The nature of transmission resonances in plasmonic metallic gratings

TL;DR

This paper analyzes transmission resonances in plasmonic metallic gratings using Fourier modal method, revealing interactions between waveguide modes and surface plasmons, and characterizing resulting resonances and band gaps.

Contribution

It provides a detailed analysis of the transmission resonances in silver gratings, highlighting the interaction between Fabry-Perot modes and surface plasmons, and describing the nature of observed resonances.

Findings

Identification of TM-Fabry-Perot branch supported by waveguide modes.

Observation of band gaps formed at surface plasmon crossings.

Characterization of long-lived ridge resonances as guided-mode resonances.

Abstract

Using the Fourier modal method (FMM) we report our analysis of the transmission resonances of a plasmonic grating with sub-wavelength period and extremely narrow slits for wavelengths of the incoming, transverse magnetic (TM)-polarized, radiation ranging from 240nm to 1500nm and incident angles from 0 degree to 90 degree. In particular, we study the case of a silver grating placed in vacuo. Consistent with previous studies on the topic, we highlight that the main mechanism for extraordinary transmission is a TM-Fabry-Perot (FP) branch supported by waveguide modes inside each slit. The TM-FP branch may also interact with surface plasmons (SPs) at the air/Ag interface through the reciprocal lattice vectors of the grating, for periods comparable with the incoming wavelength. When the TM-FP branch crosses a SP branch, a band gap is formed along the line of the SP dispersion. The gap has a Fano-Feshbach resonance at the low frequency band edge and a ridge resonance with extremely long lifetime at the high frequency band edge. We discuss the nature of these dispersion features, and in particular we describe the ridge resonance in the framework of guided-mode resonances (GMRs). In addition, we elucidate the connection of the coupling between the TM-FP branch and SPs within the Rayleigh condition. We also study the peculiar characteristics of the field localization and the energy transport in two topical examples.