Observation of geometrical resonance in optical throughput of very small aperture lasers associated with surface plasmons

Very small aperture lasers utilizing single rectangular apertures with geometrically resonant surface plasmons are experimentally characterized. Bimetal layers of tungsten and silver were employed to realize narrow apertures in a focused ion beam etching process. The tungsten overlayer acts as a mask suppressing aperture opening induced by the tail of a gallium ion beam. We observed strong enhancements in the optical output of narrow apertures (<45 nm) fabricated by single-pass mode, which we attribute to the geometrical resonance of surface plasmons within the apertures. Wider apertures (160 nm) fabricated in raster mode did not exhibit this enhancement, consistent with the explanation of geometrical resonance.