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

摘要

We examine the coupling of electromagnetic waves incident normal to a thin silver film that formsan oscillatory grating embedded between two otherwise uniform, semi-infinite half spaces. Twograting structures are considered, in one of which the midpoint of the Ag film remains fixed whereasthe thickness varies sinusoidally, while in the other the mid point oscillates sinusoidally whereas thefilm thickness remains fixed. On reducing the light wavelength from the long wavelength limit, weencounter signatures in the transmission, T, and reflection, R, coefficients associated with: (i) theshort-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 asgeneralized (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 forwavelengths corresponding to these three anomalies, particularly for the long-range plasmon modein 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 theincoming waves to be even or odd, T+R can be made to vanish for one or the other of the plasmonmodes; this corresponds to perfect coupling (impedance matching in the language of electricalengineering) between the incoming light and these modes.