Ultraviolet interband plasmonics down to the vacuum UV with ultrathin amorphous silicon nanostructures

Silicon dominates electronics, optoelectronics, photovoltaics and photonics thanks to its suitable properties, abundance, and well-developed cost-effective manufacturing processes. Recently, crystalline silicon has been demonstrated to be an appealing alternative plasmonic material, both for the infrared where free-carrier plasmons are enabled by heavy doping, and for the ultraviolet (UV) where plasmonic effects are induced by interband transitions. Herein, we demonstrate that nanostructured amorphous silicon exhibits such so-called interband plasmonic properties in the UV, as opposed to the expectation that they would only arise in crystalline materials. We report optical plasmon resonances in the 100-to-300 nm wavelength range in ultrathin nanostructures. These resonances shift spectrally with the nanostructure shape and the nature of the surrounding matrix, while their field enhancement properties turn from epsilon-near-zero plasmonic to surface plasmonic. We present a vacuum UV wavelength- and polarization-selective ultrathin film absorber design based on deeply-subwavelength anisotropically-shaped nanostructures. These findings reveal amorphous silicon as a promising material platform for ultracompact and room-temperature-processed UV plasmonic devices operating down to vacuum UV wavelengths, for applications including anticounterfeiting, data encryption and storage, sensing and detection. Furthermore, these findings raise a fundamental question on how plasmonics can be based on amorphous nanostructures.