Controlling the Polarization State of Light with Plasmonic Metal Oxide Metasurface.

Conventional plasmonic materials, namely, noble metals, hamper the realization of practical plasmonic devices due to their intrinsic limitations, such as lack of capabilities to tune in real-time their optical properties, failure to assimilate with CMOS standards, and severe degradation at increased temperatures. Transparent conducting oxide (TCO) is a promising alternative plasmonic material throughout the near- and mid-infrared wavelengths. In addition to compatibility with established silicon-based fabrication procedures, TCOs provide great flexibility in the design and optimization of plasmonic devices because their intrinsic optical properties can be tailored and dynamically tuned. In this work, we experimentally demonstrate metal oxide metasurfaces operating as quarter-waveplates (QWPs) over a broad near-infrared (NIR) range from 1.75 to 2.5 μm. We employ zinc oxide highly doped with gallium (Ga:ZnO) as the plasmonic constituent material of the metasurfaces and fabricate arrays of orthogonal nanorod pairs. Our Ga:ZnO metasurfaces provide a high degree of circular polarization across a broad range of two distinct optical bands in the NIR. Flexible broad-band tunability of the QWP metasurfaces is achieved by the significant shifts of their optical bands and without any degradation in their performance after a post-annealing process up to 450 °C.