Muhammad Faryad

The circular Bragg phenomenon

Exhibited by structurally chiral materials—such as Reusch piles, cholesteric liquid crystals (CLCs), and chiral sculptured thin films (STFs)—due to their helical nonhomogeneity along a fixed axis, the circular Bragg phenomenon is the almost total reflection of the incident light of the co-handed circular-polarization state but very little reflection of the incident light of the cross-handed circular-polarization state. Manifesting itself in spectral regimes that depend on the angle of incidence, the structural period, and the relative permittivity dyadic, the phenomenon amounts to the formation of a light pipe that bleeds energy backward under appropriate conditions. Mild dissipation and dispersion do not significantly affect the circular Bragg regime. Every structurally chiral material of sufficient thickness is essentially a circular-polarization-sensitive band-rejection filter. Cascades of these materials with or without structural defects can be used to satisfy complex filtering requirements, such as multiband, narrowband, and ultra-narrowband filtering. A shift in the circular Bragg regime due to infiltration of a chiral STF by a fluid enables optical sensing. Sources of circularly polarized light can be fabricated by embedding emission sources in CLCs and chiral STFs.

Broadband light absorption with multiple surface plasmon polariton waves excited at the interface of a metallic grating and photonic crystal.

Light incident upon a periodically corrugated metal/dielectric interface can generate surface plasmon polariton (SPP) waves. This effect is used in many sensing applications. Similar metallodielectric nanostructures are used for light trapping in solar cells, but the gains are modest because SPP waves can be excited only at specific angles and with one linear polarization state of incident light. Here we report the optical absorptance of a metallic grating coupled to silicon oxide/oxynitride layers with a periodically varying refractive index, i.e., a 1D photonic crystal. These structures show a dramatic enhancement relative to those employing a homogeneous dielectric material. Multiple SPP waves can be activated, and both s- and p-polarized incident light can be efficiently trapped. Many SPP modes are weakly bound and display field enhancements that extend throughout the dielectric layers. These modes have significantly longer propagation lengths than the single SPP modes excited at the interface of a metallic grating and a uniform dielectric. These results suggest that metallic gratings coupled to photonic crystals could have utility for light trapping in photovoltaics, sensing, and other applications.

FRACTIONAL RECTANGULAR WAVEGUIDE

Fractional curl operator is utilized to construct the solutions corresponding to fractional dual rectangular waveguides. Fractional dual rectangular waveguides may be considered as intermediate of two given waveguides, where both waveguides are related through principle of duality. Characteristic impedance of fractional waveguide is determined. Behavior of field lines in transverse plane is also investigated.

Paraboloidal Reflector in Chiral Medium Supporting Simultaneously Positive Phase Velocity and Negative Phase Velocity

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HIGH FREQUENCY EXPRESSIONS FOR THE FIELD IN THE CAUSTIC REGION OF A CYLINDRICAL REFLECTOR PLACED IN CHIRAL MEDIUM

High frequency field expressions for a two dimensional reflector are derived. The reflector is placed in a homogenous and reciprocal chiral medium. Since geometrical optics fails at caustics so Maslovs method has been used to find the field expressions which are also valid around caustics. Examples of parabolic and circular reflectors have been considered.

Grating-coupled excitation of multiple surface plasmon-polariton waves

The excitation of multiple surface plasmon-polariton (SPP) waves of different linear polarization states and phase speeds by a surface-relief grating formed by a metal and a rugate filter, both of finite thickness, was studied theoretically, using rigorous coupled-wave analysis. The incident plane wave can be either p or s polarized. The excitation of SPP waves is indicated by the presence of those peaks in the plots of absorptance vs the incidence angle that are independent of the thickness of the rugate filter. The absorptance peaks representing the excitation of s-polarized SPP waves are narrower than those representing p-polarized SPP waves. Two incident plane waves propagating in different directions may excite the same SPP wave. A line source could excite several SPP waves simultaneously.

Focal Region Field of a Paraboloidal Reflector Coated with Isotropic Chiral Medium

Maslovs method is used to derive the expressions for high frequency flelds around the focal region of a paraboloidal re∞ector coated with isotropic and homogeneous chiral medium. The fleld expressions thus calculated are solved numerically, and the results are presented in the paper. Moreover, the dependency of the electric fleld on the thickness of the coated chiral medium and its properties is also studied. The results of this study are presented in the paper, and the conclusions are drawn accordingly.

Observation of the Dyakonov-Tamm wave.

A surface electromagnetic wave called the Dyakonov-Tamm wave has been theoretically predicted to exist at the interface of two dielectric materials at least one of which is both anisotropic and periodically nonhomogeneous. For experimental confirmation, a prism-coupled configuration was used to excite Dyakonov-Tamm waves guided by the interface of a dense thin film of magnesium fluoride and a chiral sculptured thin film of zinc selenide. The excitation was indicated by a reflection dip (with respect to the angle of incidence in the prism-coupled configuration) that is independent of the polarization state of the incident light as well as the thicknesses of both partnering materials beyond some thresholds. Applications to optical sensing and long-range on-chip communication are expected.

Excitation of multiple surface-plasmon-polariton waves guided by the periodically corrugated interface of a metal and a periodic multilayered isotropic dielectric material

The excitation of multiple surface-plasmon-polariton (SPP) waves guided by the periodically corrugated interface of a homogeneous metal and a periodic multilayered isotropic dielectric (PMLID) material was studied theoretically. The solution of the underlying canonical boundary-value problem (with a planar interface) indicates that multiple SPP waves of different polarization states, phase speeds, and attenuation rates can be guided by the periodically corrugated interface. Accordingly, the boundary-value problem was formulated using rigorous coupled-wave analysis and solved using a numerically stable algorithm. A linearly polarized plane wave was considered obliquely incident on a PMLID material of finite thickness and backed by a metallic surface-relief grating. The total reflectance, total transmittance, and the absorptance were calculated as functions of the incidence angle for different numbers of unit cells in the PMLID material of fixed period. The excitation of SPP waves was indicated by those peaks in the absorptance curves that were independent of the number of unit cells, and these peaks were also correlated with the solutions of a dispersion equation obtained from the canonical boundary-value problem.

Optimization of the absorption efficiency of an amorphous-silicon thin-film tandem solar cell backed by a metallic surface-relief grating.

The rigorous coupled-wave approach was used to compute the plane-wave absorptance of a thin-film tandem solar cell with a metallic surface-relief grating as its back reflector. The absorptance is a function of the angle of incidence and the polarization state of incident light; the free-space wavelength; and the period, duty cycle, the corrugation height, and the shape of the unit cell of the surface-relief grating. The solar cell was assumed to be made of hydrogenated amorphous-silicon alloys and the back reflector of bulk aluminum. The incidence and the grating planes were taken to be identical. The AM1.5 solar irradiance spectrum was used for computations in the 400-1100 nm wavelength range. Inspection of parametric plots of the solar-spectrum-integrated (SSI) absorption efficiency and numerical optimization using the differential evolution algorithm were employed to determine the optimal surface-relief grating. For direct insolation, the SSI absorption efficiency is maximizable by appropriate choices of the period, the duty cycle, and the corrugation height, regardless of the shape of the corrugation in each unit cell of the grating. A similar conclusion also holds for diffuse insolation, but the maximum efficiency for diffuse insolation is about 20% smaller than for direct insolation. Although a tin-doped indium-oxide layer at the front and an aluminum-doped zinc-oxide layer between the semiconductor material and the backing metallic layer change the optimal depth of the periodic corrugations, the optimal period of the corrugations does not significantly change.