Claudio I. Valencia

On the transmission of diffuse light through thick slits

We study the transmission of diffuse light through thick slits. For perfectly conducting slits and in-plane s-polarized illumination, the transmittance curves present a staircaselike behavior as a function of the aperture width, where the steps mark the appearance of new propagation modes. In contrast, with p-polarized illumination the transmittance increases linearly with the aperture width, with only some perturbations in the positions that correspond to the appearance of new modes. Out-of-plane incidence and more realistic assumptions about the slit, such as finite conductivity and roughness, are also discussed.

Anomalous reflection in a metallic plate with subwavelength grooves of circular cross section

Resonant features in the response of finite arrays of rectangular grooves ruled on a metallic plate have been reported in connection with the excitation of phase resonances. These anomalies are generated by a particular arrangement of the magnetic field phases inside the subwavelength grooves when the structure is illuminated by a p-polarized electromagnetic wave. We show that this kind of resonance is also present for grooves of circular cross section and appear as sharp peaks in the specular response, the number of which increases with the number of grooves in the structure. A significant intensification of the field within the grooves is also found for these particular phase configurations. The dependence of the response on the geometrical parameters of the structure is analyzed in detail, in order to consider these structures for potential applications such as frequency selectors and polarizers.

Nonlinear electromagnetic response of corrugated metallic gratings

We describe a theoretical formalism to study the second-harmonic generation in periodically corrugated surfaces illuminated by a plane wave. The incident wave vector is contained in the plane perpendicular to the grating grooves. Our analysis is based on the most general expression for the nonlinear polarization of a homogeneous and isotropic medium. The diffraction problem is solved using a Rayleigh method, and the numerical technique is illustrated by examples for which the nonlinear susceptibilities are calculated with a free-electron model.

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, for both 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.

Phase resonances induced by a subwavelength particle near a surface with two cavities

It is well known that finite groove gratings with subwavelength features exhibit phase resonances, which are associated with a particular distribution of the magnetic field phase within the cavities and are characterized by a significant enhancement of the internal field. For a flat surface with identical grooves under symmetrical conditions of incidence, it was shown that a minimum of three cavities is required to excite a phase resonance. In this paper we show that by approaching a particle to the surface, this requirement is removed and the particle enables the excitation of phase resonances even in a system of two identical cavities under normal incidence. The influence of the position and the radius of the particle in the reflected far field response, as well as in the near and internal field, is analyzed. The possibility of exciting phase resonances in this system opens up new means for the design of sensing devices.

Influence of finite conductivity on the excitation of phase resonances in metallic surfaces with cavities of circular cross sections

Phase resonances have been investigated in the last few years, not only because of their striking features, such as extremely high quality factor and huge enhancement of the electromagnetic field inside cavities/grooves, but also for their promising applications. However, taking into account that these resonances are more efficiently excited in highly conducting structures, most of the studies have been devoted to explore this phenomenon at wavelengths in the infrared or larger, using different approaches for the boundary conditions. In this paper, we investigate the validity of the perfect conductor approximation and the surface impedance boundary condition to appropriately represent the electromagnetic response of a metallic surface comprising a finite number of subwavelength cavities of circular cross sections. Far- and near-field plots are shown and analyzed in order to investigate the validity ranges and discuss to what extent phase resonances can be excited at shorter wavelengths in these structures.

Coherent effects in the sum-frequency generation at randomly rough surfaces

We study theoretically the nonlinear generation and scattering of sum-frequency electromagnetic radiation at randomly rough metallic surfaces. The approach permits the calculation of the scattered sum-frequency field for surfaces whose profiles are invariant in one direction, but are otherwise quite arbitrary, and we assume a fairly general form for the nonlinear polarization. The surfaces studied have a relatively large roughness scale and high slopes, which leads to substantial amounts of multiple scattering. We find that the mean angular distribution of the scattered sum-frequency light displays a well-defined minimum in a direction that depends on the angles of incidence of the excitation fields and their frequencies. The observed features are due to destructive interference between waves that have been multiply scattered in the valleys of the surface.