Perfect coupling of light to surface plasmons with ultra-narrow linewidths

TL;DR

This paper investigates how electromagnetic waves couple to surface plasmons on a thin silver film with oscillatory gratings, revealing conditions for perfect coupling and ultra-narrow linewidths in plasmonic resonances.

Contribution

It introduces a novel configuration of oscillatory gratings on silver films to achieve perfect coupling and ultra-narrow plasmon linewidths, advancing plasmonic control techniques.

Findings

Identification of signatures for short- and long-range surface plasmons and electromagnetic diffraction anomalies.

Enhanced energy density at the film surface at plasmon resonance wavelengths.

Achieving perfect coupling by adjusting grating amplitude and phase of incident waves.

Abstract

We examine the coupling of electromagnetic waves incident normal to a thin silver film that forms an oscillatory grating embedded between two otherwise uniform, semi-infinite half spaces. Two grating structures are considered, in one of which the mid point of the Ag film remains fixed whereas the thickness varies sinusoidally, while in the other the mid point oscillates sinusoidally whereas the film thicknesses remains fixed. On reducing the light wavelength from the long wavelength limit, we encounter signatures in the transmission, T, and reflection, R, coefficients associated with: i) the short-range surface plasmon mode, ii) the long-range surface plasmon mode, and iii) electromagnetic diffraction tangent to the grating. The first two features can be regarded as generalized (plasmon) Wood's anomalies whereas the third is the first-order conventional (electromagnetic) Wood's anomaly. The energy density at the film surface is enhanced for wavelengths corresponding to these three anomalies, particularly for the long range plasmon mode in thin films. When exciting the silver film with a pair of waves incident from opposite directions, we find that by adjusting the grating oscillation amplitude and fixing the relative phase of the incoming waves to be even or odd, T+R can be made to vanish for one or the other of the plasmon modes; this corresponds to perfect coupling (impedance matching in the language of electrical engineering) between the incoming light and these modes.