Light in a subwavelength slit in a metal: Propagation and reflection

The propagation and reflection of TM light in a subwavelength slit in a real metal are analyzed. The results for the real metal differ significantly from that of a perfect electric conductor due to the existence of surface plasmons. As the slit becomes narrower or the relative permittivity of the metal becomes less negative, the effective index of the mode in the slit increases and the amplitude and phase of the reflection of the mode at the free-space boundary are modified. To see the influence of these changes for the real metal, it is shown that significant changes in the film thickness, greater than a factor of 2 for noble metals in the visible spectrum and 20-nm slits, are required to maintain the Fabry-Perot resonance at a constant wavelength as the relative permittivity is varied.