Metasurfaces have been investigated for various applications ranging from beam steering, focusing, to polarization conversion. Along with passive metasurfaces, significant efforts are also being made to design metasurfaces with tunable optical response. Among various approaches, voltage tuning is of particular interest because it creates the possibility of integration with electronics. In this work, we demonstrate voltage tuning of reflectance from a complementary metasurface strongly coupled to an epsilon-near-zero (ENZ) mode in an ultrathin semiconductor layer. Our approach involves electrically controlling the carrier concentration of the ENZ layer to modulate the polaritonic coupling between the dipole resonances of the metasurface and the ENZ mode for modulating the reflectance of the metasurface. The hybrid structure we fabricate is similar to MOSCAP configuration where the complementary metasurface offers a continuous gold top layer for biasing and positive/negative bias to the metasurface leads to accumulation/depletion of carriers in the ENZ layer beneath it. We optimized our structure by using InGaAs as the ENZ material because of its high mobility and low effective mass. This allowed us to reduce the doping requirement and thereby reduce the ionized impurity scattering as well as the reverse bias required to deplete the ENZ layer. For low leakage and efficient modulation of carrier density, we used Hafnia as the gate dielectric. We further added a reflecting backplane below the ENZ layer to enhance the interaction and by applying bias, we achieved spectral shifts of 500 nm and amplitude modulation of 11% of one of the polariton branches at 14 µm.
Raktim Sarma, Salvatore Campione, Michael Goldflam, Joshua Shank, Sean Smith, Jinhyun Noh, Peide Ye, Michael Sinclair, Ganapathi Subramania, Isaac Ruiz, Stephen Howell, Joel Wendt, and Igal Brener, "Voltage tuning of reflectance from a strongly coupled metasurface-semiconductor hybrid structure (Conference Presentation)," Proc. SPIE 10721, Active Photonic Platforms X, 107211E (Presented at SPIE Nanoscience + Engineering: August 22, 2018; Published: 17 September 2018); https://doi.org/10.1117/12.2321256.5836038488001.
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