arXiv subjects

Ricardo A. Depine

Publications and source records attributed to Ricardo A. Depine.

14 recordsLinked to original sources

arXiv2022

Terahertz lasing conditions of radiative and nonradiative propagating plasmon modes in graphene-coated cylinders

There is increasing interest in filling the gap of miniaturized terahertz/mid-infrared radiation sources and, particularly, in incorporating these sources into micro/nanophotonic circuits. By using rigorous electromagnetic methods, we investigate the lasing conditions and the electric-tunability of radiative and non radiative propagating surface-plasmon modes in cylinders made of active materials coated with a graphene layer. A detailed analysis of the lasing condition of different surface-plasmon modes shows that there is an abrupt change in the gain required when modes become nonradiative. Although radiative modes, subject to both radiation and ohmic losses, are expected to require more gain compensation than nonradiative modes, we find that, counterintuitively, gain compensation is greater for nonradiative modes. This is explained in terms of a change in the distribution of fields that occurs when the character of modes switches from plasmonic to photonic. Finally, we assess the feasibility of our proposal by using a realist gain medium and showing that a relatively low population inversion is required for the stimulated emission of the studied system.

physics.optics↗

arXiv2021

Dispersion properties of plasmonic sub-wavelength elliptical wires wrapped with graphene

One fundamental motivation to know the dispersive, or frequency dependent characteristics of localized surface plasmons (LSPs) supported by elliptical shaped particles wrapped with graphene sheet, as well as their scattering characteristics when these elliptical LSPs are excited, is related with the design of plasmonic structures capable to manipulate light at sub-wavelength scale. The anisotropy imposed by the ellipse eccentricity can be used as a geometrical tool for controlling plasmonic resonances. Unlike metallic case, where the multipolar eigenmodes are independent of each other, we find that the induced current on graphene boundary couples multipolar eigenmodes with the same parity. In the long wavelength limit, a recursive relation equation for LSPs in term of the ellipse eccentricity parameter is derived and explicit solutions at lowest order are presented. In this approximation, we obtain analytical expressions for both the anisotropic polarizability tensor elements and the scattered power when LSPs are excited by plane wave incidence.

physics.optics↗

arXiv2017

Green formulation for studying electromagnetic scattering from graphene-coated wires of arbitrary section

We present a rigorous electromagnetic method based on Green's second identity for studying the plasmonic response of graphene-coated wires of arbitrary shape. The wire is illuminated perpendicular to its axis by a monochromatic electromagnetic wave and the wire substrate is homogeneous and isotropic. The field is expressed everywhere in terms of two unknown source functions evaluated on the graphene coating which can be obtained from the numerical solution of a coupled pair of inhomogeneous integral equations. To assess the validity of the Green formulation, the scattering and absorption efficiencies obtained numerically in the particular case of circular wires are compared with those obtained from the multipolar Mie theory. An excellent agreement is observed in this particular case, both for metallic and dielectric substrates. To explore the effects that the break of the rotational symmetry of the wire section introduces in the plasmonic features of the scattering and absorption response, the Green formulation is applied to the case of graphene-coated wires of elliptical section. As might be expected from symmetry arguments, we find a two-dimensional anisotropy in the angular optical response of the wire, particularly evident in the frequency splitting of multipolar plasmonic resonances. The comparison between the spectral position of the enhancements in the scattering and absorption efficiency spectra for low-eccentricity elliptical and circular wires allows us to guess the multipolar order of each plasmonic resonance. We present calculations of the near field distribution for different frequencies which explicitly reveal the multipolar order of the plasmonic resonances. They also confirm the previous guess and serve as a further test on the validity of the Green formulation.

physics.optics↗

arXiv2015

Tunable plasmonic enhancement of light scattering and absorption in graphene-coated subwavelength wires

The electromagnetic response of subwavelength wires coated with a graphene monolayer illuminated by a linearly polarized plane waves is investigated. The results show that the scattering and extintion cross-sections of the coated wire can be dramatically enhanced when the incident radiation resonantly excites localized surface plasmons. The enhancements occur for p--polarized incident waves and for excitation frequencies that correspond to complex poles in the coefficients of the multipole expansion for the scattered field. By dynamically tuning the chemical potential of graphene, the spectral position of the enhancements can be chosen over a wide range.

physics.optics↗

arXiv2015

Critical coupling layer thickness for positive or negative Goos-Hänchen shifts near the excitation of backward surface polaritons in Otto-ATR systems

A theoretical analysis of the lateral displacement (Goos-Hänchen shift) of spatially limited beams reflected from Attenuated Total Reflection (ATR) devices in the Otto configuration is presented when backward surface plasmon polaritons are excited at the interface between a positive refractive index slab and a semiinfinite metamaterial with negative refractive index. First, the stationary phase approximation and a phenomenological model based on the properties of the complex poles and zeroes of the reflection coefficient are used to demonstrate that: i) the excitation of backward surface waves can lead to both negative and positive (and not exclusively negative) Goos-Hänchen shifts, and ii) the sign of the shift depends on whether the value of the coupling layer thickness is higher or lower than a critical value characteristic of the ATR structure. Second, these findings are verified through rigorous calculations of the spatial structure of the reflected beam. For incident beams with a Gaussian profile, the lateral shift calculated as the first moment of the field distribution of the reflected beam agrees quite well with the predictions of approximate analysis. Near the resonant excitation of the backward surface plasmon polariton, large (negative or positive) Goos-Hänchen shifts are obtained, along with a splitting of the reflected beam.

physics.optics↗

arXiv2011

Influence of the sign of the refractive index in the reflectivity of a metamaterial surface with localized roughness

To study the scattering properties of metamaterials, we generalize two scattering methods developed for conventional (non-magnetic) isotropic materials to the case of materials with arbitrary values (positive or negative) of magnetic permeability and electric permittivity. The generalized methods are used to study the changes in the reflectivity of a metamaterial surface with localized roughness when the relative refractive index changes sign. Our results show that, unlike the case of a plane surface whose reflectivity is unaffected by the change of sign of the relative refractive index, in rough surfaces the change of sign is manifested in the reflectivity, even for very low roughness, particularly in observation directions away from the specular direction.

physics.optics↗

arXiv2008

Optical response of a self-standing monolayer of dielectric spheres

An analysis of the optical response of periodically arrayed monolayers composed of dielectric spheres with low refractive index is herein presented. The reflectance spectra of two-dimensional square and triangular lattices are obtained by means of the vector Korringa-Kohn-Rostoker method, both spectra showing very similar qualitative features for photon energies below the onset of diffraction spots. In this energy region, the same number of peaks of unitary amplitude in the reflectance spectra are predicted for both kinds of monolayers, suggesting that this must be a universal feature independent of the particular geometry of the lattice. The origin of these high reflectance peaks is investigated. It is found that the resonances of TM and TE modes due to dipolar, quadrupolar and octupolar interaction inside the monolayer are largely responsible for the peak structure observed in the reflectance spectra.

physics.optics↗

arXiv2007

On metallic gratings coated conformally with isotropic negative-phase-velocity materials

Application of the differential method (also called the C method) to plane-wave diffraction by a perfectly conducting, sinusoidally corrugated metallic grating coated with a linear, homogeneous, isotropic, lossless dielectric-magnetic material shows that coating materials with negative index of refraction may deliver enhanced maximum nonspecular reflection efficiencies in comparison to coating materials with positive index of refraction.

physics.optics↗

arXiv2006

Zero permeability and zero permittivity band gaps in 1D metamaterial photonic crystals

We consider layered heterostructures combining ordinary positive index materials and dispersive metamaterials. We show that these structures can exhibit a new type of photonic gap around frequencies where either the magnetic permeability μor the electric permittivity εof the metamaterial is zero. Although the interface of a semi-infinite medium with zero refractive index (a condition attained either when μ= 0 or when ε= 0) is known to give full reflectivity for all incident polarizations, here we show that a gap corresponding to μ= 0 occurs only for TE polarized waves, whereas a gap corresponding to ε= 0 occurs only for TM polarized waves. These band gaps are scale-length invariant and very robust against disorder, although they may disappear for the particular case of propagation along the stratification direction.

physics.optics↗

arXiv2005

Classification of dispersion equations for homogeneous dielectric-magnetic uniaxial materials

The geometric representation at a fixed frequency of the wavevector (or dispersion) surface $ω(\vec k)$ for lossless, homogeneous dielectric--magnetic uniaxial materials is explored, when the elements of the relative permittivity and permeability tensors of the material can have any sign. Electromagnetic plane waves propagating inside the material can exhibit dispersion surfaces in the form of ellipsoids of revolution, hyperboloids of one sheet, or hyperboloids of two sheets. Furthermore, depending on the relative orientation of the optic axis, the intersections of these surfaces with fixed planes of propagation can be circles, ellipses, hyperbolas, or straight lines. The obtained understanding is used to study the reflection and refraction of electromagnetic plane waves due to a planar interface with an isotropic medium.

physics.optics↗

arXiv2005

Phase resonances in obliquely illuminated compound gratings

The existence of phase resonances in obliquely illuminated, perfectly conducting compound gratings is investigated. The diffraction problem of a p-polarized plane wave impinging on the structure is solved using the modal approach. The results show that even under oblique illumination, where no symmetry is imposed by the incident field, there are resonant wavelengths that are clearly associated with a certain degree of symmetry in the phase distribution of the magnetic field inside the cavities. New configurations of this phase distribution take place, that were not allowed under normal incidence conditions. It was found that the interior field is intensified in the resonances, and the specularly reflected efficiency is maximized. In particular, this efficiency is optimized for Littrow mount.

physics.optics↗

arXiv2005

Uniaxial dielectric mediums with hyperbolic dispersion relations

The dispersion relations for conventional uniaxial dielectric mediums may be characterized as elliptical or elliptical-like, according to whether the medium is nondissipative or dissipative, respectively. However, under certain constitutive parameter regimes, the dispersion relations may be hyperbolic or hyperbolic-like. We investigate planewave propagation in a hyperbolic/hyperbolic-like uniaxial dielectric medium. For both evanescent and nonevanescent propagation, the phase velocity is found to be positive with respect to the time-averaged Poynting vector. A conceptualization of a hyperbolic--like uniaxial medium as a homogenized composite medium is presented.

physics.optics↗

arXiv2004

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.

physics.optics↗

arXiv2004

Diffraction gratings of isotropic negative phase-velocity materials

Diffraction of electromagnetic plane waves by the gratings made by periodically corrugating the exposed planar boundaries of homogeneous, isotropic, linear dielectric--magnetic half--spaces is examined. The phase velocity vector in the diffracting material can be either co-parallel or anti-parallel to the time-averaged Poynting vector, thereby allowing for the material to be classified as of either the positive or the negative negative phase-velocity (PPV or NPV) type. Three methods used for analyzing dielectric gratings - the Rayleigh-hypothesis method, a perturbative approach, and the C formalism - are extended here to encompass NPV gratings by a careful consideration of field representation inside the refracting half--space. Corrugations of both symmetric as well as asymmetric shapes are studied, as also the diversity of grating response to the linear polarization states of the incident plane wave. The replacement of PPV grating by its NPV analog affects only nonspecular diffraction efficiencies when the corrugations are shallow, and the effect on specular diffraction efficiencies intensifies as the corrugations deepen. Whether the type of the refracting material is NPV or PPV is shown to affect surface wave propagation as well as resonant excitation of surface waves.

physics.optics↗