Enhanced transmission via evanescent-to-propagating conversion in metallic nanoslits: role of Rayleigh anomalies

Abstract

We analyze the enhanced transmission phenomenon in subwavelength slit structures near a dielectric interface. In particular, we investigate the influence of Rayleigh anomalies in the spectral position as well as in the bandwidth of Fabry-Perot resonances excited on such structures. We consider the cases of propagating and evanescent incidence, i.e., when the metallic structure is illuminated from the dielectric medium side with an incidence angle larger than the critical angle. We show that Rayleigh anomalies strongly interact with Fabry-Perot resonances, and make them deviate from the spectral positions predicted by the infinitely thin slit model. To get physical insight into this problem, we develop a simplified electromagnetic model and show that there is a close correspondence between the transmitted response of the structure and the behavior of certain function that depends on the geometrical and the illumination parameters. Our results suggest that Rayleigh anomalies strongly modify the electromagnetic response of the structure due to the existence of surface waves that modify the coupling condition between the fields inside and outside the slits. Besides, we show that even in absence of Fabry-Perot resonances, it is possible to produce enhanced transmission by taking advantage of the pseudoperiodicity condition of the fields.

Striking properties of light transmitted by a double period nanoslit structure under evanescent incidence

“…For the parameters of figure 2(a) the normalized resonant wavelengths are λ = d 2.6, 1.3, 0.86, 0.65, 0.52, etc, and for figure 2(b) these values are λ = d 1.8, 0.9, 0.6, 0.45, etc. It can be noticed that the FP maxima are slightly shifted to longer wavelengths, as already observed for regular gratings [28]. However, in the dual-period case the enhancement bands are widened (compare with figure 2 of [28]), which allows a better tunability and also implies a larger overall power transmitted to the far field.…”

“…It can be noticed that the FP maxima are slightly shifted to longer wavelengths, as already observed for regular gratings [28]. However, in the dual-period case the enhancement bands are widened (compare with figure 2 of [28]), which allows a better tunability and also implies a larger overall power transmitted to the far field. Besides, in the simple grating case, Rayleigh anomalies strongly modify the FP transmission maxima, i.e., the evanescent waves generated at these particular wavelengths modify the propagation conditions at the boundary, which, in turn, produce significant changes in the transmitted response [28].…”

“…For θ 0 larger than the critical angle (θ c ) at which the total internal reflection (TIR) condition is enabled, the pseudoperiodicity condition imposed by the periodicity of the slits' array allows transmission to the far field. It has been recently shown that regular slit gratings illuminated by evanescent incidence exhibit enhanced transmission for certain ranges of wavelengths and incidence angles, and that FP resonances and Rayleigh anomalies play an important role in the generation of the intensification effect [28]. We have also reported that the distance between the grating and the dielectric interface is of relevant importance for this phenomenon.…”

“…However, in the dual-period case the enhancement bands are widened (compare with figure 2 of [28]), which allows a better tunability and also implies a larger overall power transmitted to the far field. Besides, in the simple grating case, Rayleigh anomalies strongly modify the FP transmission maxima, i.e., the evanescent waves generated at these particular wavelengths modify the propagation conditions at the boundary, which, in turn, produce significant changes in the transmitted response [28]. On the other hand, in the case of dual-period structures, FP resonances seem to prevail over Rayleigh anomalies, and the transmission response is not as significantly affected.…”

“…This aspect is of great interest since it opens up new possibilities for the design of efficient optical components such as tunable filters and plasmonic devices. In a recent paper we investigated the enhanced transmission phenomenon in simple periodic subwavelength slit structures under propagating and evanescent incidence [28]. We found that Rayleigh anomalies interact with the FP resonances, and make them deviate from the spectral positions predicted by the infinitely thin slit model.…”