Few-layer bifunctional metasurfaces enabling asymmetric and symmetric polarization-plane rotation at the subwavelength scale.

We introduce and numerically validate the concept of few-layer bifunctional metasurfaces comprising two arrays of quasiplanar subwavelength resonators and a middle grid (array of rectangular holes) that offer both symmetric and asymmetric transmissions connected, respectively, with symmetric and asymmetric polarization-plane rotation functionalities. The proposed structures are thinner than λ / 7 and free of diffractions. Usually, the structure's symmetry or asymmetry, i.e. unbroken or broken spatial inversion symmetries, are considered for metasurfaces as prerequisites of the capability of symmetric or asymmetric conversion of linearly polarized waves, respectively. Due to the achieved adjustment of the resonances enabling the rotation of the polarization plane simultaneously for both orthogonal polarizations of the incident wave, the symmetric polarization-plane rotation functionality can be obtained within one subwavelength band, whereas the asymmetric polarization-plane rotation functionality associated with the asymmetric transmission is obtained within another subwavelength band. This combination of the functionalities in one subdiffraction structure is possible due to the optimal choice of the grid parameters, since they may strongly affect the coupling between the two resonator arrays. Although normal incidence is required for the targeted bifunctionality, the variations of the incidence angle can also be exploited for the enrichment of the overall functional capability. Variations of the polarization angle give another important degree of freedom. The connection between the polarization-angle dependence of cross-polarized transmission and capability of symmetric and asymmetric polarization-plane rotation functionalities is highlighted. The feasible designs of the bifunctional metasurfaces are discussed.

Quasi-bound states in the continuum for electromagnetic induced transparency and strong excitonic coupling.

Advancing on previous reports, we utilize quasi-bound states in the continuum (q-BICs) supported by a metasurface of TiO2 meta-atoms with broken inversion symmetry on an SiO2 substrate, for two possible applications. Firstly, we demonstrate that by tuning the metasurface's asymmetric parameter, a spectral overlap between a broad q-BIC and a narrow magnetic dipole resonance is achieved, yielding an electromagnetic induced transparency analogue with a 50 µs group delay. Secondly, we have found that, due to the strong coupling between the q-BIC and WS2 exciton at room temperature and normal incidence, by integrating a single layer of WS2 to the metasurface, a 37.9 meV Rabi splitting in the absorptance spectrum with 50% absorption efficiency is obtained. These findings promise feasible two-port devices for visible range slow-light characteristics or nanoscale excitonic coupling.

A novel method to eliminate the symmetry dependence of fiber coils for shupe mitigation.

It is a well-known fact that interferometric fiber optic gyroscopes (IFOGs) are easily distorted by thermal effects and distortion results in the degradation of the performance of these sensors. Changing the fiber coil geometry, increasing the winding symmetry, adding fiber buffer layers around the fiber coil, using different modulation methods for multifunctional integrated optic chips, and using special types of fibers, such as photonic crystal fibers, are some alternative solutions for preventing this degradation. This paper, theoretically and experimentally, investigates not only how different types of fiber coil winding methods behave under different rates of temperature change but also presents a novel method, to the best of our knowledge, to eliminate the Shupe effect, without violating the simplest IFOG scheme. This method rules out the importance of the winding symmetry epochally and the need of any extra treatment for the fiber coil to increase the thermal performance of the system. Regardless of the symmetry of the fiber coil winding, the rate error due to the Shupe effect can be reduced to about ± 0 . 05 ∘ / h for any rate of temperature change with this new method according to the experimental results.

Exploring localized ENZ resonances and their role in superscattering, wideband invisibility, and tunable scattering.

While the role and manifestations of the localized surface plasmon resonances (LSPRs) in anomalous scattering, like superscattering and invisibility, are quite well explored, the existence, appearance, and possible contribution of localized epsilon-near-zero (ENZ) resonances still invoke careful exploration. In this paper, that is done along with a comparison of the resonances of two types in the case of thin-wall cylinders made of lossy and loss-compensated dispersive materials. It is shown that the localized ENZ resonances exist and appear very close to the zero-permittivity regime, i.e., at near-zero but yet negative permittivity that is similar to the ENZ modes in thin planar films. Near- and far-field characteristics of the superscattering modes are investigated. The results indicate that the scattering regimes arising due to LSPRs and localized ENZ resonances are distinguishable in terms of the basic field features inside and around the scatterer and differ in their contribution to the resulting scattering mechanism, e.g., in terms of the occupied frequency and permittivity ranges as well as the sensitivity to the wall thickness variations. When the losses are either weak or tend to zero due to the doping with gain enabling impurities, the sharp peaks of the scattering cross-section that are yielded by the resonances can be said to be embedded into the otherwise wide invisibility range. In the case of lossy material, a wide and continuous invisibility range is shown to appear not only due to a small total volume of the scatterer in the nonresonant regime, but also because high-Q superscattering modes are suppressed by the losses. For numerical demonstration, indium antimonide, a natural lossy material, and a hypothetical, properly doped material with the same real part of the permittivity but lower or zero losses are considered. In the latter case, variations of permittivity with a control parameter can be adjusted in such a way that transitions from one superscattering mode to another can be achieved. In turn, transition from the strong-scattering to the invisibility regime is possible even for the original lossy material. The basic properties of the studied superscattering modes may be replicable in artificial structures comprising natural low-loss materials.

Rational design of an acceptor-chromophore-relay-catalyst tetrad assembly for water oxidation.

We report the step-by-step synthesis of a precious metal-free acceptor-chromophore-relay-catalyst tetrad assembly that exhibits a turnover frequency (TOF) of 7.5 × 10-3 s-1 under neutral conditions. Transient absorption spectroscopic studies indicate that upon fullerenol incorporation into the investigated complexes, charge separation efficiency increases considerably.

Low-Pressure Deuterium Storage on Palladium-Coated Titanium Nanofilms: A Versatile Model System for Tritium-Based Betavoltaic Battery Applications.

Deuterium (D2(g)) storage of Pd-coated Ti ultra-thin films at relatively low pressures is fine-tuned by systematically controlling the thicknesses of the catalytic Pd overlayer, underlying Ti ultra-thin film domain, D2(g) pressure (PD2), duration of D2(g) exposure, and the thin film temperature. Structural properties of the Ti/Pd nanofilms are investigated via XRD, XPS, AFM, SEM, and TPD to explore new structure-functionality relationships. Ti/Pd thin film systems are deuterated to obtain a D/Ti ratio of up to 1.53 forming crystallographically ordered titanium deuteride (TiDx) phases with strong Tix+-Dy- electronic interactions and high thermal stability, where >90% of the stored D resides in the Ti component, thermally desorbing at >460 °C in the form of D2(g). Electronic interaction between Pd and D is weak, yielding metallic (Pd0) states where D storage occurs mostly on the Pd film surface (i.e., without forming ordered bulk PdDx phases) leading to the thermal desorption of primarily DOH(g) and D2O(g) at <265 °C. D-storage typically increases with increasing Ti film thickness, PD2, T, and t, whereas D-storage is found to be sensitive to the thickness and the surface roughness of the catalytic Pd overlayer. Optimum Pd film thickness is determined to be 10 nm providing sufficient surface coverage for adequate wetting of the underlying Ti film while offering an appropriate number of surface defects (roughness) for D immobilization and a relatively short transport pathlength for efficient D diffusion from Pd to Ti. The currently used D-storage optimization strategy is also extended to a realistic tritium-based betavoltaic battery (BVB) device producing promising ß-particle emission yields of 164 mCi/cm2, an open circuit potential (VOC) of 2.04 V, and a short circuit current (ISC) of 7.2 nA.

Manipulating Intermetallic Charge Transfer for Switchable External Stimulus-Enhanced Water Oxidation Electrocatalysis.

Electrocatalytic processes involving the oxygen evolution reaction (OER) present a kinetic bottleneck due to the existence of linear-scaling relationships, which bind the energies of the different intermediates in the mechanism limiting optimization. Here, we offer a way to break these scaling relationships and enhance the electrocatalytic activity of a Co-Fe Prussian blue modified electrode in OER by applying external stimuli. Improvements of ≈11 % and ≈57 % were achieved under magnetic field (0.2 T) and light irradiation (100 mW cm-2 ), respectively, when working at fixed overpotential, η=0.6 V at pH 7. The observed enhancements strongly tie in with the intermetallic charge transfer (IMCT) intensity between Fe and Co sites. Density Functional Theory simulations suggest that tuning the IMCT can lead to a change of the OER mechanism to an external stimuli-sensitive spin crossover-based pathway, which opens the way for switchable electrocatalytic devices.

Phase-change Fano resonator for active modulation of thermal emission.

Optical modulation of heat emission using spectrally selective infrared (IR) metasurface nanoantenna designs has found potential applications in various fields, including radiative cooling and thermal camouflage. While radiative cooling requires emitters to emit within atmospheric transmissive windows (mainly located at 8-14 µm), thermal camouflage structures have to operate within the non-transmissive window (5-8 µm) to hide an object from thermal imaging systems and cameras. Therefore, a passive nanoantenna structure cannot satisfy both conditions simultaneously. In this paper, we propose an adaptive nanoantenna emitter made of samarium nickelate (SmNiO3) phase change material to cover both functionalities with a single Fano resonator-based design. As the temperature rises, the thermal signature of the nanoantenna at the transmissive window is suppressed; therefore, a better camouflage performance is achieved. The dynamic tunability of switching from radiative cooling to thermal camouflage of the proposed Fano resonator-based design is quantitatively demonstrated using emissive power calculations under different conditions.

High-Figure-of-Merit Biosensing and Enhanced Excitonic Absorption in an MoS2-Integrated Dielectric Metasurface.

Among the transitional metal dichalcogenides (TMDCs), molybdenum disulfide (MoS2) is considered an outstanding candidate for biosensing applications due to its high absorptivity and amenability to ionic current measurements. Dielectric metasurfaces have also emerged as a powerful platform for novel optical biosensing due to their low optical losses and strong near-field enhancements. Once functionalized with TMDCs, dielectric metasurfaces can also provide strong photon-exciton interactions. Here, we theoretically integrated a single layer of MoS2 into a CMOS-compatible asymmetric dielectric metasurface composed of TiO2 meta-atoms with a broken in-plane inversion symmetry on an SiO2 substrate. We numerically show that the designed MoS2-integrated metasurface can function as a high-figure-of-merit (FoM=137.5 RIU-1) van der Waals-based biosensor due to the support of quasi-bound states in the continuum. Moreover, owing to the critical coupling of the magnetic dipole resonances of the metasurface and the A exciton of the single layer of MoS2, one can achieve a 55% enhanced excitonic absorption by this two-port system. Therefore, the proposed design can function as an effective biosensor and is also practical for enhanced excitonic absorption and emission applications.

2D Network overtakes 3D for photocatalytic hydrogen evolution.

3-Dimensional (3D) cyanide coordination polymers, typically known as Prussian blue Analogues (PBAs), have received great attention in catalysis due to their stability, easily tuned metal sites, and porosity. However, their high crystallinities and relatively low number of surface-active sites significantly hamper their intrinsic catalytic activities. Herein, we report the utilization of a 2-dimensional (2D) layered cobalt tetracyanonickelate, [Co-Ni], for the reduction of protons to H2. Relying on its exposed facets, layered morphology, and abundant surface-active sites, [Co-Ni] can efficiently convert water and sunlight to H2 in the presence of a ruthenium photosensitizer (Ru PS) with an optimal evolution rate of 30 029 ± 590 µmol g-1 h-1, greatly exceeding that of 3D Co-Fe PBA [Co-Fe] and Co-Co PBA [Co-Co]. Furthermore, [Co-Ni] retains its structural integrity throughout a 6 hour photocatalytic cycle, which is confirmed by XPS, PXRD, and Infrared analysis. This recent work reveals the excellent morphologic properties that promote [Co-Ni] as an attractive catalyst for the hydrogen evolution reaction (HER).

"Plug and Play" Photosensitizer-Catalyst Dyads for Water Oxidation.

We present a simple and easy-to-scale synthetic method to plug common organic photosensitizers into a cyanide-based network structure for the development of photosensitizer-water oxidation catalyst (PS-WOC) dyad assemblies for the photocatalytic water oxidation process. Three photosensitizers, one of which absorbs red light similar to P680 in photosystem II, were utilized to harvest different regions of the solar spectrum. Photosensitizers are covalently coordinated to CoFe Prussian blue structures to prepare PS-WOC dyads. All dyads exhibit steady water oxidation catalytic activities throughout a 6 h photocatalytic experiment. Our results demonstrate that the covalent coordination between the PS and WOC group not only enhances the photocatalytic activity but also improves the robustness of the organic PS group. The photocatalytic activity of "plug and play" dyads relies on several structural and electronic parameters, including the position of the energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the PS with respect to the HOMO level of the catalytic site, the intensity and wavelength of the absorption band of the PS, and the number of catalytic sites.

Transmissive terahertz metasurfaces with vanadium dioxide split-rings and grids for switchable asymmetric polarization manipulation.

Metasurfaces containing arrays of thermally tunable metal-free (double-)split-ring meta-atoms and metal-free grids made of vanadium dioxide (VO[Formula: see text]), a phase-change material can deliver switching between (1) polarization manipulation in transmission mode as well as related asymmetric transmission and (2) other functionalities in the terahertz regime, especially when operation in the transmission mode is needed to be conserved for both phases of VO[Formula: see text]. As the meta-atom arrays function as arrays of metallic subwavelength resonators for the metallic phase of VO[Formula: see text], but as transmissive phase screens for the insulator phase of VO[Formula: see text], numerical simulations of double- and triple-array metasurfaces strongly indicate extreme scenarios of functionality switching also when the resulting structure comprises only VO[Formula: see text] meta-atoms and VO[Formula: see text] grids. More switching scenarios are achievable when only one meta-atom array or one grid is made of VO[Formula: see text] components. They are enabled by the efficient coupling of the geometrically identical resonator arrays/grids that are made of the materials that strongly differ in terms of conductivity, i.e. Cu and VO[Formula: see text] in the metallic phase.

Light-Driven Water Oxidation with Ligand-Engineered Prussian Blue Analogues.

The elucidation of the ideal coordination environment of a catalytic site has been at the heart of catalytic applications. Herein, we show that the water oxidation activities of catalytic cobalt sites in a Prussian blue (PB) structure could be tuned systematically by decorating its coordination sphere with a combination of cyanide and bidentate pyridyl groups.  K0.1[Co(bpy)]2.9[Fe(CN)6]2 ([Cobpy-Fe]), K0.2[Co(phen)]2.8[Fe(CN)6]2 ([Cophen-Fe]), {[Co(bpy)2]3[Fe(CN)6]2}[Fe(CN)6]1/3 ([Cobpy2-Fe]), and {[Co(phen)2]3[Fe(CN)6]2}[Fe(CN)6]1/3 Cl0.11 ([Cophen2-Fe]) were prepared by introducing bidentate pyridyl groups (phen: 1,10-phenanthroline, bpy: 2,2'-bipyridine) to the common synthetic protocol of Co-Fe Prussian blue analogues. Characterization studies indicate that [Cobpy2-Fe] and [Cophen2-Fe] adopt a pentanuclear molecular structure, while [Cobpy-Fe] and [Cophen-Fe] could be described as cyanide-based coordination polymers with lower-dimensionality and less crystalline nature compared to the regular Co-Fe Prussian blue analogue (PBA), K0.1Co2.9[Fe(CN)6]2 ([Co-Fe]). Photocatalytic studies reveal that the activities of [Cobpy-Fe] and [Cophen-Fe] are significantly enhanced compared to those of [Co-Fe], while molecular [Cobpy2-Fe] and [Cophen2-Fe] are inactive toward water oxidation. [Cobpy-Fe] and [Cophen-Fe] exhibit upper-bound turnover frequencies (TOFs) of 1.3 and 0.7 s-1, respectively, which are ∼50 times higher than that of [Co-Fe] (1.8 × 10-2 s-1). The complete inactivity of [Cobpy2-Fe] and [Cophen2-Fe] confirms the critical role of aqua coordination to the catalytic cobalt sites for oxygen evolution reaction (OER). Computational studies show that bidentate pyridyl groups enhance the susceptibility of the rate-determining Co(IV)-oxo species to the nucleophilic water attack during the critical O-O bond formation. This study opens a new route toward increasing the intrinsic water oxidation activity of the catalytic sites in PB coordination polymers.

Fano resonance in a dolomite phase-change multilayer design for dynamically tunable omnidirectional monochromatic thermal emission.

In this Letter, we unveil the unprecedented optical phonon response of CaMg(CO3)2 (dolomite) thin film in the design of a planar ultra-narrowband mid-infrared (MIR) thermal emitter. Dolomite (DLM) is a carbonate mineral composed of calcium magnesium carbonate, which can inherently accommodate highly dispersive optical phonon modes. Utilizing strong interference in the Al-DLM bilayer, a lithography-free planar thermal emitter is realized with near-unity omnidirectional emission at a specific resonance wavelength of 7.12 µm. Further incorporation of embedded vanadium dioxide (VO2) phase change material (PCM) enables the excitation of hybrid Fano resonances with dynamic spectral tunability. The findings of this study can have multiple applications, ranging from biosensing and gas sensing to thermal emission.

Optimization of the annealed proton exchange method with controlled annealing for multifunctional integrated optical chip production.

The main objective of our study is to develop a new approach to the annealed proton exchange (APE) method for the fabrication of the multifunctional integrated optical chip (MIOC) used in fiber-optic gyro systems and to eliminate the loss of time and material, especially in mass production applications. In this work, self-polarized waveguides, which are the basic components of a MIOC device, were produced by the APE method and studied. With the developed method, controlled annealing trials have been carried out from a certain region on the LiNbO3 substrate used in waveguide production, and the annealing time specific to the annealing process was determined. By utilizing a special setup for the hot acid process, the proton exchange process was accomplished without a sudden temperature change of the substrate. Using prism coupling measurements of the fabricated waveguides, annealing times were determined to obtain index change values suitable for 45%-50% optical throughput. Mode profiles of devices with high optical throughput that were produced by the proposed method were measured, and it was seen that devices from different proton exchange runs had similar profiles. As a result, many undamaged substrates were fabricated, and their optical quality was found to be within the expected values.

Electrodeposited cobalt hexacyanoferrate electrode as a non-enzymatic glucose sensor under neutral conditions.

A CoFe Prussian blue analogue (CoFe PB) modified FTO electrode, prepared via a facile electrodeposition method, is investigated as a non-enzymatic glucose sensor under neutral conditions. The electrode exhibits a linear detection of glucose in the 0.1-8.2 mmol/L range with a detection limit of 67 µM, a sensitivity of 18.69 µA/mM.cm2, and a fast response time of less than 7 s under neutral conditions. Its stability is confirmed with both electrochemical experiments and characterization studies performed on the pristine and post-mortem electrode. We also conducted a comprehensive electrochemical analysis to elucidate the identity of the active site and the glucose oxidation mechanism on the Prussian blue surface.

Adaptive visible and short-wave infrared camouflage using a dynamically tunable metasurface.

In this Letter, a dynamically tunable metasurface, which is based on antimony trisulfide, is introduced. In this structure, first a metal-insulator-metal (MIM) nanocavity is optimized in a way that, upon phase change, the visible response switches from a transmissive colored window into a reflective mirror. Later, an indium tin oxide nanoantenna is integrated on the MIM cavity to provide antireflection in the short-wave infrared (SWIR) range for SWIR mode thermal camouflage. The MIM host acts as a thermally tunable substrate to tune the SWIR response of the design.

Multi-spectral infrared camouflage through excitation of plasmon-phonon polaritons in a visible-transparent hBN-ITO nanoantenna emitter.

In an ideal platform for camouflage compatible cooling, the thermal emitter should be a spectrally selective antenna to radiate its heat buildup without being detected by thermal cameras. Moreover, to keep its visual appearance and to minimize solar induced heating, the structure should be visibly transparent. In this Letter, to achieve the visually invisible mid-infrared (MIR) camouflage-cooling feature, a metasurface design based on an indium-doped tin oxide (ITO)-hexagonal boron nitride (hBN) heterostructure is proposed. The proposed ITO-hBN nanoantenna shows spectrally selective broadband absorptions in near-infrared (NIR) and non-transmissive (MIR) windows, while it is dominantly non-emissive in other ranges. The camouflage ability of the structure in the targeted wavelengths is demonstrated using power calculations.

Fiber-optic gyroscope for the suppression of a Faraday-effect-induced bias error.

Inertial rotation sensors, interferometric fiber-optic gyroscopes (IFOGs), are widely used in military and industrial applications due to their high sensitivity and stability. In this Letter, a new, to the best of our knowledge, fiber coil design is proposed to reduce magnetic field sensitivity without adding any optical components or electronic algorithms to the IFOG system. It is shown that this design can be applied without disturbing the simplest IFOG structure. Considering the fact that the magnetic field has an invertible effect on polarization, the compensation of the Faraday-effect-induced bias error has been demonstrated theoretically and experimentally by allowing two different polarizations to travel inside the fiber coil. According to the experimental results, the bias error was reduced approximately 20 times from ±9.6∘/h/mT to ±0.5∘/h/mT.