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Tom G. Mackay

Publications and source records attributed to Tom G. Mackay.

At least 55 records · Page 3Linked to original sources
arXiv2013

On the sensitivity of directions which support Voigt wave propagation in infiltrated biaxial dielectric materials

Voigt wave propagation (VWP) was considered in a porous biaxial dielectric material which was infiltrated with a material of refractive index $n_a$. The infiltrated material was regarded as a homogenized composite material in the long-wavelength regime and its constitutive parameters were estimated using the extended Bruggeman homogenization formalism. In our numerical studies, the directions which support VWP were found to vary by as much as $300^\circ$ per RIU as the refractive index $n_a$ was varied. The sensitivities achieved were acutely dependent upon the refractive index $n_a$ and the degrees of anisotropy and dissipation of the porous biaxial material. The orientations, shapes and sizes of the particles which constitute the infiltrating material and the porous biaxial material exerted only a secondary influence on the maximum sensitivities achieved. Also, for the parameter ranges considered, the degree of porosity of the biaxial material had little effect on the maximum sensitivities achieved. These numerical findings bode well for the possible harnessing of VWP for optical sensing applications.

physics.optics↗
arXiv2012

Vector spherical wavefunctions for orthorhombic dielectric-magnetic material with gyrotropic-like magnetoelectric properties

Vector spherical wavefunctions were derived in closed-form to represent time-harmonic electromagnetic fields in an orthorhombic dielectric-magnetic material with gyrotropic-like magnetoelectric properties. These wavefunctions were used to formulate the T matrix for scattering by a three-dimensional object composed of the chosen material. Furthermore, a closed-form, coordinate-free expression of the dyadic Green function for the chosen material was derived. Expressions ascertained for the singularity behavior will be useful for formulating volume integral equations for scattering inside the chosen material. A bilinear expansion of the dyadic Green function was obtained in terms of the derived vector spherical wavefunctions.

physics.optics↗
arXiv2012

On the sensitivity of generic porous optical sensors

A porous material was considered as a platform for optical sensing. It was envisaged that the porous material was infiltrated by a fluid which contains an agent to be sensed. Changes in the optical properties of the infiltrated porous material provide the basis for detection of the agent to be sensed. Using a homogenization approach based on the Bruggeman formalism, wherein the infiltrated porous material was regarded as a homogenized composite material, the sensitivity of such a sensor was investigated. For the case of an isotropic dielectric porous material of relative permittivity $ε^a$ and an isotropic dielectric fluid of relative permittivity $ε^b$, it was found that the sensitivity was maximized when there was a large contrast between $ε^a$ and $ε^b$; the maximum sensitivity was achieved at mid-range values of porosity. Especially high sensitivities may be achieved for $ε^b$ close to unity when $ε^a >> 1$, for example. Furthermore, higher sensitivities may be achieved by incorporating pores which have elongated spheroidal shapes.

physics.optics↗
arXiv2012

Bruggeman formalism vs. `Bruggeman formalism': Particulate composite materials comprising oriented ellipsoidal particles

Two different formalisms for the homogenization of composite materials containing oriented ellipsoidal particles of isotropic dielectric materials are being named after Bruggeman. Numerical studies reveal clear differences between the two formalisms which may be exacerbated: (i) if the component particles become more aspherical, (ii) at mid-range values of the volume fractions, and (iii) if the homogenized component material is dissipative. The correct Bruggeman formalism uses the correct polarizability density dyadics of the component particles, but the other formalism does not.

physics.optics↗
arXiv2011

On columnar thin films as platforms for surface-plasmonic-polaritonic optical sensing: higher-order considerations

The ability to tailor the porosity and optical properties of columnar thin films (CTFs) renders them promising platforms for optical sensing. In particular, surface-plasmon-polariton (SPP) waves, guided by the planar interface of an infiltrated CTF and a thin layer of metal, may be harnessed to detect substances that penetrate the void regions in between the columns of a CTF. This scenario was investigated theoretically using a higher-order homogenization technique, based on an extended version of the second-order strong-permittivity-fluctuation theory, which takes into account the size of the component particles which make up the infiltrated CTF and the statistical distribution of these particles. Our numerical studies revealed that as the size of the component particles increases and as the correlation length that characterizes their distribution increases: (i) the phase speed of the SPP wave decreases and the SPP wave's attenuation increases; (ii) the SPP wave's penetration into the CTF decreases; (iii) the angle of incidence required to excite the SPP wave in a modified Kretschmann configuration increases; (iv) the sharpness of the SPP trough in the graph of reflectance versus angle of incidence increases; and (v) the sensitivity to changes in refractive index of the infiltrating fluid decreases.

physics.optics↗
arXiv2011

Towards an experimental realization of affinely transformed linearized QED vacuum via inverse homogenization

Within the framework of quantum electrodynamics (QED), vacuum is a nonlinear medium which can be linearized for a rapidly time-varying electromagnetic field with a small amplitude subjected to a magnetostatic field. The linearized QED vacuum is a uniaxial dielectric-magnetic medium for which the degree of anisotropy is exceedingly small. By implementing an affine transformation of the spatial coordinates, the degree of anisotropy may become sufficiently large as to be readily perceivable. The inverse Bruggeman formalism can be implemented to specify a homogenized composite material (HCM) which is electromagnetically equivalent to the affinely transformed QED vacuum. This HCM can arise from remarkably simple component materials; for example, two isotropic dielectric materials and two isotropic magnetic materials, randomly distributed as oriented spheroidal particles.

physics.optics↗
arXiv2011

A simple construction for a cylindrical cloak via inverse homogenization

An effective cylindrical cloak may be conceptualized as an assembly of adjacent local neighbourhoods, each of which is made from a homogenized composite material (HCM). The HCM is required to be a certain uniaxial dielectric-magnetic material, characterized by positive-definite constitutive dyadics. It can arise from the homogenization of remarkably simple component materials, such as two isotropic dielectric-magnetic materials, randomly distributed as oriented spheroidal particles. By carefully controlling the spheroidal shape of the component particles, a high degree of HCM anisotropy may be achieved, which is necessary for the cloaking effect to be realized. The inverse Bruggeman formalism can provide estimates of the shape and constitutive parameters for the component materials, as well as their volume fractions.

physics.optics↗
arXiv2011

Voigt waves in homogenized particulate composites based on isotropic dielectric components

Homogenized composite materials (HCMs) can support a singular form of optical propagation, known as Voigt wave propagation, while their component materials do not. This phenomenon was investigated for biaxial HCMs arising from nondissipative isotropic dielectric component materials. The biaxiality of these HCMs stems from the oriented spheroidal shapes of the particles which make up the component materials. An extended version of the Bruggeman homogenization formalism was used to investigate the influence of component particle orientation, shape and size, as well as volume fraction of the component materials, upon Voigt wave propagation. Our numerical studies revealed that the directions in which Voigt waves propagate is highly sensitive to the orientation of the component particles and to the volume fraction of the component materials, but less sensitive to the shape of the component particles and less sensitive still to the size of the component particles. Furthermore, whether or not such an HCM supports Voigt wave propagation at all is critically dependent upon the size of the component particles and, in certain cases, upon the volume fraction of the component materials.

physics.optics↗
arXiv2011

Towards a realization of Schwarzschild-(anti-)de Sitter spacetime as a particulate metamaterial

While metamaterials offer the potential to realize Tamm mediums which represent vacuous spacetime subjected to gravitational fields, practical formulations for suitable metamaterials have not hitherto been developed. This matter is addressed by establishing a metamaterial formulation for the Tamm medium representing Schwarzschild-(anti-)de Sitter spacetime. Our formulation is remarkably simple and does not involve a complex nanostructure of the type that is often associated with metamaterials. Instead it is based on the homogenization of only isotropic dielectric and isotropic magnetic component mediums, which are distributed randomly as oriented spheroidal particles. Using the inverse Bruggeman homogenization formalism, we demonstrated that a wide range of constitutive parameter values for the Tamm medium may be accessed through varying the particle shape, volume fraction or relative permittivity and relative permeability of the component mediums. The presented formulation is appropriate for the regions of spacetime which lie outside the event horizon for Schwarzschild spacetime and inside the event horizon for de Sitter spacetime; there are no such restrictions for anti-de Sitter spacetime.

physics.optics↗
arXiv2010

Ray trajectories for Alcubierre spacetime

The Alcubierre spacetime was simulated by means of a Tamm medium which is asymptotically identical to vacuum and has constitutive parameters which are ontinuous functions of the spatial coordinates. Accordingly, the Tamm medium is amenable to physical realization as a nanostructured metamaterial. A comprehensive characterization of ray trajectories in the Tamm medium was undertaken, within the geometric-optics regime. Propagation directions corresponding to evanescent waves were identified: these occur in the region of the Tamm medium which corresponds to the warp bubble of the Alcubierre spacetime, especially for directions perpendicular to the velocity of the warp bubble at high speeds of that bubble. Ray trajectories are acutely sensitive to the magnitude and direction of the warp bubble's velocity, but rather less sensitive to the thickness of the transition zone between the warp bubble and its background. In particular, for rays which travel in the same direction as the warp bubble, the latter acts as a focusing lens, most notably at high speeds.

physics.optics↗
arXiv2010

On chemiluminescent emission from an infiltrated chiral sculptured thin film

The theory describing the far-field emission from a dipole source embedded inside a chiral sculptured thin film (CSTF), based on a spectral Green function formalism, was further developed to allow for infiltration of the void regions of the CSTF by a fluid. In doing so, the extended Bruggeman homogenization formalism--which accommodates constituent particles that are small compared to wavelength but not vanishingly small--was used to estimate the relative permittivity parameters of the infiltrated CSTF. For a numerical example, we found that left circularly polarized (LCP) light was preferentially emitted through one face of the CSTF while right circularly polarized (RCP) light was preferentially emitted through the opposite face, at wavelengths within the Bragg regime. The centre wavelength for the preferential emission of LCP/RCP light was red shifted as the refractive index of the infiltrating fluid increased from unity, and this red shift was accentuated when the size of the constituent particles in our homogenization model was increased. Also, the bandwidth of the preferential LCP/RCP emission regime decreased as the refractive index of the infiltrating fluid increased from unity.

physics.optics↗
arXiv2010

A Plethora of Negative-Refraction Phenomenons in Relativistic and Non-Relativistic Scenarios

In accordance with Snel's law of refraction, whether a plane wave is refracted in the negative sense or positive sense at a planar boundary between two homogenous mediums is determined solely by the orientation of the real parts of the wavevectors involved. Thus, negative refraction should be distinguished from the associated but independent phenomenons of negative phase velocity, counterposition and negative deflection of energy flux. None of these phenomenons is Lorentz covariant.

physics.optics↗
arXiv2010

Ray trajectories for a spinning cosmic string and a manifestation of self-cloaking

A study of ray trajectories was undertaken for the Tamm medium which represents the spacetime of a cosmic spinning string, under the geometric-optics approximation. Our numerical studies revealed that: (i) rays never cross the string's boundary; (ii) the Tamm medium supports evanescent waves in regions of phase space that correspond to those regions of the string's spacetime which could support closed timelike curves; and (iii) a spinning string can be slightly visible while a non-spinning string is almost perfectly invisible.

physics.optics↗
arXiv2010

Surface-plasmon-polariton waves guided by the uniformly moving planar interface of a metal film and dielectric slab

We explored the effects of relative motion on the excitation of surface-plasmon-polariton (SPP) waves guided by the planar interface of a metal film and a dielectric slab, both materials being isotropic and homogeneous. Electromagnetic phasors in moving and non-moving reference frames were related directly using the corresponding Lorentz transformations. Our numerical studies revealed that, in the case of a uniformly moving dielectric slab, the angle of incidence for SPP-wave excitation is highly sensitive to (i) the ratio $β$ of the speed of motion to speed of light in free space and (ii) the direction of motion. When the direction of motion is parallel to the plane of incidence, the SPP wave is excited by $p$-polarized (but not $s$-polarized) incident plane waves for low and moderate values of $β$, while at higher values of $β$ the total reflection regime breaks down. When the direction of motion is perpendicular to the plane of incidence, the SPP wave is excited by $p$-polarized incident plane waves for low values of $β$, but $s$-polarized incident plane waves at moderate values of $β$, while at higher values of $β$ the SPP wave is not excited. In the case of a uniformly moving metal film, the sensitivity to $β$ and the direction of motion is less obvious.

physics.optics↗
arXiv2010

On limitations of the Bruggeman formalism for inverse homogenization

The Bruggeman formalism provides an estimate $\eps^{Br}_{hcm}$ of the relative permittivity of a homogenized composite material (HCM), arising from two component materials with relative permittivities $\eps_a$ and $\eps_b$. It can be inverted to provide an estimate of $\eps_a$, from a knowledge of $\eps^{Br}_{hcm}$ and $\eps_b$. Numerical studies show that the inverse Bruggeman estimate $\eps_a$ can be physically implausible when (i) $Re\{\eps^{Br}_{hcm}\}/Re\{\eps_b\} > 0 $ and the degree of HCM dissipation is moderate or greater; or (ii) $Re\{\eps^{Br}_{hcm}\}/Re\{\eps_b\} < 0 $ regardless of the degree of HCM dissipation. Furthermore, even when the inverse Bruggeman estimate is not obviously implausible, huge discrepancies can exist between this estimate and the corresponding estimate provided by the inverse Maxwell Garnett formalism.

physics.optics↗
arXiv2010

Modeling chiral sculptured thin films as platforms for surface-plasmonic-polaritonic optical sensing

Biomimetic nanoengineered metamaterials called chiral sculptured thin films (CSTFs) are attractive platforms for optical sensing because their porosity, morphology and optical properties can be tailored to order. Furthermore, their ability to support more than one surface-plasmon-polariton (SPP) wave at a planar interface with a metal offers functionality beyond that associated with conventional SPP--based sensors. An empirical model was constructed to describe SPP-wave propagation guided by the planar interface of a CSTF--infiltrated with a fluid which supposedly contains analytes to be detected--and a metal. The inverse Bruggeman homogenization formalism was first used to determine the nanoscale model parameters of the CSTF. These parameters then served as inputs to the forward Bruggeman homogenization formalism to determine the reference relative permittivity dyadic of the infiltrated CSTF. By solving the coresponding boundary-value problem for a modified Kretschmann configuration, the characteristics of the multiple SPP modes at the planar interface were investigated as functions of the refractive index of the fluid infiltrating the CSTF and the rise angle of the CSTF. The SPP sensitivities thereby revealed bode well for the implementation of fluid-infiltrated CSTFs as SPP-based optical sensors.

physics.optics↗
arXiv2009

Empirical model of optical sensing via spectral shift of circular Bragg phenomenon

Setting up an empirical model of optical sensing to exploit the circular Bragg phenomenon displayed by chiral sculptured thin films (CSTFs), we considered a CSTF with and without a central twist defect of $π/2$ radians. The circular Bragg phenomenon of the defect-free CSTF, and the spectral hole in the co-polarized reflectance spectrum of the CSTF with the twist defect, were both found to be acutely sensitive to the refractive index of a fluid which infiltrates the void regions of the CSTF. These findings bode well for the deployment of CSTFs as optical sensors.

physics.optics↗