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The perturbation of a magnetostatic field by a toroid made of a homogeneous anisotropic magnetic material was formulated using the solutions of the Laplace equation in the toroidal coordinate system. That was straightforward in the region outside the toroid, but an affine coordinate transformation had to be employed inside the toroid. The coefficients of the series expansion of the perturbation potential in terms of appropriate toroidal basis functions were related to the coefficients of the series expansion of the source potential in terms of appropriate toroidal basis functions by a transition matrix. As a result of the solution of this novel problem, the consequences of material anisotropy on perturbing the magnetostatic field are clearly evident in the region near the toroid.

The circular Bragg phenomenon is the circular-polarization-state-selective reflection of light in a spectral regime called the circular Bragg regime. In continuation of an expository review on this phenomenon published in 2014, an album of theoretical results is provided in this decadal update. Spectral variations of intensity-dependent and phase-dependent observable quantities in the reflection and transmission half-spaces are provided in relation to the polarization state and the direction of propagation of the incident plane wave., Comment: 20 pages, 11 figures

The geometric phase has acquired interest for optical devices such as achromatic phase shifters, spatial light modulators, frequency shifters, and planar lenses for wavefront engineering. Numerical work with Reusch piles with a large number of layers per period suggests that the geometric phase of the reflected/transmitted plane wave may be sensitive to nanoscale morphological differences, and the geometric-phase spectrum could contain signatures of morphological details that depend on the fabrication technique.

Closed-form expressions were recently derived for depolarization dyadics for truncated spheres and truncated spheroids, and the formalism was extended to truncated ellipsoids. These results were exploited to develop an implementation of the Maxwell Garnett homogenization formalism for the relative permittivity parameters of homogenized composite mediums (HCMs) arising from an isotropic host medium impregnated with isotropic inclusions that are truncated spheres, spheroids, and ellipsoids. In so doing, the anisotropy of the HCM was related to the geometry of the inclusions: in general, the more the shape of the inclusions deviated from spherical, the greater was the degree of anisotropy exhibited by the HCM.

A slab made of a dielectric structurally chiral medium (DSCM) strongly reflects the co-handed circularly polarized plane wave, but not the cross-handed circularly polarized plane wave, in a spectral regime called the circular Bragg regime. The effect of inserting a central phase defect in a DSCM slab with a modest number of structural periods is a spectral reflection hole in the circular Bragg regime, for co-handed incidence only. However, if the incident plane wave is left-circularly polarized, the geometric phase of the transmitted plane wave contains evidence of both the circular Bragg regime and the spectral reflection hole, regardless of the structural handedness of the DSCM. This evidence is indicative of the type of phase defect. The effect of inserting a central phase defect in a DSCM slab with a large number of structural periods is a spectral transmission hole in the circular Bragg regime, for cross-handed incidence only. The spectral transmission hole may be difficult to observe experimentally because of absorption inside the DSCM slab, but it will still be evident in the geometric phase of the transmitted plane wave, if the incident plane wave is left-circularly polarized., Comment: 8 pages, 8 figures

Depolarization dyadics play a central role in theoretical studies involving scattering from small particles and homogenization of particulate composite materials. Closed-form expressions for depolarization dyadics have been developed for truncated spheres and truncated spheroids, and the formalism has been extended to truncated ellipsoids; the evaluation of depolarization dyadics for this latter case requires numerical integration. The H\"{o}lder continuity condition has been exploited to fix the origin of the coordinate system for the evaluation of depolarization dyadics.These results will enable theoretical studies involving scattering from small particles and homogenization of particulate composite materials to accommodate particles with a much wider range of shapes than was the case hitherto.

Polymer banknotes are being increasingly adopted to replace older banknotes. Since banknotes are forensically important substrates for fingermark detection and identification, we present a single-step process to enhance the visualization of fingermarks on banknotes using columnar thin films (CTFs) of nickel. This single-step vacuum technique enhances the quality grade of fingermarks maximally, whether the fingermarks are aged for one or seven days before CTF deposition. This work represents progress over currently available sequences of diverse techniques for enhancing fingermarks on polymer banknotes., Comment: 11 pages, 3 figures

CDs and DVDs are forensically important substrates for latent fingermarks. Upright columnar thin films (CTFs) grown conformally over these substrates were shown to significantly enhance the visual quality of fingermarks. Enhancement to maximum possible grade for visual quality occurred, even for the samples that were developed three days after the fingermarks were placed on them. The CTFs were deposited by directing a collimated vapor flux of an evaporant material towards the substrate placed at an angle and made to rotate rapidly., Comment: 12 pages, 4 figures

The propagation of surface-plasmon-polariton (SPP) waves at the planar interface of a metal and a dielectric material was investigated for a dielectric material with strongly temperature-dependent constitutive properties. The metal was silver and the dielectric material was vanadium multioxide impregnated with a combination of active dyes. Depending upon the volume fraction of vanadium multioxide, either attenuation or amplification of the SPP waves may be achieved; the degree of attenuation or amplification is strongly dependent on both the temperature and whether the temperature is increasing or decreasing. At intermediate volume fractions of vanadium multioxide, for a fixed temperature, a SPP wave may experience attenuation if the temperature is increasing but experience amplification if the temperature is decreasing.

The planewave response of a bicontrollable metasurface absorber with graphene-patched pixels was simulated in the X band using commercial software. Each square meta-atom is a 4x4 array of 16 pixels, some patched with graphene and the others unpatched. The pixels are arranged on a PVC skin which is placed on a copper-backed YIG substrate. Graphene provides electrostatic controllability and YIG provides magnetostatic controllability. Our design delivers absorptance equal to or in excess of 0.9 over a 100-MHz spectral regime in the X band, with 360 MHz/kA magnetostatic controllabity rate and 1 Hz m/V electrostatic controllability rate. Notably, electrostatic control via graphene in the GHz range is novel., Comment: 9 pages, 4 figures, 1 table

A Huygens bianisotropic medium is a linear homogeneous medium for which the Huygens principle can be formulated. When a bounded 3D scattering object composed of a linear bianisotropic medium, whether homogeneous or not, is embedded in a Huygens bianisotropic medium, the excess field phasors inside that object act as volume current densities, and the tangential components of the internal field phasors on the surface of the same object act as surface current densities, to radiate identical field phasors in the external region.

The Bruggeman homogenization formalism was used to numerically investigate the dielectric properties of a columnar thin film (CTF) made from vanadium dioxide. For visible and near-infrared wavelengths, the CTF is electromagnetically equivalent to a homogeneous orthorhombic material. Over the 58 deg C -- 72 deg C temperature range, the eigenvalues of the CTF's relative permittivity dyadic are highly sensitive to temperature, and vary according to whether the CTF is being heated or cooled. The anisotropy revealed through the eigenvalues, and the anisotropy of the associated hysteresis, were investigated in relation to temperature for CTFs of different porosities and columnar cross sections. When the free-space wavelength is 800 nm, the CTF is a dissipative dielectric material that exhibits temperature-dependent anisotropy and anisotropic hysteresis. In contrast, when the free-space wavelength is 1550 nm, the CTF can be either a dissipative dielectric material, a hyperbolic material or a metal-like material, depending on the temperature and the porosity of the CTF. As the porosity of the CTF decreases from 0.55 to 0.3, the anisotropy of the CTF becomes more pronounced, as does the anisotropy of the hysteresis. Only relatively modest variations in anisotropy and hysteresis arise in response to varying the columnar cross-sectional shape, as compared to the variations induced by varying the porosity.

Vanadium dioxide (VO2) transforms from purely monoclinic to purely tetragonal on being heated from 58 deg C to 72 deg C, the transformation being reversible but hysteretic. Electromagnetically, VO2 transforms from a dissipative dielectric to another dissipative dielectric if the free-space wavelength is less than 1100 nm, but from a dissipative dielectric to a plasmonic metal (or vice versa) if the free-space wavelength exceeds 1100 nm. Calculating the extinction, total scattering, absorption, radiation-pressure, back-scattering, and forward-scattering efficiencies of a VO2 sphere, we found clear signatures of thermal hysteresis in (i) the forward-scattering, back-scattering, and absorption efficiencies for free-space wavelength less than 1100 nm, and (ii) the forward-scattering, back-scattering, total scattering, and absorption efficiencies for free-space wavelength more than 1100 nm. Vacuum and null-permittivity quasistates occur between 58 deg C and 72 deg C, when tetragonal VO2 is a plasmonic metal, once each on the heating branch and once each on the cooling branch of thermal hysteresis. But none of the six efficiencies show significant differences between the two quasistates., Comment: 11 pages, 4 figures

We conceptualized three-array scalable metasurfaces comprising only three thin strip grids and numerically demonstrated their characteristics in the mid-infrared spectral regime for switchable polarization manipulation and related asymmetric diodelike transmission (AT). A few or all of the grids were taken to be made of VO$_2$, a phase-change material. For each proposed metasurface, two effective structures and, therefore, two different functionalities exist, corresponding to the metallic and insulating phases of VO$_2$. The achieved scenarios of functionality switching that depend on the VO$_2$ phase are shown to significantly depend in the way in which VO$_2$ is incorporated to the metasurface. Switchable bands of polarization manipulation are up to 40 THz wide. The AT band can be modulated when Fabry--Perot (anti-)resonances come into play., Comment: 6 pages, 4 figures

The homogenization of vaterite impregnated with gold nanoparticles was accomplished using the extended Maxwell Garnett formalism. The extended formalism takes into account the intrinsic anisotropy of vaterite as well as the size, shape, and orientation of the nanoparticles. Size-dependent permittivity was used for the gold nanoparticles. Numerical studies revealed that the homogenized composite material's permittivity parameters are acutely sensitive to the size, shape, orientation, and volume fraction of the gold nanoparticles.

The extended boundary condition method (EBCM) was formulated for the perturbation of a source electric potential by a 3D object composed of a homogeneous anisotropic dielectric medium whose relative permittivity dyadic is positive definite. The formulation required the application of Green's second identity to the exterior region to deduce the electrostatic counterpart of the Ewald--Oseen extinction theorem. The electric potential inside the object was represented using a basis obtained by implementing an affine bijective transformation of space to the Gauss equation for the electric field. The EBCM yields a transition matrix that depends on the geometry and the composition of the 3D object, but not on the source potential.

The boundary-value problem for the perturbation of an electric potential by a homogeneous anisotropic dielectric sphere in vacuum was formulated. The total potential in the exterior region was expanded in series of radial polynomials and tesseral harmonics, as is standard for the Laplace equation. A bijective transformation of space was carried out to formulate a series representation of the potential in the interior region. Boundary conditions on the spherical surface were enforced to derive a transition matrix that relates the expansion coefficients of the perturbation potential in the exterior region to those of the source potential. Far from the sphere, the perturbation potential decays as the inverse of the distance squared from the center of the sphere, as confirmed numerically.

Ordinarily, a trimaterial structure comprising a sufficiently thin metal film interposed between two homogeneous dielectric materials guides compound plasmon-polariton (CPP) waves, for which the fields on both sides of the metal film decay exponentially with distance from the nearest metal/dielectric interface. However, if one of the dielectric materials is anisotropic then the trimaterial structure can guide an exceptional CPP wave for a particular propagation direction. On the side of the metal film occupied by the anisotropic dielectric material, the fields of the exceptional CPP wave decay as the product of a linear function and an exponential function of the distance from the nearest metal/dielectric interface. The canonical boundary-value problem for CPP-wave propagation has been analyzed and solved numerically; thereby, the spatial field profiles for exceptional CPP waves for a uniaxial-dielectric/metal/isotropic-dielectric structure have been established.

A stepwise chirping of the periodicity of a chiral sculptured thin film is shown to considerably enhance the bandwidth of the Bragg regime, thereby extending the frequency range of operation as a circular{polarization filter.

Radars often use correlation of received signals with transmitted signals to identify targets. However, when a target translates at a high uniform speed, the correlation of the transmitted and received signals depends strongly on the target's velocity, even after the two-way Doppler shift has been removed. This is true whether the target is made of highly dispersive material or not.

An exceptional surface wave can propagate in an isolated direction, when guided by the planar interface of two homogeneous dielectric partnering mediums of which at least one is anisotropic, provided that the constitutive parameters of the partnering mediums satisfy certain constraints. Exceptional surface waves are distinguished from unexceptional surface waves by their localization characteristics: the fields of an exceptional surface wave in the anisotropic partnering medium decay as a combined linear-exponential function of distance from the interface, whereas the decay is purely exponential for an unexceptional surface wave. If both partnering mediums are anisotropic then a doubly exceptional surface wave can exist for an isolated propagation direction. The decay of this wave in both partnering mediums is governed by a combined linear-exponential function of distance from the interface.

Power-conversion efficiency is a critical factor for the wider adoption of solar-cell modules. Thin-film solar cells are cheap and easy to manufacture, but their efficiencies are low compared to crystalline-silicon solar cells and need to be improved. A thin-film solar cell with two absorber layers (instead of only one), with bandgap energy graded in both, can capture solar photons in a wider spectral range. With a 300-nm-thick CIGS~absorber layer and an 870-nm-thick CZTSSe~absorber layer, an efficiency of $34.45\%$ is predicted by a detailed optoelectronic model, provided that the grading of bandgap energy is optimal in both absorber layers., Comment: 4 pages, 3 figures

Guided by the planar interface of two dissimilar linear, homogeneous mediums, a Voigt surface wave arises due to an exceptional point of either of the two matrixes necessary to describe the spatial characteristics in the direction normal to the planar interface. There is no requirement for either or both partnering mediums to be dissipative, unlike a Voigt plane wave which can propagate only in a dissipative medium.

The Lorenz--Mie formulation of electromagnetic scattering by a homogeneous, isotropic, dielectric-magnetic sphere was extended to incorporate topologically insulating surface states characterized by a surface admittance $\gamma$. Closed-form expressions were derived for the expansion coefficients of the scattered field phasors in terms of those of the incident field phasors. These expansion coefficients were used to obtain analytical expressions for the total scattering, extinction, forward scattering, and backscattering efficiencies of the sphere. Resonances exist for relatively low values of $\gamma$, when the sphere is either nondissipative or weakly dissipative. For large values of $\gamma$, the scattering characteristics are close to that of a perfect electrically conducting sphere, regardless of whether the sphere is composed of a dissipative or nondissipative material, and regardless of whether that material supports planewave propagation with positive or negative phase velocity.

An optoelectronic optimization was carried out for an AlGaAs solar cell containing (i) an n-AlGaAs absorber layer with a graded bandgap and (ii) a periodically corrugated Ag backreflector combined with localized ohmic Pd-Ge-Au backcontacts. The bandgap of the absorber layer was varied either sinusoidally or linearly. An efficiency of 33.1% with the 2000-nm-thick n-AlGaAs absorber layer is predicted with linearly graded bandgap along with silver backreflector and localized ohmic backcontacts, in comparison to 27.4% efficiency obtained with homogeneous bandgap and a continuous ohmic backcontact. Sinusoidal grading of the bandgap {is predicted to enhance} the maximum efficiency to 34.5%. Thus, grading the bandgap of the absorber layer, along with a periodically corrugated Ag backreflector and localized ohmic Pd-Ge-Au backcontacts can help realize ultrathin and high-efficient AlGaAs solar cells for terrestrial applications.

A coupled optoelectronic model was implemented along with the differential evolution algorithm to assess the efficacy of grading the bandgap of the CZTSSe layer for enhancing the power conversion efficiency of thin-film CZTSSe solar cells. Both linearly and sinusoidally graded bandgaps were examined, with the molybdenum backreflector in the solar cell being either planar or periodically corrugated. Whereas an optimally graded bandgap can dramatically enhance the efficiency, the effect of periodically corrugating the backreflector is modest at best. An efficiency of 21.74% is predicted with sinusoidal grading of a 870-nm-thick CZTSSe layer, in comparison to 12.6% efficiency achieved experimentally with a 2200-nm-thick homogeneous CZTSSe layer. High electron-hole-pair generation rates in the narrow-bandgap regions and a high open-circuit voltage due to a wider bandgap close to the front and rear faces of the CZTSSe layer are responsible for the high enhancement of efficiency.

Very weak left/right asymmetry in reflection and transmission is offered by a layer of a topological insulator on top of a layer of an anisotropic dielectric material, but it can be enhanced very significantly by using a periodic multilayer of both types of materials. This is an attractive prospect for realizing one-way terahertz devices, because both types of materials can be grown using standard physical-vapor-deposition techniques.

The propagation of Dyakonov-Tamm (DT) surface waves guided by the planar interface of two nondissipative materials $A$ and $B$ was investigated theoretically and numerically, via the corresponding canonical boundary-value problem. Material $A$ is a homogeneous uniaxial dielectric material whose optic axis lies at an angle $\chi$ relative to the interface plane. Material $B$ is an isotropic dielectric material that is periodically nonhomogeneous in the direction normal to the interface. The special case was considered in which the propagation matrix for material $A$ is non-diagonalizable because the corresponding surface wave-named the Dyakonov-Tamm-Voigt (DTV) surface wave-has unusual localization characteristics. The decay of the DTV surface wave is given by the product of a linear function and an exponential function of distance from the interface in material $A$; in contrast, the fields of conventional DT surface waves decay only exponentially with distance from the interface. Numerical studies revealed that multiple DT surface waves can exist for a fixed propagation direction in the interface plane, depending upon the constitutive parameters of materials $A$ and $B$. When regarded as functions of the angle of propagation in the interface plane, the multiple DT surface-wave solutions can be organized as continuous branches. A larger number of DT solution branches exist when the degree of anisotropy of material $A$ is greater. If $\chi = 0^\circ$ then a solitary DTV solution exists for a unique propagation direction on each DT branch solution. If $\chi > 0^\circ$, then no DTV solutions exist. As the degree of nonhomogeneity of material $B$ decreases, the number of DT solution branches decreases., Comment: arXiv admin note: text overlap with arXiv:1910.10972

We study the convergence properties of the two-dimensional Rigorous Coupled Wave Approach (RCWA) for p-polarized monochromatic incident light. The RCWA is a semi-analytical numerical method that is widely used to solve the boundary-value problem of scattering by a grating. The approach requires the expansion of all electromagnetic field phasors and the relative permittivity as Fourier series in the spatial variable along the direction of the periodicity of the grating. In the direction perpendicular to the grating periodicity, the domain is discretized into thin slices and the actual relative permittivity is replaced by an approximation. The approximate relative permittivity is chosen so that the solution of the Maxwell equations in each slice can be computed without further approximation. Thus, there is error due to the approximate relative permittivity as well as the trucation of the Fourier series. We show that the RCWA embodies a Galerkin scheme for a perturbed problem, and then we use tools from the Finite Element Method to show that the method converges with increasing number of retained Fourier modes and finer approximations of the relative permittivity. Numerical examples illustrate our analysis, and suggest further work.

The incorporation of materials with controllable electromagnetic constitutive parameters allows the conceptualization and realization of controllable metasurfaces. With the aim of formulating and investigating a tricontrollable metasurface for efficiently absorbing terahertz radiation, we adopted a pixel-based approach in which the meta-atoms are biperiodic assemblies of discrete pixels. We patched some pixels with indium antimonide (InSb) and some with graphene, leaving the others unpatched. The bottom of each meta-atom was taken to comprise a metal-backed substrate of silicon nitride. The InSb-patched pixels facilitate the thermal and magnetic control modalities, whereas the graphene-patched pixels facilitate the electrical control modality. With proper configuration of patched and unpatched pixels and with proper selection of the patching material for each patched pixel, the absorptance spectrums of the pixelated metasurface were found to contain peak-shaped features with maximum absorptance exceeding 0.95, full-width-at-half-maximum bandwidth of less than 0.7~THz, and the maximum-absorptance frequency lying between 2~THz and 4~THz. The location of the maximum-absorptance frequency can be thermally, magnetically, and electrically controllable. The lack of rotational invariance of the optimal meta-atom adds mechanical rotation as the fourth control modality.

Solutions of the boundary-value problem for electromagnetic waves guided by a layer of a homogeneous and isotropic (metal or dielectric) material sandwiched between a structurally chiral material (SCM) and a periodically multi-layered isotropic dielectric (PMLID) material were numerically obtained and analyzed. If the sandwiched layer is sufficiently thick, the two bimaterial interfaces decouple from each other, and each interface may guide one or more electromagnetic surface waves (ESWs) by itself. Depending on the constitution of the two materials that partner to form an interface, the ESWs can be classified as surface-plasmon-polarition (SPP) waves, Tamm waves, Dyakonov-Tamm waves, or Uller-Zenneck waves. When the sandwiched layer is sufficiently thin, the ESWs for single bimaterial interfaces coalesce to form compound guided waves (CGWs). The phase speeds, propagation distances, and spatial profiles of the electromagnetic fields of CGWs are different from those of the ESWs. The energy of a CGW is distributed in both the SCM and the PMLID material, if the sandwiched layer is sufficiently thin. Some CGWs require the sandwiched layer to have a minimum thickness. Indeed, the coupling between the two faces of the sandwiched layer is affected by the ratio of the thickness of the sandwiched layer to the skin depth in that material and the rates at which the fields of the ESWs guided individually by the two interfaces decay away from their respective guiding interfaces.

Dyakonov-Voigt (DV) surface waves guided by the planar interface of (i) material $A$ which is a uniaxial dielectric material specified by a relative permittivity dyadic with eigenvalues $\epsilon^s_A$ and $\epsilon^t_A$, and (ii) material $B$ which is an isotropic dielectric material with relative permittivity $\epsilon_B$, were numerically investigated by solving the corresponding canonical boundary-value problem. The two partnering materials are generally dissipative, with the optic axis of material $A$ being inclined at the angle $\chi \in [ 0^\circ, 90^\circ ]$ relative to the interface plane. No solutions of the dispersion equation for DV surface waves exist when $\chi=90^\circ$. Also, no solutions exist for $\chi \in ( 0^\circ, 90^\circ )$, when both partnering materials are nondissipative. For $\chi \in [ 0 ^\circ, 90^\circ )$, the degree of dissipation of material $A$ has a profound effect on the phase speeds, propagation lengths, and penetration depths of the DV surface waves. For mid-range values of $\chi$, DV surface waves with negative phase velocities were found. For fixed values of $\epsilon^s_A$ and $\epsilon^t_A$ in the upper-half-complex plane, DV surface-wave propagation is only possible for large values of $\chi$ when $| \epsilon_B|$ is very small.

Theoretical formalisms for the homogenization of particulate composites are identified as following either the direct scattering approach (DSA) or the indirect scattering approach (ISA). Both approaches can take inclusion size-dependence and distribution statistics into account. However, the DSA is generally limited to mediums with direction-independent constitutive properties and inclusions with simple shapes, but the ISA is not hobbled thus.

The concept of local homogenization is invoked in the visible and the sub-visible frequency regimes to estimate the permittivity dyadics of a thin-film helicoidal bianisotropic mediums (TFHBMs)~---~a canonical class of sculptured thin films with helicoidally periodic microstructure~---~made of a metal. Examination of the components of the permittivity dyadics, as well as of the co- and the cross-polarized reflectances of axially excited, metallic TFHBM halfspaces, reveals that three different absorbing-dielectric-to-metal transitions will occur as the metallic volumetric fraction is increased, but only two will be optically sensed using normally incident plane waves. The first transition is show to virtually eliminate the circular Bragg phenomenon characteristically displayed by axially excited, dielectric TFHBMs.

Near-field phase-shifting contact lithography is modeled to characterize electromagnetic absorption in a photoresist layer with one face in contact with a quartz binary phase-shift mask. The broadband ultraviolet illumination is represented as a frequency-spectrum of normally incident plane waves. A rigorous coupled-wave analysis is carried out to determine the absorption spectrum of the photoresist layer. The specific absorption rate in the photoresist layer is calculated and examined in relation to the geometric parameters. Columnar features in the photoresist layer are of higher quality on broadband illumination in contrast to monochromatic illumination, in conformity with some recent experimental results. Feature resolution and profile are noticeably affected by the depth of the grooves in the phase-shift mask. Ideally, the feature linewidth can be less than about 100 nm for broadband illumination in the transverse-magnetic mode. These conclusions are subject to modification by the photochemistry-wavelength characteristics of the photoresist.

The introduction of either a central layer defect or a central twist defect in a periodic structurally chiral material generates circular-polarization-sensitive spectral holes in the remittance spectrums for normally incident plane waves. We propose and theoretically establish here the third method to generate such spectral holes using two-section chiral sculptured thin films (STFs). Both sections of the proposed device have the same periodicity and handedness, but their dielectric properties are different and related in a specific way. The concept of pseudoisotropy is highly relevant for the production of the proposed device.

A chiral sculptured thin film is fabricated from patinal titanium oxide using the serial bideposition technique. Axial transmittance spectrums are measured over a spectral region encompassing the Bragg regime for axial excitation. The same spectrums are calculated using a nominal structure-property relationship model and the parameter space of the model is explored for best fits of the calculated and measured transmittances. Ambiguity arising on calibrating the model against axial transmittance measurements is shown to be resolvable using non--axial transmittance measurements.

The power conversion efficiency of an ultrathin CIGS solar cell was maximized using a coupled optoelectronic model to determine the optimal bandgap grading of the nonhomogeneous CIGS layer in the thickness direction. The bandgap of the CIGS layer was either sinusoidally or linearly graded, and the solar cell was modeled to have a metallic backreflector corrugated periodically along a fixed direction in the plane. The model predicts that specially tailored bandgap grading can significantly improve the efficiency, with much smaller improvements due to the periodic corrugations. An efficiency of 27.7% with the conventional 2200-nm-thick CIGS layer is predicted with sinusoidal bandgap grading, in comparison to 22% efficiency obtained experimentally with homogeneous bandgap. Furthermore, the inclusion of sinusoidal grading increases the predicted efficiency to 22.89% with just a 600-nm-thick CIGS layer. These high efficiencies arise due to a large electron-hole-pair generation rate in the narrow-bandgap regions and the elevation of the open-circuit voltage due to a wider bandgap in the region toward the front surface of the CIGS layer. Thus, bandgap nonhomogeneity, in conjunction with periodic corrugation of the backreflector, can be effective in realizing ultrathin CIGS solar cells that can help overcome the scarcity of indium.

The canonical boundary-value problem for surface-plasmon-polariton (SPP) waves guided by the planar interface of a dielectric material and a plasmonic material was solved for cases wherein either partnering material could be a uniaxial material with optic axis lying in the interface plane.Numerical studies revealed that two different SPP waves, with different phase speeds, propagation lengths, and penetration depths, can propagate in a given direction in the interface plane; in contrast, the planar interface of isotropic partnering materials supports only one SPP wave for each propagation direction. Also, for a unique propagation direction in each quadrant of the interface plane, it was demonstrated that a new type of SPP wave--called a surface-plasmon-polariton-Voigt (SPP-V) wave--can exist. The fields of these SPP-V waves decay as the product of a linear and an exponential function of the distance from the interface in the anisotropic partnering material; in contrast, the fields of conventional SPP waves decay only exponentially with distance from the interface. Explicit analytic solutions of the dispersion relation for SPP-V waves exist and help establish constraints on the constitutive-parameter regimes for the partnering materials that support SPP-V-wave propagation.

We theoretically expanded the capabilities of optical sensing based on surface plasmon resonance in a prism-coupled configuration by incorporating artificial neural networks (ANNs). We used calculations modeling the situation in which an index-matched substrate with a metal thin film and a porous chiral sculptured thin film (CSTF) deposited successively on it is affixed to the base of a triangular prism. When a fluid is brought in contact with the exposed face of the CSTF, the latter is infiltrated. As a result of infiltration, the traversal of light entering one slanted face of the prism and exiting the other slanted face of the prism is affected. We trained two ANNs with differing structures using reflectance data generated from simulations to predict the refractive index of the infiltrant fluid. The best predictions were a result of training the ANN with simpler structure. With realistic simulated-noise, the performance of this ANN is robust.

A design tool was formulated for optimizing the efficiency of inorganic, thin-film, photovoltaic solar cells. The solar cell can have multiple semiconductor layers in addition to antireflection coatings, passivation layers, and buffer layers. The solar cell is backed by a metallic grating which is periodic along a fixed direction. The rigorous coupled-wave approach is used to calculate the electron-hole-pair generation rate. The hybridizable discontinuous Galerkin method is used to solve the drift-diffusion equations that govern charge-carrier transport in the semiconductor layers. The chief output is the solar-cell efficiency which is maximized using the differential evolution algorithm to determine the optimal dimensions and bandgaps of the semiconductor layers.

Two interpretations of the Bruggeman approach for the homogenization of isotropic chiral mixtures are shown to lead to different results. Whereas the standard interpretation is shown to yield the average-polarizability-density approach, a recent interpretation turns out to deliver a null-excess-polarization approach. The difference between the two interpretations arises from differing treatments of the local field

A numerical investigation was undertaken to elucidate the propagation of electromagnetic surface waves guided by the planar interface of two temperature-sensitive materials. One partnering material was chosen to be isotropic and the other to be anisotropic. Both partnering materials were engineered composite materials, based on the temperature-sensitive semiconductor InSb. At low temperatures the anisotropic partnering material is a non-hyperbolic uniaxial material; as the temperature is raised this material becomes a hyperbolic uniaxial material. At low temperatures, a solitary Dyakonov wave propagates along any specific direction in a range of directions parallel to the planar interface. At high temperatures, up to three different surface waves can propagate in certain directions parallel to the planar interface; one of these surface waves propagates with negative phase velocity (NPV). At a fixed temperature, the range of directions for NPV propagation decreases uniformly in extent as the volume fraction of InSb in the isotropic partnering material decreases. At a fixed volume fraction of InSb in the isotropic partnering material, the angular range for NPV propagation varies substantially as the temperature varies.

We study the convergence properties of the two-dimensional Rigorous Coupled Wave Approach (RCWA) for s-polarized monochromatic incident light. The RCWA is widely used to solve electromagnetic boundary-value problems where the relative permittivity varies periodically in one direction, i.e., scattering by a grating. This semi-analytical approach expands all the electromagnetic field phasors as well as the relative permittivity as Fourier series in the spatial variable along the direction of periodicity, and also replaces the relative permittivity with a stairstep approximation along the direction normal to the direction of periodicity. Thus, there is error due to Fourier truncation and also due to the approximation of grating permittivity. We prove that the RCWA is a Galerkin scheme, which allows us to employ techniques borrowed from the Finite Element Method to analyze the error. An essential tool is a Rellich identity that shows that certain continuous problems have unique solutions that depend continuously on the data with a continuity constant having explicit dependence on the relative permittivity. We prove that the RCWA converges with an increasing number of retained Fourier modes and with a finer approximation of the grating interfaces. Numerical results show that our convergence results for increasing the number of retained Fourier modes are seen in practice, while our estimates of convergence in slice thickness are pessimistic.

Degradation of metal-insulator-semiconductor (MIS) capacitors of gold/Parylene-C/Pentacene under constant voltage stress (CVS) was investigated to explore the electrical stability and reliability of Parylene C as a gate dielectric in flexible electronics. A stress voltage of fixed magnitude as high as 20 V, both negative and positive in polarity, was applied to each MIS capacitor at room temperature for a fixed duration as long as 10 s. The CVS effects on the capacitance-voltage curve-shift, the time-dependent leakage current, and the time-dependent dielectric breakdown were measured and analyzed. CVS is observed to induce charge in Parylene-C and its interfaces with gold and Pentacene. The net induced charge is positive and negative for, respectively, negative and positive gate bias polarity during CVS. The magnitude of the charge accumulated following positive gate CVS is significantly higher than that following negative gate bias CVS in the range of 4 to 25 nC cm$^{-2}$. In contrast, the leakage current during the negative gate stress is three orders of magnitude higher than that during the positive gate stress for the same bias stress magnitude. The charge buildup and leakage current are due to the trapping of electrons and holes near the Parylene-C/Pentacene interface as well as in the Parylene-C layer. Before the application of the CVS, a dielectric breakdown occurs at an electric field of 1.62 MV cm$^{-1}$. After the application of the CVS, the breakdown voltage decreases and the density of the trapped charges increases as the stress voltage increases in magnitude, with the polarity of the trapped charges opposite to that of the stress voltage. The magnitude and direction of the capacitance-voltage curve-shift depend on the trapping and recombination of electrons and holes in the Parylene-C layer and in the proximity of the Parylene-C/Pentacene interface during CVS., Comment: 11 pages, 4 figures

The so-called electromagnetic ghost waves are simply electromagnetic nonuniform plane waves, whose association with both propagating and evanescent fields has long been known, even for isotropic dielectric materials that are non-dissipative.

Absorption of terahertz waves by a metasurface comprising a biperiodic array of pixellated meta-atoms on top of a dielectric substrate backed by a perfect electric conductor was simulated using a commercial software, with either all or a few of the pixels in every meta-atom patched with graphene. Absorptances as high as 0.99 over a 1.16-THz-wide spectral regime for normal incidence were found for a metasurface comprising meta-atoms with only a few pixels patched with graphene, regardless of the polarization state. In comparison to metasurface absorbers comprising two graphene layers separated by an insulator mounted on a metal-backed substrate, graphene need was thereby reduced by two thirds. The number and dimensions of the pixels in a meta-atom can be altered to fit spectral requirements., Comment: 14 pages

The extended boundary condition method can be used to study planewave scattering by an ellipsoid composed of an orthorhombic dielectric-magnetic material whose relative permittivity dyadic is a scalar multiple of its relative permeability dyadic. The scattered and internal field phasors can be expanded in terms of appropriate vector spherical wavefunctions with unknown expansion coefficients, whereas the incident field phasors can be similarly expanded but with known expansion coefficients. The scattered-field coefficients are related to the incident-field coefficients through a matrix. The scattering, absorption, and extinction efficiencies were calculated thereby in relation to the propagation direction and the polarization state of the incident plane wave, the constitutive-anisotropy parameters, and the nonsphericity parameters of the ellipsoid, when the eigenvectors of the real permittivity dyadic are aligned along the three semi-axes of the ellipsoid. As the electrical size of the ellipsoid increases, multiple lobes appear in the scattering pattern. The total scattering efficiency can be smaller than the absorption efficiency for some configurations of the incident plane wave but not necessarily for others. The nonsphericity of the object has a stronger influence on the total scattering efficiency than on the absorption efficiency. The forward-scattering efficiency increases monotonically with the electrical size for all configurations of the incident plane wave, and so does the backscattering efficiency for some configurations. For other configurations, the backscattering efficiency has an undulating behavior with increase in electrical size, and is highly affected by the shape and the constitutive anisotropy of the ellipsoid.

The simple Lorentz-nonreciprocal medium described by the constitutive relations ${\bf D}=\epsilon_o\epsilon_r{\bf E]-{\bf\Gamma}\times{\bf H}$ and ${\bf B}=\mu_o\mu_r{\bf H]-{\bf\Gamma}\times{\bf E}$ is inspired by a specific spacetime metric, $\bf\Gamma$ being the magnetoelectric-gyrotopy vector. Field representations in this medium can be obtained from those for the isotropic dielectric-magnetic medium. When a plane wave is incident on a half space occupied by the Lorentz-nonreciprocal medium with magnetoelectric gyrotopy, theory shows that the transverse component of the magnetoelectric-gyrotopy vector is responsible for a rotation about the normal axis; furthermore, left/right reflection asymmetry is exhibited. Additionally, left/right transmission asymmetry is exhibited by a planar slab composed of the Lorentz-nonreciprocal medium with magnetoelectric gyrotopy. The left/right asymmetries are of interest for one-way devices.

The simplest Lorentz-nonreciprocal medium has the constitutive relations (${\bf D} =\epso {\bf E} -{\bf \Gamma}\times {\bf H}$ and ${\bf B} =\muo {\bf H} + {\bf \Gamma}\times{\bf E}$). Scattering by a three-dimensional object composed of this medium was investigated using the extended boundary condition method. Scattering by this object in free space must be attributed to non-zero $\Gamma=\vert\bf \Gamma\vert$. The differential scattering efficiency is immune to the transformation of the incident toroidal electric field phasor into a poloidal electric field phasor, or \textit{vice versa}, and a consequence of this source-invariance is the polarization-state invariance of the differential scattering efficiency when the irradiating field is a plane wave. Both the total scattering and forward-scattering efficiencies of an ellipsoid composed of the simplest Lorentz-nonreciprocal medium are maximum when the plane wave is incident in a direction {coparallel (but not antiparallel)} to $\bf \Gamma$, and the backscattering efficiency is minimum when $\bf \Gamma$ is {parallel} to the incidence direction. The total scattering and the forward-scattering efficiencies are maximum when the incidence direction is parallel to the largest semi-axis of the ellipsoid if the incidence direction is coparallel (but not antiparallel) to $\bf \Gamma$. Lorentz nonreciprocity in an object is thus intimately connected to the shape of that object in affecting the scattered field., Comment: 22 pages, 8 figures