Chapter Three - Ultraviolet and visible metasurfaces

Metasurfaces are an array of periodic subwavelength nanostructures that resonantly couple to the incident light. Such nanostructured surfaces can reproduce the functions of bulk optics, and on occasions, offer new functionalities that are not possible with conventional diffractive optics. Metallic metasurfaces, employing resonant oscillation of surface-plasmons, can confine light in the nanoscale gaps, so-called hot spots, with extreme sensitivity to the refractive index of the environment. However, the price of such characteristics is the high ohmic losses of metallic nanoparticles. On the other hand, high-index dielectric and semiconductor metasurfaces are lossless. They can stimulate Mie resonances in a multipolar fashion, applicable to both linear and nonlinear regimes, although they produce much weaker hot spots. The benefits of metallic or dielectric metasurfaces have been a subject of debate in the last decade. In this seminar, I review my journey in employing metallic to dielectric and semiconductor metasurfaces in both linear and nonlinear regimes. I will discuss the benefits of each of them for certain applications ranging from ultra-sensitive detection to near-infrared imaging.

Bismuth-based perovskite has become the best candidate material for non-lead perovskite because of its low toxicity and good moisture stability. The structural diversity of bismuth halide perovskites gives them many photoelectric properties, such as nonlinear optical properties, photochromic effects and photoelectric effects. However, bismuth halide perovskite materials with different stoichiometric ratios have different light absorption properties and carrier transport processes. In order to search for high stability and excellent optical properties of bismuth halide perovskite materials, this paper synthesized Rb3BiBr6 materials by using rubidium bromide and bismuth bromide as precursors, and further studied the morphology and optical properties of the materials by scanning electron microscopy, UV-VIS absorption spectroscopy, ultraviolet photoelectron spectroscopy and nanosecond transient absorption spectroscopy. The results show that the material has a wide light absorption range and a long carrier transfer process, indicating that the bismuth-based perovskite material has great application value in the field of photocatalysis and photodetector.

Optical metasurfaces composed of designed Mie-resonant semiconductor nanoparticles arranged in a plane offer unique opportunities for controlling the properties of light fields [1]. Such metasurfaces can impose a spatially variant phase shift onto an incident light field, thereby providing control over its wave front with high transmittance efficiency. They can also e.g. act as polarizing optical elements, exhibit tailored nonlinear optical properties, or manipulate spontaneous emission processes of nanoscale emitters integrated in the metasurface architecture. However, the optical response of most semiconductor metasurfaces realized so far was permanently encoded into the metasurface structure during fabrication. Recently, a growing amount of research is concentrating on obtaining dynamic control of their optical response, with the aim of creating metasurfaces with functionalities that can be tuned, switched or programmed on demand. This talk will provide an overview of our recent advances in actively tunable Mie-resonant semiconductor metasurfaces. In particular, by integrating silicon metasurfaces into a liquid-crystal (LC) cell, we can tune their linear-optical transmittance and reflectance spectra by application of a voltage [2]. In our work, we utilize a LC photoalignment material [3] during the assembly of the LC metasurfaces, leading to a drastic improvement of the tuning performance and reproducibility. Based on this method, we demonstrate electrical tuning of LC-infiltrated dielectric metasurfaces at near-infrared and visible wavelengths. We show that these metasurfaces can be tuned into and out of the so-called Huygens' regime of spectrally overlapping electric and magnetic dipolar resonances, which is characterized by near-unity resonant transmission, by application of an external voltage. In particular, we demonstrate tuning of the metasurface transmission from nearly opaque to nearly transparent at 1070 nm. Furthermore, making use of the strong modulation of the metasurface response in combination with patterned electrodes, we experimentally demonstrate a transparent metasurface display device operating in the visible spectral range. However, while the integration of silicon metasurfaces into nematic LC cells represents an efficient and versatile tuning approach showing large resonance shifts and strong tuning contrast, the switching times that can be achieved based on this approach are limited. Thus, as an alternative tuning mechanism allowing for ultrafast operation, we consider the transient changes of the optical properties of semiconductor materials when optically pumped by femtosecond laser pulses. These changes can lead to pronounced changes of the resonance condition for semiconductor metasurfaces at an ultrafast time scale. Our recent progress in ultrafast switching and tuning of semiconductor metasurfaces based on different material platforms and different physical mechanisms occurring at an ultrafast time scale will be discussed [4,5]. Furthermore, strategies to translate ultrafast tuning of metasurface resonances to ultrafast control of more complex metasurface functionalities such as wavefront shaping will be outlined. [1] I. Staude & J. Schilling, Nature Photon. 11, 274 (2017). [2] A. Komar et al., Appl. Phys. Lett. 110(7), 071109 (2017). [3] I. I. Rushnova et al., Opt. Commun. 413, 179 (2018). [4] M. R. Shcherbakov et al., Nano Lett. 15, 6985 (2015). [5] M. R. Shcherbakov et al., Nat. Commun. 8, 17 (2017).

Lithium tantalate (LiTaO3, LT), structurally similar to lithium niobate, possesses a broad spectrum of optoelectronic properties that hold significant promise for integrated photonics applications. Due to its larger bandgap and higher optical damage threshold, LT can be employed for efficient nonlinear optical frequency conversion in the ultraviolet (UV) wavelength range. Here, we report on fabrication of monolithic, high-quality LT metasurfaces, composed of periodic arrays of LT truncated square pyramids, created by partially etching of a LT thin film layer using a focused ion beam milling process. These pyramidal structures introduce discrete translational symmetry, which allows for the folding of guided-mode dispersion into the first Brillouin zone, leading to the formation of guided mode resonances with Q-factors that can be easily controlled by the height of LT pyramids. The measurements show a Q-factor up to approximately 640 at 786 nm. By leveraging the strong field localization of these high-Q resonances, we demonstrate enhanced second-harmonic generation at 393 nm with an absolute conversion efficiency of 0.042%, establishing a new benchmark for the UV harmonic generation in dielectric metasurfaces. Moreover, our fabrication technique can be readily adapted to a wide range of other material platforms, opening new avenues for the development of high-quality UV metasurfaces and advanced nonlinear photonic meta-devices.

Spectral albedos of open water, nilas, nilas with frost flowers, slush, and first-year ice with both thin and thick snow cover were measured in the East Antarctic sea-ice zone during the Sea Ice Physics and Ecosystems eXperiment II (SIPEX II) from September to November 2012, near 65°S, 120°E. Albedo was measured across the ultraviolet (UV), visible and near-infrared (nIR) wavelengths, augmenting a dataset from prior Antarctic expeditions with spectral coverage extended to longer wavelengths, and with measurement of slush and frost flowers, which had not been encountered on the prior expeditions. At visible and UV wavelengths, the albedo depends on the thickness of snow or ice; in the nIR the albedo is determined by the specific surface area. The growth of frost flowers causes the nilas albedo to increase by 0.2–0.3 in the UV and visible wavelengths. The spectral albedos are integrated over wavelength to obtain broadband albedos for wavelength bands commonly used in climate models. The albedo spectrum for deep snow on first-year sea ice shows no evidence of light-absorbing particulate impurities (LAI), such as black carbon (BC) or organics, which is consistent with the extremely small quantities of LAI found by filtering snow meltwater. Estimated BC mixing ratios were in the range 0.1–0.5 ng of carbon per gram of snow.

Two-dimensional metamaterials (metasurfaces) have led to many exciting phenomena both in linear and nonlinear optics. In this talk I will present an overview of some recent results for both metallic and dielectric metasurfaces. Record second order nonlinearities can be obtained when metallic metasurfaces are coupled with resonant electronic transitions in semiconductors such as intersubband transitions. Additionally, since the nonlinear unit in this case is a single resonator coupled to the semiconductor heterostructure, additional functionality can be obtained at the second harmonic beam. This phenomena can be described as a phased-array source. Using this principle, we have created beam and polarization splitters operating at the second harmonic wavelength. This is new functionality that has no counterpart in conventional nonlinear optical materials. Another interesting case is the combination of all-dielectric metasurfaces with nonlinear optical phenomena, both bulk and surface enhanced. All-dielectric metasurfaces provide a platform to engineer magnetic and electric resonant modes in wavelength-scale nanoresonators with very low loss. Fabricating such dielectric metasurfaces from different types of semiconductors can be used to enhance their second and third order nonlinearities by several orders of magnitudes.

The high demand to understand the optical, electronic, and structure properties of materials has fostered to extend the investigation down to shorter wavelengths in the far ultraviolet (FUV) and extreme ultraviolet (EUV) range. This has pushed scientists to investigate and design new optical tools as wave retarder (QWR) which, coupled with other techniques, can provide valuable information about physical, like magnetic and optical properties of materials. We have designed and studied an EUV polarimetric apparatus based on multilayer structures as QWR with a protective capping layer to avoid oxidation and contamination to improve stability and reflectivity efficiency. This device works within a suitably wide spectral range (88-160 nm) where some important spectral emission lines are as the hydrogen Lyman alpha 121.6 and Oxygen VI (103.2 nm) lines. Such design could be particularly useful as analytical tools in EUV-ellipsometry field. The system can be a relatively simple alternative to Large Scale Facilities and can be applied to test optical components by deriving their efficiency and their phase effect, i.e. determining the Mueller Matrix terms, and even to the analysis of optical surface and interface properties of thin films. In addition, the phase retarder element could be used in other experimental applications for generating EUV radiation beams of suitable polarization or for their characterization.

BackgroundYttrium-stabilized zirconia (YSZ) and alumina are the most commonly used dental esthetic crown materials. This study aimed to provide detailed information on the comparison between yttrium-stabilized zirconia (YSZ) and alumina, the two materials most often used for esthetic crowns in dentistry.MethodologyThe ground-state energy of the materials was calculated using the Cambridge Serial Total Energy Package (CASTEP) code, which employs a first-principles method based on density functional theory (DFT). The electronic exchange–correlation energy was evaluated using the generalized gradient approximation (GGA) within the Perdew (Burke) Ernzerhof scheme.ResultsOptimization of the geometries and investigation of the optical properties, dynamic stability, band structures, refractive indices, and mechanical properties of these materials contribute to a holistic understanding of these materials. Geometric optimization of YSZ provides important insights into its dynamic stability based on observations of its crystal structure and polyhedral geometry, which show stable configurations. Alumina exhibits a distinctive charge, kinetic, and potential (CKP) geometry, which contributes to its interesting structural framework and molecular-level stability. The optical properties of alumina were evaluated using pseudo-atomic computations, demonstrating its responsiveness to external stimuli. The refractive indices, reflectance, and dielectric functions indicate that the transmission of light by alumina depends on numerous factors that are essential for the optical performance of alumina as a material for esthetic crowns. The band structures of both the materials were explored, and the band gap of alumina was determined to be 5.853 eV. In addition, the band structure describes electronic transitions that influence the conductivity and optical properties of a material. The stability of alumina can be deduced from its bandgap, an essential property that determines its use as a dental material. Refractive indices are vital optical properties of esthetic crown materials. Therefore, the ability to understand their refractive-index graphs explains their transparency and color distortion through how the material responds to light..The regulated absorption characteristics exhibited by YSZ render it a highly attractive option for the development of esthetic crowns, as it guarantees minimal color distortion.ConclusionThe acceptability of materials for esthetic crowns is strongly determined by mechanical properties such as elastic stiffness constants, Young's modulus, and shear modulus. YSZ is a highly durable material for dental applications, owing to its superior mechanical strength.

Accurate estimation of the absorption coefficient (ag) for chromophoric dissolved organic matter (CDOM) over ultraviolet (UV) and short visible radiation wavelengths (with λ = 275–450 nm) is crucial to provide a robust assessment of the biogeochemical significance of UV in the global ocean. Using a training data set spanning a variety of water types from the clearest open ocean to dynamic inshore waters, a novel algorithm to accurately resolve CDOM absorption spectra from ocean color is presented. Employing a suite of multivariate statistical approaches (principal component analysis, cluster analysis, and multiple linear regression), this new algorithm was developed with matched field data for CDOM spectra and remote sensing reflectance (Rrs) at Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) bands. Freed from any presupposition about CDOM spectral shape or conventional spectral extrapolations from visible data, our algorithm allows direct retrieval of a fully resolved CDOM absorption spectrum over UV wavelengths from visible Rrs and further enables a global scale view of the dynamics of CDOM over different water types. Accuracy of ag retrieval is good, with a mean absolute percent difference for ag in the UV of ∼25%. With fully resolved spectra, maps of calculated CDOM spectral slopes (S275–295, S350–400) and slope ratios (SR) are presented with the potential to provide new information about the chemical composition (e.g., molecular weight and aromaticity), sources, transformation, and cycling pathways of CDOM on global as well as regional scales. The new algorithm will contribute to improved accuracy for photochemical and photobiological rate calculations from ocean color.

Light-matter interaction between quantum emitters and optical cavities plays a vital role in fundamental quantum photonics and the development of optoelectronics. Resonant metasurfaces are proven to be an efficient platform for tailoring the spontaneous emission (SE) of the emitters. In this work, we study the interplay between quasi-2D perovskites and dielectric TiO2 metasurfaces. The metasurface, functioning as an open cavity, enhances electric fields near its plane, thereby influencing the emissions of the perovskite. This is verified through angle-resolved photoluminescence (PL) studies. We also conducted reflectivity measurements and numerical simulations to validate the coupling between the quasi-2D perovskites and photonic modes. Notably, our work introduces a spatial mapping approach to study Purcell enhancement. Using fluorescence lifetime imaging microscopy (FLIM), we directly link the PL and lifetimes of the quasi-2D perovskites in spatial distribution when positioned on the metasurface. This correlation provides unprecedented insights into emitter distribution and emitter-resonator interactions. The methodology opens a new (to the best of our knowledge) approach for studies in quantum optics, optoelectronics, and medical imaging by enabling spatial mapping of both PL intensity and lifetime, differentiating between uncoupled quantum emitters and those coupled with different types of resonators.

A core shell structure CdSe/ZnS quantum dots were prepared by an organic coverage approach and the effect of coverage amount of ZnS on the optical properties of the quantum dots materials was investigated. Furthermore, the core shell quantum dots materials were modified with thiols, the influence of modification of aromatic thiol of 1,4-benzenedimethanethiol and aliphatic thiol of 1,8-octanedithiol on the optical properties of the materials were studied, respectively. It was found that the optical properties of core-shell structure CdSe/ZnS quantum dots were enhanced significantly with the coverage of ZnS and the modification of thiols. The quantum yield and strength of photoluminescence were increased with the increase of covered amount of ZnS, also, the modification of thiols could enhance the strength of photoluminescence and the stability of the quantum materials, generally the optical properties of modified quantum materials were improved with the increase of thiols concentration, but as the concentration of thiols increased until to a certain extent, then the optical properties were not varied. 31P solid-state NMR spectra revealed that the trioctylphosphine oxide ligand on CdSe/ZnS quantum dots’surface might be partial replaced by thiols molecules, which results in the enhancement of optical properties and the better stability.

Introduction. Radiative heat transfer in porous insulating materials at high temperatures was investigated in [1–3], and compound heat transfer under unsteady-state conditions in a material was investigated in [4]. Radiation transfer in the gray medium approximation substantially limits the adequacy of the model used in [5]. The present work, which is a continuation of [5], was performed by us with a view to account for the dependence of the optical properties of gas-permeable materials on radiation wavelength. The article describes a computational investigation of compound heat and mass transfer in a semitransparent gas-permeable material in the presence of phase transition and oxygen chemisorption in pores. An innovation in the formulation of the problem is accounting for the interaction of the radiation fi elds of optical inhomogeneities (pores and the particles intruded into a material) when calculating the material�c s optical properties. The optical properties of particles are usually calculated by the Mie theory [6], but at high concentrations the radiation fi elds of individual particles overlap, which is not taken into account in the Mie theory and leads to a large error in the calculated properties. To avoid the occurrence of such an error, in the present work the optical properties at a high particle concentration are calculated by means of the Maxwell–Garnett approximation [7] that accounts for the interaction of the radiation fi elds of the indicated inhomogeneities. Physical Model. We studied quartz that contained pores fi lled with air and vanadium dioxide particles in which 2 nd -order dielectric–metal phase transformations occur with heat liberation during cooling and heat absorption upon heating. Heat exchange with the surrounding medium proceeds by radiation and convection. We consider the processes of heat and mass transfer in a thin gas-permeable plane layer in one-dimensional approximation under conditions where radiative heat transfer is comparable with heat conduction in the solid skeleton and gas convection in the pores. Under conditions of heating or cooling, 2 nd -order dielectric–metal phase transition occurs in the vanadium dioxide particles, which is accompanied by oxygen chemisorptions and by a change in the crystalline structure of particles, with simultaneous liberation or absorption of heat in the material. These processes can be modeled by volumetric heat and mass sources that are introduced into the effective heat capacity of the material [5]. The absence of large pressure gradients in the gas leads to a relatively slow gas fi ltration, which allows us to assume that thermal equilibrium was achieved between the solid skeleton and the gas. The problem formulated is solved in the following approximation: the material under study is considered as a set of three continua inserted into one another. The components of this set are the continua of air in the pores, quartz (the material skeleton), and of the vanadium dioxide particles introduced into the skeleton. The thermophysical properties of continuous components are the effective properties in relation to the corresponding properties of a real material. Gas fi ltration in a material is modeled as the motion of a continuous air component characterized by a velocity fi eld and by the fi eld of specifi c air fl ow rates. Radiative transfer in the system considered occurs through the solid skeleton and gas in the pores. In modeling the radiative transfer, we took into account the processes of the radiation absorption, emission, and scattering. We assumed that the optical properties of a material were dependent on the wavelength. The effective heat capacity of the material depends on temperature and

The electrical and optical properties of materials which are characterized by narrow bands in the vicinity of the Fermi energy are discussed. For such materials, electronic correlations and the electron-phonon coupling must be considered explicitly. Correlations in $f$ bands and in extremely narrow $d$ bands can be handled in the ionic limit of the Hubbard Hamiltonian. It is shown that free carriers in such bands form small polarons which contribute to conduction only by means of thermally activated hopping. Wider bands may also exist near the Fermi energy. Carriers in these bands may form large polarons and give a bandlike contribution to conductivity. A model is proposed for determining the density of states of pure stoichiometric crystals, beginning with the free-ion energy levels, and taking into account the Madelung potential, screening and covalency effects, crystalline-field stabilizations, and overlap effects. Exciton states are considered explicitly. The Franck-Condon principle necessitates the construction of different densities of states for electrical conductivity and optical absorption. Because of the bulk of experimental data presently available, the model is applied primarily to NiO. The many-particle density of states of pure stoichiometric NiO is calculated and is shown to be in agreement with the available experimental data. When impurities are present or nonstoichiometry exists, additional transitions must be discussed from first principles. The case of Li-doped NiO is discussed in detail. The calculations are consistent with the large mass of experimental information on this material. It is concluded that the predominant mechanism for conduction between 200 and 1000 \ifmmode^\circ\else\textdegree\fi{}K is the transport of hole-like large polarons in the oxygen $2p$ band. A method for representing the many-particle density of states on an effective one-electron diagram is discussed. It is shown that if correlations are important, donor or acceptor levels cannot be drawn as localized levels in the energy gap when multiple conduction or valence bands are present. This result comes about because extrinsic ionization energies of two correlated bands differ by an energy which bears no simple relation to the difference in energies of the intrinsic excitations, which are conventionally used to determine the relative positions of the bands.

Metasurfaces enable flat optical elements by leveraging optical resonances in metallic or dielectric nanoparticles to obtain accurate control over the amplitude and phase of the scattered light. While highly efficient, these resonances are static and difficult to tune actively. Exciton resonances in atomically thin 2D semiconductors provide a novel and uniquely strong resonant light–matter interaction, which presents a new opportunity for optical metasurfaces. Their resonant properties are intrinsic to the band structure of the material, do not rely on nanoscale patterns, and are highly tunable using external stimuli. In this tutorial, we present the role that exciton resonances can play for atomically thin optics. We describe the essentials of metasurface physics and provide background on exciton physics and a comprehensive overview of excitonic materials. Excitons demonstrate to provide new degrees of freedom and enhanced light–matter interactions in hybrid metasurfaces through coupling with metallic and dielectric metasurfaces. Using the high sensitivity of excitons to the medium's electron density, the first demonstrations of electrically tunable nanophotonic devices and atomically thin optical elements are also discussed. The future of excitons in metasurfaces looks promising, while the main challenge lies in large-area growth and precise integration of high-quality materials.

In recent years, dielectric and metallic nanoscale metasurfaces are attracting growing attention and are being used for variety of applications. Resulting from the ability to introduce abrupt changes in optical properties at nanoscale dimensions, metasurfaces enable unprecedented control over light's different degrees of freedom, in an essentially two-dimensional configuration. Yet, the dynamic control over metasurface properties still remains one of the ultimate goals of this field. Here, we demonstrate the optical resonant interaction between a form birefringent dielectric metasurface made of silicon and alkali atomic vapor to control and effectively tune the optical transmission pattern initially generated by the nanoscale dielectric metasurface. By doing so, we present a controllable metasurface system, the output of which may be altered by applying magnetic fields, changing input polarization, or shifting the optical frequency. Furthermore, we also demonstrate the nonlinear behavior of our system taking advantage of the saturation effect of atomic transition. The demonstrated approach paves the way for using metasurfaces in applications where dynamic tunability of the metasurface is in need, for example, for scanning systems, tunable focusing, real time displays, and more.