Finite Difference Time Domain Method For Grating Structures

International audienceThe aim of this chapter is to present the principle of the FDTD method when applied to the resolution of Maxwell equations. Centered finite differences are used to approximate the value of both time and space derivatives that appear in these equations. The convergence criteria in addition to the boundary conditions (periodic or absorbing ones) are given. The special case of bi-periodic structures illuminated at oblique incidence is solved with the SFM (split field method) technique. In all cases, the dispersion of an eventual metallic media is taken into account through an analytical frequency dependence of the dielectric permittivity. Applications dealing with extraordinary optical transmission are presented and show new results allowing enhanced transmission by excitation of the TEM, a cutoff-less, guided mode of annular aperture arrays engraved into thick silver film

Enhanced transmission through subwavelength metallic coaxial apertures by excitation of the TEM mode

We present here the first theoretical demonstration of enhanced transmission (up to 90%) through annular aperture arrays engraved into opaque metallic plates thanks to the excitation of the TEM guided mode inside each coaxial cavity. The generation of this peculiar mode is obtained, first, by illuminating the structure under oblique incidence and, second, by considering a TM polarization. Analytical demonstration is performed to confirm the involvement of these two conditions for the emergence of this guided mode. The originality of this configuration comes, first, from the fact that the TEM mode has no cut-off wavelength and, second, from the fact that the transmission peak position is independent of the angle of incidence

Spatiotemporal sub-wavelength near-field light localization

International audienceThe control and localization of light at sub-wavelength scale are theoretically demonstrated with a very simple sub-wavelength dimension structure. This is demonstrated through a peculiar structure that can support localized modes which are not linked to any plasmon resonance. It is based on the acronym "FEMTO” that is designed using 26 sub-wavelength rectangular apertures engraved into perfectly conducting metal screen. A polarization-sensitive guided mode through these nano-apertures is at the origin of the light localization. Consequently, sub-wavelength light spots can be achieved with very simple structures illuminated by temporally shaped plane waves. Three parameters are temporally controlled for this purpose: the polarization, the wavelength and the amplitude of the incident beam. It is also demonstrated that replacing the perfect conductor by a real metal with dispersion leads to accentuate both the light confinement and its localization. These results open the path to the conception of optical nano-structures dedicated to sub-wavelength light addressing

Split-field FDTD method for oblique incidence study of periodic dispersive metallic structures

International audienceThe study of periodic structures illuminated by a normally incident plane wave is a simple task that can be numerically simulated by the finite-difference time-domain (FDTD) method. On the contrary, for off-normal incidence, a widely modified algorithm must be developed in order to bypass the frequency dependence appearing in the periodic boundary conditions. After recently implementing this FDTD algorithm for pure dielectric materials, we here extend it to the study of metallic structures where dispersion can be described by analytical models. The accuracy of our code is demonstrated through comparisons with already-published results in the case of 1D and 3D structures

Three-dimensional finite-difference time-domain algorithm for oblique incidence with adaptation of perfectly matched layers and nonuniform meshing: Application to the study of a radar dome

International audienceThe three-dimensional finite-difference time-domain 3D-FDTD method is developed and implemented in the case of oblique incidence in order to study biperiodic structures that are finished according to the third direction. The perfectly matched layer PML is adapted to the developed algorithm. The electromagnetic fields of Maxwell's equations in the main grid and in the PML media are transferred from the E-H domain to the mapped P-Q domain. The modified Maxwell's equations are implemented by the split-field method SFM. Several tests are made and presented in order to verify and demonstrate the accuracy of our codes. The obtained results are in good agreement with published ones obtained by other methods. The originality of this paper comes, first from the fact that it brings a complete development of the used algorithm, and second, from the study of the spectral response of a radar dome based on annular aperture arrays perforated into a perfect conductor plate

Superfocusing and Light Confinement by Surface Plasmon Excitation Through Radially Polarized Beam

International audienceWe theoretically demonstrate the possibility of obtaining nanosources through an original schema based on the generation of the radially polarized surface plasmon mode of a cylindrical metallic tip. This mode has no cutoff radius and can propagate along the tip walls until its nanometric-sized apex. Instead of radiating from the tip end, the guided mode will give rise to a nanospotlight via the well-known antenna effect. 3D calculations demonstrate that both surface plasmon-guided mode and antenna effect are directly involved in the light confinement. Near-field optical microscopy can benefit significantly from this kind of probe because the sample does not need to be directly illuminated