Diana C. Skigin

Extraordinary Transmission Through Arrays of Slits: A Circuit Theory Model

Extraordinary transmission and other interesting related phenomena for 1-D periodic arrays of slits (compound diffraction gratings) have recently been the object of intense research in the optics and solid state physics communities. This case should be differentiated from the extraordinary transmission through arrays of small apertures on metal screens since small holes only support below-cutoff modes, whereas slits can also support transverse electromagnetic modes without cutoff frequency. In this paper, an equivalent-circuit approach is proposed to account for the most relevant details of the behavior of slit-based periodic structures: extraordinary transmission peaks, Fabry-Perot resonances, and transmission dips observed in compound structures. The proposed equivalent-circuit model, based on well-established concepts of waveguide and circuit theory, provides a simple and accurate description of the phenomenon that is appropriate for educational purposes, as well as for the design of potential devices based on the behavior of the structures under study.

Scattering from metallic surfaces having a finite number of rectangular grooves

A modal theory is presented for solving the problem of electromagnetic scattering from a surface consisting of a finite number of one-dimensional rectangular grooves in a metallic plane. The incident plane wave can be polarized with either its electric or its magnetic field along the grooves. The formalism is applicable to perfectly conducting materials and to real metals with high (but finite) conductivity. Particular attention is paid to the changes appearing in the scattering pattern when the conductivity of the structure is changed from an infinite value (perfect conductor) to a finite value (highly conducting metal). The excitation of surface waves when the incident wave is p polarized is illustrated in some numerical examples that demonstrate the differences between the spectral amplitudes corresponding to s and p polarizations.

Experimental demonstration of phase resonances in metallic compound gratings with subwavelength slits in the millimeter wave regime

We provide experimental evidence of phase resonances in metallic periodic structures in which each period comprises several subwavelength slits of the same width. We have analyzed and measured the response of these structures in the millimeter wave regime and show that phase resonances are characterized by a remarkable minimum in the transmission response, as predicted by numerical calculations. We compare experimental with numerical results, obtaining a very good agreement between them. This experimental confirmation encourages research in compound structures and their multiple potential applications, such as frequency selective surfaces.

Superdirective radiation from finite gratings of rectangular grooves

In this paper, the superdirective property of arrays comprising a finite number of rectangular grooves is studied by using the modal approach, which is a simple but powerful technique for analyzing these gratings. Numerical results show that when a specific characteristic mode of the structure referred to as the /spl pi/ mode is excited, the intensity of the field scattered in the specular direction exhibits a maximum, which becomes sharper and narrower as the number of grooves is increased. The far-field patterns exhibit superdirectivity at these resonant frequencies as evidenced by their beamwidths that are narrower than those expected from apertures of comparable size. The model-based parameter estimation (MBPE) technique has been employed to help locate extremely narrow resonances that are characteristic of superdirective arrays and its usefulness has been demonstrated.

Bandwidth control of forbidden transmission gaps in compound structures with subwavelength slits

Phase resonances in transmission compound structures with subwavelength slits produce sharp dips in the transmission response. For all equal slits, the wavelengths of these sharp transmission minima can be varied by changing the width or the length of all the slits. In this paper we show that the width of the dip, i.e., the frequency range of minimum transmittance, can be controlled by making at least one slit different from the rest within a compound unit cell. In particular, we investigate the effect that a change in the dielectric filling, or in the length of a single slit, produces in the transmission response. We also analyze the scan angle behavior of these structures by means of band diagrams and compare them with previous results for all-equal slit structures.

Diffraction by dual-period gratings

The dynamical characteristics of dual-period perfectly conducting gratings are explored. Gratings with several grooves (reflection) or slits (transmission) within each period are considered. A scalar approach is proposed to derive the general characteristics of the diffracted response. It was found that compound gratings can be designed to cancel as well as to intensify a given diffraction order. These preliminary estimations for finite gratings are validated by numerical examples for infinitely periodic reflection and transmission gratings with finite thickness, performed using an extension of the rigorous modal method to compound gratings, for both polarization cases.

Phase resonances in compound metallic gratings

We study the phase resonances that appear in infinite conducting gratings comprising a finite number of grooves in each period (compound gratings), when illuminated by a p-polarized plane wave. In particular, we investigate a surface that separates a lossy conductor from a dielectric isotropic medium. The resonances appear when a particular distribution of the phase of the electromagnetic field inside the cavities takes place, and are identified as peaks in the specularly reflected efficiency. These resonances are accompanied by an intensification of the internal field. The diffraction problem is solved by using the modal method. We use the surface impedance boundary condition, which has been proven to be reliable for metals with high conductivity, and simplifies the numerical treatment.

Optical extinction spectroscopy used to characterize metallic nanowires

We present a method for sizing metallic nanowires through the analysis of the extinction spectra of the scattered light when the wires are illuminated alternatively with p- and s-polarization waves. The method is applied to isolated silver nanowires in air or immersed in index matching oil. The dielectric function of silver is affected by the size of the cylinders, and its influence on the extinction spectra near the plasmon resonance or near the dip position is considered. Due to the size of the nanocylinders, it is necessary to include two different permittivities in the electromagnetic model to analyse the behaviour of the material under different polarization incidences. This introduces anisotropy in the system, which comprises isotropic cylinders. The behaviour of the extinction spectra for p-waves allows us to determine the wire radii, taking into account the plasmon peak position for radii larger than 7 nm, or alternatively, by using the contrast between maximum and minimum intensity near the plasmon frequency, for radii lower than 5 nm. For s-waves, although no plasmon peak appears, we can determine the radii by analysing the contrast between the ridge of the spectra near 260–275 nm and the minimum near 320–330 nm for radii larger than 10 nm, or analysing the slope in the spectra over 350 nm, for radii below 10 nm. The present study shows that spectral extinction is a very simple and inexpensive technique that can be useful for characterizing the radius of nanocylinders when electron microscopy (TEM or SEM) is not available.

The multilayer modal method for electromagnetic scattering from surfaces with several arbitrarily shaped grooves

The problem of electromagnetic scattering from a perfectly conducting corrugated surface having a finite number of arbitrarily shaped grooves is solved by means of the multilayer modal method. The R-matrix propagation algorithm is used to improve numerical stability for deep surfaces. Comparisons with the results obtained by the integral method are shown, among other numerical examples.

Coupling of evanescent s-polarized waves to the far field by waveguide modes in metallic arrays

In this paper we show that enhanced optical transmission through a 1D periodic slit array comprising real metallic cylinders is possible for s-polarization when the array is near a dielectric interface. We investigate the behaviour of this structure under s-polarized illumination, in which case surface plasmons are not excited. Numerical results show that the transmitted intensity appears as a periodic function of the slit depth, for propagating as well as for evanescent incidence, suggesting that this behaviour is related to the excitation of waveguide modes in the slits. In particular, the coupling of evanescent to propagating electromagnetic waves is investigated. It is shown that s-polarized evanescent waves generated at the interface can be transformed into propagating waves if the optical width of the slits allows propagation of the first waveguide mode. As the interface approaches the array, the transmitted intensity increases for evanescent incidence.