Nano Lett. 14, 1659–1664 (2014)
Channel plasmons guided by sharp V-shaped grooves in metal surfaces are known to have relatively low propagation losses. Recent work has revealed promising applications for such plasmons, including nano-optical coupling and nanofocusing; however, efficient excitation of the modes remains a challenge. Previously, this had usually been achieved by end-fire excitation (light incident on a waveguide termination) with either a focused laser beam or butt-coupling an optical fibre. However, the efficiency is typically low (∼10% or less), because of the poor spatial overlap between optical modes, impedance mismatch, and polarization and positioning issues. Now, Cameron Smith and colleagues in Denmark have fabricated high-quality V-grooves having tilted 'mirrors' at their ends, which can couple bulk waves that are approximately normally incident on their surfaces. Although the idea of employing normally incident coupling and using mirrors to scatter or couple light into V-grooves is not entirely new, the in-coupling efficiency of over 50% (incident wavelength, 811 nm) is impressive. An out-coupling efficiency of 75% was achieved. The researchers attribute this high efficiency to the structure's shape and the smooth gold surface obtained by using crystallographic etching and thermal SiO2 growth processes.
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