. These findings have important implications for the development of future theoretical models of the strong-field ion emission from silica nanoparticles, and the application of near-field imaging and reaction dynamics of functional groups on isolated nanoparticles.
Hu X, Lo TW, Mancini A, et al., 2022, Near-field nano-spectroscopy of strong mode coupling in phonon-polaritonic crystals, APPLIED PHYSICS REVIEWS, Vol: 9, ISSN: 1931-9401
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- Citations: 2
Luo S, Mancini A, Wang F, et al., 2022, High-Throughput Fabrication of Triangular Nanogap Arrays for Surface-Enhanced Raman Spectroscopy, ACS NANO, Vol: 16, Pages: 7438-7447, ISSN: 1936-0851
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Zhang W, Dagar R, Rosenberger P, et al., 2022, All-optical nanoscopic spatial control of molecular reaction yields on nanoparticles, OPTICA, Vol: 9, Pages: 551-560, ISSN: 2334-2536
Stefancu A, Nan L, Zhu L, et al., 2022, Controlling Plasmonic Chemistry Pathways through Specific Ion Effects, ADVANCED OPTICAL MATERIALS, Vol: 10, ISSN: 2195-1071
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- Citations: 3
Tilmann B, Pandeya AK, Grinblat G, et al., 2022, Ultrafast sub‐100 fs all‐optical modulation and efficient third‐harmonic generation in Weyl semimetal niobium phosphide thin films, Advanced Materials, Vol: 34, Pages: 1-7, ISSN: 0935-9648
Since their experimental discovery in 2015, Weyl semimetals generated a large amount of attention due their intriguing physical properties that arise from their linear electron dispersion relation and topological surface states. In particular in the field of nonlinear (NL) optics and light harvesting, Weyl semimetals have shown outstanding performances and achieved record NL conversion coefficients. In this context, we perform first steps towards Weyl semimetal nanophotonics by thoroughly characterizing the linear and NL optical behavior of epitaxially grown niobium phosphide (NbP) thin films, covering the visible to near-infrared regime of the electromagnetic spectrum. Despite the measured high linear absorption, third-harmonic generation studies demonstrate high conversion efficiencies up to 10-4%, that can be attributed to the topological electron states at the surface of the material. Furthermore, nondegenerate pump-probe measurements with sub-10 fs pulses reveal a maximum modulation depth of about 1%, completely decaying within 100 fs and therefore suggesting the possibility of developing devices based on NbP with all-optical switching bandwidths of up to 10 THz. Altogether, our work reveals promising NL optical properties of Weyl semimetal thin films that are outperforming bulk crystals of the same material, laying the grounds for nanoscale applications, enabled by top-down nanostructuring, such as light-harvesting, on-chip frequency conversion and all-optical processing.
Possmayer T, Tilmann B, Maia LJQ, et al., 2022, Second to fifth harmonic generation in individual beta-barium borate, OPTICS LETTERS, Vol: 47, Pages: 1826-1829, ISSN: 0146-9592
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