Silver film with cross-shaped nanoapertures is plasmonic quarter-wave plate

Researchers at the University of Melbourne (Victoria, Australia) and Casix (Fuzhou, China) are computationally modeling plasmonic optical wave plates that consist of a periodic array of cross-shaped nanoapertures in a silver film. For example, they have designed a quarter-wave plate for operation at a specified wavelength in the infrared. The wave-plate quality is created by making the cross shapes asymmetric.

The 2D structure—which was modeled using multiphysics software by COMSOL (Burlington, MA)—has a silver film thickness of 140 nm, an array period d in both dimensions of 400 nm, and crosses with a constant linewidth w of 40 nm, but with differing line lengths l_x and l_y. The period is large enough to keep localized aperture resonances well separated from resonances arising from surface-plasmon excitation. In one instance, l_y is held fixed at 210 nm and l_x is varied from 170 to 250 nm. The resulting transmission spectrum has two peaks that arise from the excitation of the resonance in each arm of the cross. The analysis showed that patterns with l_x of 193 nm and 229 nm would behave as partially transmitting quarter-wave plates at 710 nm (left circularly polarized) and 760 nm (right circularly polarized), respectively. Contact Ann Roberts at [email protected].