We report the experimental realization of an optical metamaterial composed of a hexagonal array of coaxial plasmonic metal/insulator/metal waveguides that shows strong polarization-independent optical mode index dispersion in the ultraviolet/blue. The metamaterial is composed of silicon coaxes with a well-defined diameter in the range of 150-168 nm with extremely thin sidewalls (13-15 nm), embedded in a silver film, fabricated using a combination of electron beam lithography, physical vapor deposition, reactive ion etching, and focused ion beam polishing. Using a Mach-Zehnder interferometer the phase advance is measured on several metamaterial samples with different dimensions in the UV/visible part of the spectrum. For all geometries the spectral features as well as the geometry dependence of the data correspond well with numerical finite-difference time domain simulations and the calculated waveguide dispersion diagram, showing a negative mode index between 440 and 500 nm.

Nano Lett.
Photonic Materials

van de Haar, M., Maas, R. C., Schokker, A. H., & Polman, A. (2014). Experimental realization of a polarization-independent ultraviolet/visible coaxial plasmonic metamaterial. Nano Lett., 14(11), 6356–6360. doi:10.1021/nl5028183