arXiv subjects

Akhlesh Lakhtakia

Publications and source records attributed to Akhlesh Lakhtakia.

At least 73 records · Page 4Linked to original sources

arXiv2018

Optimization of nonhomogeneous indium-gallium-nitride Schottky-barrier thin-film solar cells

A two-dimensional model was developed to simulate the optoelectronic characteristics of indium-gallium-nitride (InGaN), thin-film, Schottky-barrier solar cells. The solar cells comprise a window, designed to reduce the reflection of incident light, Schottky-barrier and ohmic front electrodes, an n-doped InGaN wafer, and a metallic periodically corrugated back-reflector (PCBR). The ratio of indium to gallium in the wafer varies periodically throughout the thickness of the absorbing layer of the solar cell. Thus, the resulting InGaN wafer's optical and electrical properties are made to vary periodically. This material nonhomogeneity could be physically achieved by varying the fractional composition of indium and gallium during deposition. Empirical models for indium nitride and gallium nitride were combined using Vegard's law to determine the optical and electrical constitutive properties of the alloy. The nonhomogeneity of the electrical properties of the InGaN aids in the separation of the excited electron-hole pairs, while the periodicities of optical properties and the back-reflector enable the incident light to couple to multiple guided wave modes. The profile of the resulting charge-carrier-generation rate when the solar cell is illuminated by the AM1.5G spectrum was calculated using the rigorous coupled-wave approach. The steady-state drift-diffusion equations were solved using COMSOL, which employs finite-volume methods, to calculate the current density as a function of the voltage. Mid-band Shockley-Read-Hall, Auger, and radiative recombination rates were taken to be the dominant methods of recombination. The model was used to study the effects of the solar-cell geometry and the shape of the periodic material nonhomogeneity on efficiency. The solar-cell efficiency was optimized using the differential evolution algorithm.

physics.app-ph↗

arXiv2018

Optimization approach for optical absorption in three-dimensional structures including solar cells

The rigorous coupled-wave approach (RCWA) and the differential evolution algorithm (DEA) were coupled in a practicable approach to maximize absorption in optical structures with three-dimensional morphology. As a model problem, optimal values of four geometric parameters and the bandgaps of three i-layers were found for an amorphous-silicon, multi-terminal, thin-film tandem solar cell comprising three p-i-n junctions with a metallic hexagonally corrugated back-reflector. When the optical short-circuit current density was chosen as the figure of merit to be maximized, only the bandgap of the topmost i-layer was significant and the remaining six parameters played minor roles. While this configuration would absorb light very well, it would have poor electrical performance. This is because the optimization problem allows for the thicknesses and bandgaps of the semiconductor layers to change. We therefore devised another figure of merit that takes into account bandgap changes by estimating the open-circuit voltage. The resulting configuration was found to be optimal with respect to all seven variable parameters. The RCWA+DEA optimization approach is applicable to other types of photovoltaic solar cells as well as optical absorbers, with the choice of the figure of merit being vital to a successful outcome.

physics.app-ph↗

arXiv2018

Chiral Sculptured Thin Films for Circular Polarization of Mid-Wavelength Infrared Light

Being an assembly of identical upright helixes, a chiral sculptured thin film (CSTF) exhibits the circular Bragg phenomenon and can therefore be used as a circular- polarization filter in a spectral regime called the circular Bragg regime. This has been already demonstrated in the near-infrared and short-wavelength infrared regimes. If two CSTFs are fabricated in identical conditions to differ only in the helical pitch, and if both are made of a material whose bulk refractive index is constant in a wide enough spectral regime, then the center wavelengths of the circular Bragg regimes of the two CSTFs must be in the same ratio as their helical pitches by virtue of the scale invariance of the frequency-domain Maxwell postulates. This theoretical result was confirmed by measuring the linear-transmittance spectrums of two zinc-selenide CSTFs with helical pitches in the ratio 1:7:97. The center wavelengths were found to be in the ratio 1:7:1, the deviation from the ratio of helical pitches being explainable at least in part be- cause the bulk refractive index of zinc selenide decreased a little with wavelength. We concluded that CSTFs can be fabricated to function as circular-polarization filters in the mid-wavelength infrared regime.

physics.optics↗

arXiv2018

Simultaneous existence of amplified and attenuated Dyakonov surface waves

The propagation of Dyakonov surface waves guided by the planar interface of (i) an isotropic dielectric material and (ii) a homogenized uniaxial dielectric composite material comprising both passive and active component materials was theoretically investigated, under the assumption that the optic axis of the uniaxial partnering material lies wholly in the interface plane. For a certain range of the volume fraction of the active component material, the uniaxial partnering material is neither wholly dissipative nor wholly active and the Dyakonov surface waves propagating in certain directions amplify but the Dyakonov surface waves propagating in other directions attenuate.

physics.optics↗

arXiv2018

Experimental and theoretical investigation of the co-occurrence of linear and circular dichroisms for oblique incidence of light on chiral sculptured thin films

Theory shows that a slab of a dielectric structurally chiral material (DSCM) exhibits both linear and circular dichroisms because of its anisotropy and structural chirality, for normal as well as oblique incidence. This conclusion was confirmed by fabricating a chiral sculptured thin film and measuring the spectra of its reflectances and transmittances, both linear and circular. Signatures of the circular Bragg phenomenon are evident in the spectra of all reflectances, transmittances, absorptances, and dichroisms. Reversal of the structural handedness of a DSCM and rotation of the projection of the direction of propagation of the incident light clockwise instead of counterclockwise about the axis of helicoidal nonhomogeneity simultaneously changes the sign of circular dichroism but has no effect on linear dichroism.

physics.optics↗

arXiv2017

Electromagnetic Pulse Scattering by a Spacecraft Nearing Light Speed

Humans will launch spacecraft that travel at an appreciable fraction of the speed of light. Spacecraft traffic will be tracked by radar. Scattering of pulsed electromagnetic fields by an object in uniform translational motion at relativistic speed may be computed using the frame-hopping technique. Pulse scattering depends strongly on the velocity, shape, orientation, and composition of the object. The peak magnitude of the backscattered signal varies by many orders of magnitude depending on whether the object is advancing toward or receding away from the source of the interrogating signal. The peak magnitude of the backscattered signal goes to zero as the object recedes from the observer at a velocity very closely approaching light speed, rendering the object invisible to the observer. The energy scattered by an object in motion may increase or decrease relative to the energy scattered by the same object at rest. Both the magnitude and sign of the change depend on the velocity of the object, as well as on its shape, orientation, and composition. In some cases, the change in total scattered energy is greatest when the object is moving transversely to the propagation direction of the interrogating signal, even though the Doppler effect is strongest when the motion is parallel or antiparallel to the propagation direction.

physics.optics↗

arXiv2017

Bicontrollable Terahertz Metasurface with Subwavelength Scattering Elements of Two Different Materials

Transmission of a normally incident plane wave through a metasurface with bicontrollable subwavelength scattering elements was simulated using a commercial software. Some pixels comprising the $H-shaped scattering elements were made of a magnetostatically controllable material whereas the remaining pixels were made of a thermally controllable material, the metasurface designed to operate in the terahertz spectral regime. The co-polarized transmission coefficients were found to exhibit stopbands that shift when either a magnetostatic field is applied or the temperature is increased or both. Depending on spectral location of the stopband, either the magnetostatic field gives coarse control and temperature gives fine control or vice versa. The level of magnetostatic control depends on the magnetostatic-field configuration.

physics.optics↗

arXiv2017

Polarization-state-dependent attenuation and amplification in a columnar thin film

We numerically investigated the plane-wave reflection-transmission characteristics of a columnar thin film (CTF) whose columns are made from a dissipative material but whose void regions are filled with an active material. By computing the reflectances and transmittances, we found that the CTF can simultaneously amplify $s$-polarized incident light and attenuate $p$-polarized incident light, or vice versa. This polarization-state-dependent attenuation and amplification phenomenon depends upon the angle of incidence and the thickness of the CTF.

physics.optics↗

arXiv2017

Scattering characteristics of relativistically moving concentrically layered spheres

The energy extinction cross section of a concentrically layered sphere varies with velocity as the Doppler shift moves the spectral content of the incident signal in the sphere's co-moving inertial reference frame toward or away from resonances of the sphere. Computations for hollow gold nanospheres show that the energy extinction cross section is high when the Doppler shift moves the incident signal's spectral content in the co-moving frame near the wavelength of the sphere's localized surface plasmon resonance. The energy extinction cross section of a three-layer sphere consisting of an olivine-silicate core surrounded by a porous and a magnetite layer, which is used to explain extinction caused by interstellar dust, also depends strongly on velocity. For this sphere, computations show that the energy extinction cross section is high when the Doppler shift moves the spectral content of the incident signal near either of olivine-silicate's two localized surface phonon resonances at 9.7 $μ$m and 18 $μ$m.

physics.optics↗

arXiv2017

Lorentz Invariance of Absorption and Extinction Cross Sections of a Uniformly Moving Object

The energy absorption and energy extinction cross sections of an object in uniform translational motion in free space are Lorentz invariant, but the total energy scattering cross section is not. Indeed, the forward-scattering theorem holds true for co-moving observers but not for other inertial observers. If a pulsed plane wave with finite energy density is incident upon an object, the energies scattered, absorbed, and removed from the incident signal by the object are finite. The difference between the energy extinction cross section and the sum of the total energy scattering and energy absorption cross sections for a non-co-moving inertial observer can be either negative or positive, depending on the object's velocity, shape, size, and composition. Calculations for a uniformly translating, solid, homogeneous sphere show that all three cross sections go to zero as the sphere recedes directly from the source of the incident signal at speeds approaching c, whether the material is a plasmonic metal (e.g., silver) or simply a dissipative dielectric material (e.g., silicon carbide).

physics.optics↗

arXiv2017

High-phase-speed Dyakonov-Tamm surface waves

Two numerical studies, one based on a canonical boundary-value problem and the other based on a reflection-transmission problem, revealed that the propagation of high-phase-speed Dyakonov-Tamm (HPSDT) surface waves may be supported by the planar interface of a chiral sculptured thin film and a dissipative isotropic dielectric material. Furthermore, many distinct HPSDT surface waves may propagate in a specific direction, each with a phase speed exceeding the phase speed of plane waves that can propagate in the isotropic partnering material. Indeed, for the particular numerical example considered, the phase speed of an HPSDT surface wave could exceed the phase speed in bulk isotropic dielectric material by a factor of as much as 10.

physics.optics↗

arXiv2017

Signatures of thermal hysteresis in Tamm-wave propagation

We numerically solved the boundary-value problem for Tamm waves (which may also be classified as Uller-Zenneck waves here) guided by the planar interface of a homogeneous isotropic dissipative dielectric (HIDD) material and a periodically multilayered isotropic dielectric material. The HIDD material was chosen to be VO${}_2$ which, at optical wavelengths, has a temperature-dependent refractive index with a hysteresis feature, i.e., the temperature-dependence of its refractive index varies depending upon whether the temperature is increasing or decreasing. A numerical code was implemented to extract solutions of the dispersion equation at a fixed wavelength for both $p$- and $s$-polarization states over the temperature range [50,80] degrees. A multitude of Tamm waves of both linear polarization states were found, demonstrating a clear demarcation of the heating and cooling phases in terms of wavenumbers and propagation distances. Thereby, the signatures of thermal hysteresis in Tamm-wave propagation were revealed.

physics.optics↗

arXiv2017

Temperature-mediated transition from Dyakonov-Tamm surface waves to surface-plasmon-polariton waves

The effect of changing the temperature on the propagation of electromagnetic surface waves (ESWs), guided by the planar interface of a homogeneous isotropic temperature-sensitive material (namely, InSb) and a temperature-insensitive structurally chiral material (SCM) was numerically investigated in the terahertz frequency regime. As the temperature rises, InSb transforms from a dissipative dielectric material to a dissipative plasmonic material. Correspondingly, the ESWs transmute from Dyakonov-Tamm surface waves into surface-plasmon-polariton waves. The effects of the temperature change are clearly observed in the phase speeds, propagation distances, angular existence domains, multiplicity, and spatial profiles of energy flow of the ESWs. Remarkably large propagation distances can be achieved; in such instances the energy of an ESW is confined almost entirely within the SCM. For certain propagation directions, simultaneous excitation of two ESWs with (i) the same phase speeds but different propagation distances or (ii) the same propagation distances but different phase speeds are also indicated by our results.

physics.optics↗

arXiv2017

Non-exhibition of Bragg phenomenon by chevronic sculptured thin films: experiment and theory

The unit cell of a chevronic sculptured thin film (ChevSTF) comprises two identical columnar thin films (CTFs) except that the nanocolumns of the first are oriented at an angle χand nanocolumns of the second are oriented at an angle π-χwith respect to the interface of the two CTFs. A ChevSTF containing 10 unit cells was fabricated using resistive-heating physical vapor deposition of zinc selenide. Planewave reflectance and transmittance spectrums of this ChevSTF were measured for a wide variety of incidence conditions over the 500--900-nm range of the free-space wavelength. Despite its structural periodicity, the ChevSTF did not exhibit the Bragg phenomenon. Theoretical calculations with the CTFs modeled as biaxial dielectric materials indicated that the Bragg phenomenon would not be manifested for normal and near-normal incidence, but vestigial manifestation was possible for suffciently oblique incidence. Thus, structural periodicity does not always lead to electromagnetic periodicity that underlies the exhibition of the Bragg phenomenon.

physics.optics↗

arXiv2017

Parylene-C microfibrous thin films as phononic crystals

Phononic bandgaps of Parylene-C microfibrous thin films (muFTFs) were computationally determined by treating them as phononic crystals comprising identical microfibers arranged either on a square or a hexagonal lattice. The microfibers could be columnar,chevronic, or helical in shape, and the host medium could be either water or air. All bandgaps were observed to lie in the 0.01-to-162.9-MHz regime, for microfibers of realistically chosen dimensions. The upper limit of the frequency of bandgaps was the highest for the columnar muFTF and the lowest for the chiral muFTF. More bandgaps exist when the host medium is water than air. Complete bandgaps were observed for the columnar muFTF with microfibers arranged on a hexagonal lattice in air, the chevronic muFTF with microfibers arranged on a square lattice in water, and the chiral muFTF with microfibers arranged on a hexagonal lattice in either air or water. The softness of the Parylene-C muFTFs makes them mechanically tunable, and their bandgaps can be exploited in multiband ultrasonic filters.

physics.class-ph↗

arXiv2016

Classical electromagnetic model of surface states in topological insulators

A topological insulator is classically modeled as an isotropic dielectric-magnetic with a magnetoelectric pseudoscalar $Ψ$ existing in its bulk while its surface is charge-free and current-free. An alternative model is obtained by setting $Ψ\equiv0$ and incorporating surface charge and current densities characterized by an admittance $γ$. Analysis of plane-wave reflection and refraction due to a topological-insulator half space reveals that the parameters $Ψ$ and $γ$ arise identically in the reflection and transmission coefficients, implying that the two classical models cannot be distinguished on the basis of any scattering scenario. However, as $Ψ$ disappears from the Maxwell equations applicable to any region occupied by the topological insulator, and because surface states exist on topological insulators as protected conducting states, the alternative model must be chosen.

physics.optics↗

arXiv2015

Simultaneous amplification and attenuation in isotropic chiral materials

The electromagnetic field phasors in an isotropic chiral material (ICM) are superpositions of two Beltrami fields of different handedness. Application of the Bruggeman homogenization formalism to two-component composite materials delivers ICMs wherein Beltrami fields of one handedness attenuate whereas Beltrami waves of the other handedness amplify. One component material is a dissipative ICM, the other an active dielectric material.

physics.optics↗

arXiv2015

Gain and loss enhancement in active and passive particulate composite materials

Two active dielectric materials may be blended together to realize a homogenized composite material (HCM) which exhibits more gain than either component material. Likewise, two dissipative dielectric materials may be blended together to realize an HCM which exhibits more loss than either component material. Sufficient conditions for such gain/loss enhancement were established using the Bruggeman homogenization formalism. Gain/loss enhancement arises when (i) the imaginary parts of the relative permittivities of both component materials are similar in magnitude and (ii) the real parts of the relative permittivities of both component materials are dissimilar in magnitude.

physics.optics↗