A new condition to identify isotropic dielectric-magnetic materials displaying negative phase velocity

The derivation of a new condition for characterizing isotropic dielectric-magnetic materials exhibiting negative phase velocity, and the equivalence of that condition with previously derived conditions, are presented.Comment: 4 page

Polarization--universal rejection filtering by ambichiral structures made of indefinite dielectric--magnetic materials

An ambichiral structure comprising sheets of an anisotropic dielectric material rejects normally incident plane waves of one circular polarization (CP) state but not of the other CP state, in its fundamental Bragg regime. However, if the same structure is made of an dielectric--magnetic material with indefinite permittivity and permeability dyadics, it may function as a polarization--universal rejection filter because two of the four planewave components of the electromagnetic field phasors in each sheet are of the positive--phase--velocity type and two are of the negative--phase--velocity type.Comment: Cleaned citations in the tex

Thin-Film Metamaterials called Sculptured Thin Films

Morphology and performance are conjointed attributes of metamaterials, of which sculptured thin films (STFs) are examples. STFs are assemblies of nanowires that can be fabricated from many different materials, typically via physical vapor deposition onto rotating substrates. The curvilinear--nanowire morphology of STFs is determined by the substrate motions during fabrication. The optical properties, especially, can be tailored by varying the morphology of STFs. In many cases prototype devices have been fabricated for various optical, thermal, chemical, and biological applications.Comment: to be published in Proc. ICTP School on Metamaterials (Augsut 2009, Sibiu, Romania

Blending of nanoscale and microscale in uniform large-area sculptured thin-film architectures

The combination of large thickness ($>3$ $\mu$m), large--area uniformity (75 mm diameter), high growth rate (up to 0.4 $\mu$m/min) in assemblies of complex--shaped nanowires on lithographically defined patterns has been achieved for the first time. The nanoscale and the microscale have thus been blended together in sculptured thin films with transverse architectures. SiO$_x$ ($x\approx 2$) nanowires were grown by electron--beam evaporation onto silicon substrates both with and without photoresist lines (1--D arrays) and checkerboard (2--D arrays) patterns. Atomic self--shadowing due to oblique--angle deposition enables the nanowires to grow continuously, to change direction abruptly, and to maintain constant cross--sectional diameter. The selective growth of nanowire assemblies on the top surfaces of both 1--D and 2--D arrays can be understood and predicted using simple geometrical shadowing equations.Comment: 17 pages, 9 figure

Electromagnetic waves with negative phase velocity in Schwarzschild-de Sitter spacetime

The propagation of electromagnetic plane waves with negative phase velocity (NPV) is considered in Schwarzschild-(anti-)de Sitter spacetime. It is demonstrated that NPV propagation occurs in Schwarzschild-de Sitter spacetime at lower values of the cosmological constant than is the case for de Sitter spacetime. Furthermore, we report that neither is NPV propagation observed in Schwarzschild-anti-de Sitter spacetime, nor is it possible outside the event horizon of a Schwarzschild blackhole.Comment: Typographical error in eq (34) of EPL version is corrected in arxiv version. Europhysics Letters (accepted for publication

Depolarization regions of nonzero volume in bianisotropic homogenized composites

In conventional approaches to the homogenization of random particulate composites, the component phase particles are often treated mathematically as vanishingly small, point-like entities. The electromagnetic responses of these component phase particles are provided by depolarization dyadics which derive from the singularity of the corresponding dyadic Green functions. Through neglecting the spatial extent of the depolarization region, important information may be lost, particularly relating to coherent scattering losses. We present an extension to the strong-property-fluctuation theory in which depolarization regions of nonzero volume and ellipsoidal geometry are accommodated. Therein, both the size and spatial distribution of the component phase particles are taken into account. The analysis is developed within the most general linear setting of bianisotropic homogenized composite mediums (HCMs). Numerical studies of the constitutive parameters are presented for representative examples of HCM; both Lorentz-reciprocal and Lorentz-nonreciprocal HCMs are considered. These studies reveal that estimates of the HCM constitutive parameters in relation to volume fraction, particle eccentricity, particle orientation and correlation length are all significantly influenced by the size of the component phase particles

Theory of Dyakonov-Tamm waves at the planar interface of a sculptured nematic thin film and an isotropic dielectric material

In order to ascertain conditions for surface-wave propagation guided by the planar interface of an isotropic dielectric material and a sculptured nematic thin film (SNTF) with periodic nonhomogeneity, we formulated a boundary-value problem, obtained a dispersion equation therefrom, and numerically solved it. The surface waves obtained are Dyakonov-Tamm waves. The angular domain formed by the directions of propagation of the Dyakonov--Tamm waves can be very wide (even as wide as to allow propagation in every direction in the interface plane), because of the periodic nonhomogeneity of the SNTF. A search for Dyakonov-Tamm waves is, at the present time, the most promising route to take for experimental verification of surface-wave propagation guided by the interface of two dielectric materials, at least one of which is anisotropic. That would also assist in realizing the potential of such surface waves for optical sensing of various types of analytes infiltrating one or both of the two dielectric materials.Comment: accepted for publication in J. Opt.

Enhanced diffraction by a rectangular grating made of a negative phase--velocity (or negative index) material

The diffraction of electromagnetic plane waves by a rectangular grating formed by discrete steps in the interface of a homogeneous, isotropic, linear, negative phase--velocity (negative index) material with free space is studied using the semi--analytic C method. When a nonspecular diffracted order is of the propagating type, coupling to that order is significantly larger for a negative index material than for conventional material. The computed coupling strengths reported here are in agreement with recent experiments, and illustrate the role of evanescent fields localized at the grating interface in producing this enhanced coupling.Comment: 12 pages, 4 figure

Quantification of optical pulsed-plane-wave-shaping by chiral sculptured thin films

The durations and average speeds of ultrashort optical pulses transmitted through chiral sculptured thin films (STFs) were calculated using a finite-difference time-domain algorithm. Chiral STFs are a class of nanoengineered materials whose microstructure comprises parallel helicoidal nanowires grown normal to a substrate. The nanowires are $\sim$10-300 nm in diameter and $\sim1-10 \mu$m in length. Durations of transmitted pulses tend to increase with decreasing (free-space) wavelength of the carrier plane wave, while average speeds tend to increase with increasing wavelength. An increase in nonlinearity, as manifested by an intensity-dependent refractive index in the frequency domain, tends to increase durations of transmitted pulses and decrease average speeds. The circular Bragg phenomenon exhibited by a chiral STFs manifests itself in the frequency domain as high reflectivity for normally incident carrier plane waves whose circular polarization state is matched to the structural handedness of the film and whose wavelength falls in a range known as the Bragg regime; films of the opposite structural handedness reflect such plane waves little. This effect tends to distort the shapes of transmitted pulses with respect to the incident pulses, and such shaping can cause sharp changes in some measures of average speed with respect to carrier wavelength. A local maximum in the variation of one measure of the pulse duration with respect to wavelength is noted and attributed to the circular Bragg phenomenon. Several of these effects are explained via frequency-domain arguments. The presented results serve as a foundation for future theoretical and experimental studies of optical pulse propagation through causal, nonlinear, nonhomogeneous, and anisotropic materials.Comment: To appear in Journal of Modern Optic