arXiv subjects
Publications and source records attributed to Akhlesh Lakhtakia.
Experimentation with obliquely incident light established that all four circular reflectances of a chiral sculptured thin film backed by a metallic mirror contain strong evidence of the circular Bragg phenomenon. When the mirror is removed, strong evidence of that phenomenon is found only in the spectrum of the co-polarized and co-handed reflectance.
Anisotropic homogeneous metamaterials that are neither wholly dissipative nor wholly active at a specific frequency are permitted by classical electromagnetic theory. Well-established homogenization formalisms indicate that such a metamaterial may be realized quite simply as a random mixture of electrically small (possibly nanoscale) spheroidal particles of at least two different isotropic dielectric materials, one of which must be dissipative but the other active. The dielectric properties of this metamaterial are influenced by the volume fraction, spatial distribution, particle shape and size, and the relative permittivities of the component materials. Similar metamaterials with more complicated linear as well as nonlinear constitutive properties are possible. Dynamic control of the active component material, for example via stimulated Raman scattering, affords dynamic control of the metamaterial.
Multiple surface plasmon-polariton (SPP) waves excited at the interface of a homogeneous isotropic metal and a chiral sculptured thin film (STF) impregnated with silver nanoparticles were theoretically assessed for the multiple-SPP-waves-based sensing of a fluid uniformly infiltrating the chiral STF. The Bruggemann homogenization formalism was used in two different modalities to determine the three principal relative permittivity scalars of the silver-nanoparticle-impregnated chiral STF infiltrated uniformly by the fluid. The dynamic sensitivity increased when silver nanoparticles were present, provided their volume fraction did not exceed about 1%.
Multiple compound surface plasmon-polariton (SPP) waves can be guided by a structure consisting of a sufficiently thick layer of metal sandwiched between a homogeneous isotropic dielectric (HID) material and a dielectric structurally chiral material (SCM). The compound SPP waves are strongly bound to both metal/dielectric interfaces when the thickness of the metal layer is comparable to the skin depth but just to one of the two interfaces when the thickness is much larger. The compound SPP waves differ in phase speed, attenuation rate, and field profile, even though all are excitable at the same frequency. Some compound SPP waves are not greatly affected by the choice of the direction of propagation in the transverse plane but others are, depending on metal thickness. For fixed metal thickness, the number of compound SPP waves depends on the relative permittivity of the HID material, which can be useful for sensing applications.
After formulating the frequency-domain Maxwell equations for a homogeneous, linear, bianisotropic material occupying a bounded region, we found that the axionic piece vanishes from both the differential equations valid in the region and the boundary conditions, thereby vindicating the Post constraint. Our analysis indicates that characteristic effects that may be observed experimentally with magnetoelectric materials are not the consequences of the axionic piece but of an admittance that describes surface states.
If a dissipative anisotropic dielectric material, characterized by the permittivity matrix $\underline{\underlineε}$, supports Voigt-wave propagation, then so too does the analogous active material characterized by the permittivity matrix $\underline{\underline{\tildeε}}$, where $\underline{\underline{\tildeε}}$ is the hermitian conjugate of $\underline{\underlineε}$. Consequently, a dissipative material that supports Voigt-wave propagation can give rise to a material that supports the propagation of Voigt waves with attendant linear gain in amplitude with propagation distance, by infiltration with an active dye.
Multiple p- and s-polarized compound surface plasmon-polariton (SPP) waves at a fixed frequency can be guided by a structure consisting of a metal layer sandwiched between a homogeneous isotropic dielectric (HID) material and a periodic multilayered isotropic dielectric (PMLID) material. For any thickness of the metal layer, at least one compound SPP wave must exist. It possesses the p-polarization state, is strongly bound to the metal/HID interface when the metal thickness is large but to both metal/dielectric interfaces when the metal thickness is small. When the metal layer vanishes, this compound SPP wave transmutes into a Tamm wave. Additional compound SPP waves exist, depending on the thickness of the metal layer, the relative permittivity of the HID material, and the period and the composition of the PMLID material. Some of these are p polarized, the others being s polarized. All of them differ in phase speed, attenuation rate, and field profile, even though all are excitable at the same frequency. The multiplicity and the dependence of the number of compound SPP waves on the relative permittivity of the HID material when the metal layer is thin could be useful for optical sensing applications.
The homogenization of a composite material comprising three isotropic dielectric materials was investigated. The component materials were randomly distributed as spherical particles, with the particles of two of the component materials being coupled to form dimers. The Bruggeman and Maxwell Garnett formalisms were developed to estimate the permittivity dyadic of the homogenized composite material (HCM), under the quasi-electrostatic approximation. Both randomly oriented and identically oriented dimers were accommodated; in the former case the HCM is isotropic, whereas in the latter case the HCM is uniaxial. Representative numerical results for composite materials containing dielectric--dielectric dimers demonstrate close agreement between the estimates delivered by the Bruggeman and Maxwell Garnett formalisms. For composite materials containing metal--dielectric dimers with moderate degrees of dissipation, the estimates of the two formalisms are in broad agreement, provided that the dimer volume fractions are relatively low. In general, the effects of intradimer coupling on the estimates of the HCM permittivity are relatively modest but not insignificant, these effects being exacerbated by anisotropy when all dimers are identically oriented.
The methodologies of bioinspiration, biomimetics, and bioreplication are inevitably pointing to the incorporation of multifunctionality in engineered materials when designing ever more complex systems. Optimal multifunctionality is also the defining characteristic of metamaterials. As fibrous materials are commonly manufactured from a variety of source materials, mimumes---i.e., microfibrous multifunctional metamaterials---are industrially viable even today, as exemplified by mimumes of parylene C. The microfibrous morphology of mimumes will enhance surface-dominated effects in comparison to those evinced by bulk materials.
The morphology of a columnar thin film (CTF) of silver renders it an effectively biaxially anisotropic continuum. CTFs of silver deposited on one-dimensional gratings of photoresist showed strong blazing action and asymmetrically coupled optical radiation to surface plasmon-polariton (SPP) waves propagating only along one direction supported by either the CTF/photoresist or the CTF/air interfaces. Homogenization of the CTFs using the Bruggeman formalism revealed them to display hyperbolic dispersion, and the dispersion of SPP waves was adequately described thereby.
We considered the experimental realization of a Tamm medium that is optically equivalent to the collision of two linearly polarized gravitational plane waves as a piecewise homogeneous metamaterial. Our formulation was based on the homogenization of remarkably simple arrangements of oriented ellipsoidal nanoparticles of isotropic dielectric-magnetic mediums. The inverse Bruggeman homogenization was used to estimate the constitutive parameters, volume fractions, and shape parameters for the component mediums. The presented formulation is appropriate for the regions of spacetime where the two gravitational plane waves interact, excluding the immediate vicinity of the nonsingular Killing-Cauchy horizon at the focusing point of the two plane waves.
The excitation of multiple SPP waves as Floquet harmonics was demonstrated in structures fabricated as one-dimensional photonic crystals (PCs) on top of two-dimensional gold gratings. Each period of the PC comprised nine layers of silicon oxynitrides of different compositions, and each PC had either two or three periods. Absorptances for obliquely incident $p$- and $s$-polarized light were measured in the 500--1000-nm wavelength regime and the sharp bands in the absorptance spectra were compared with the solutions of the underlying canonical boundary-value problem. The excitation of multiple surface-plasmon-polariton (SPP) waves as Floquet harmonics was confirmed. The structures demonstrated broadband absorption with overall weak dependences on the incidence angle and the polarization state of the incident light, and has potential application for harvesting solar energy.
The Uller-Zenneck wave has been theoretically predicted to exist at the planar interface of two homogeneous dielectric materials of which only one must be dissipative. Experimental confirmation of this century-old prediction was obtained experimentally by exciting the Uller-Zenneck wave as a Floquet harmonic of non-zero order at the periodically corrugated interface of air and crystalline silicon in the 400-to-900-nm spectral regime. Application for intra-chip optical interconnects at ~850 nm appears promising.
The excitation of the Uller-Zenneck surface wave in the optical regime was theoretically investigated for planar as well as periodically corrugated interfaces of two homogeneous, isotropic dielectric materials, with only one of the two being dissipative. A practical configuration involving the planewave illumination of a planar interface of the two partnering materials was found to be unsuitable for experimental confirmation of the existence of this surface wave. But, when the interface was periodically corrugated, the Uller-Zenneck wave was found to be excited over a wide range of the angle of incidence. Air and crystalline silicon were identified as suitable partnering materials for experiments in the visible and ultraviolet spectral regimes.
An exact transition matrix was formulated for electromagnetic scattering by a vacuum-like sphere with magnetoelectric gyrotropy. Both the total scattering and forward scattering efficiencies are lower when the magnetoelectric gyrotropy vector of the sphere is co/anti-parallel to the electric field or magnetic field of an incident plane wave than when the magnetoelectric gyrotropy vector is coparallel to the propagation vector of the incident plane wave. Backscattering is absent when the propagation vector is co/anti-paralel to the magnetoelectric gyrotropy vector.
Developments that took place in the area of sculptured--thin--film technology after {\em Bianisotropics '98\/} are reviewed here. Devices addressed include: optical filters of various kinds, laser mirrors, gas--concentration sensors, optical interconnects, interlayer dielectrics, and biosensors. Pulse bleeding is related to the circular Bragg phenomenon in chiral sculptured thin films. Acoustic research is also identified.
The planar interface of an isotropic homogeneous metal and an ambichiral dielectric material can guide surface-plasmon-polariton waves. The planar interface of an isotropic, homogeneous dielectric material and an ambichiral dielectric material can guide Dyakonov-Tamm waves. In either instance, we found that, as the ambichiral partnering material evolves into a finely chiral material, the solutions of the dispersion equation for surface-wave propagation evince convergence. The convergence is faster for the surface waves with larger phase speeds than for the surface waves with smaller phase speeds.
The planar interface of an ordinary, isotropic, homogeneous, dielectric material and a hyperbolic, dielectric, structurally chiral material can support the propagation of one or multiple Dyakonov-Tamm waves, at a specified frequency and along a specified direction in the interface plane. When multiple Dyakonov-Tamm waves can exist, they differ in phase speed, propagation length, degree of localization to the interface, and spatial profiles of the associated electromagnetic fields. Dependence on the relative permittivity scalar of the isotropic partnering material suggests exploitation for optical sensing of analytes.