Hyper-Doped Silicon Nanoantennas and Metasurfaces for Tunable Infrared Plasmonics

The use of plasmonic or high refractive index dielectric nanoantennas or metasurfaces is a common strategy for increasing optical absorption and scattering and for localizing light at subwavelength dimensions. Because of their low losses and direct compatibility with large-scale production techniques from microelectronics, semiconductor nanostructures and full noble metal nanostructures are attracting increasing interest in the visible spectrum. Those techniques have limitations in the mid-infrared (MIR), namely at wavelengths ranging from 2 to 15 µm, which is the region of relevance for chemical or biological sensing and thermal imaging.<br/>Doped semiconductors^1-2, in addition to other properties like shape and size, are a promising choice for plasmonics encompassing a broad infrared spectrum, since their LSPR can be controlled by dopant concentration as well as other parameters like shape and size. Massively doped silicon (Si) nanostructure arrays are logical candidates for mass-producing highly integrated low-cost MIR plasmonic devices.<br/>In this framework, we developed all-silicon-based plasmonic metasurfaces generated by submicrometer tiny doped-Si nanodisks (Si-NDs) by processing hyper-doped overlayers of SOI substrates³. We showed that such nanostructures can support localized surface plasmon resonances that are controlled by the Si free carrier density. By modulating the free carrier concentration between 10²⁰ and 10²¹ cm^-3, the plasmon can be tuned in a spectral window between 2 and 5 µm⁴. Active dopant concentrations exceeding 10²¹cm^-3 are required to reach the NIR regime. We showed that, despite a large mismatch between the wavelength (25.5 µm) and the Si-ND diameter (100 nm), the LSPR allows for a substantial interaction with IR light, resulting in a loss in transmittance of up to 10% at the resonance⁴. We evaluated the optical characteristics of a single nanodisk as well as the entire metasurface using numerical simulations⁵. They demonstrate excellent agreement with the experimental findings and provide physical insights into the influence of nanostructure form and near-field effects on the metasurface's optical characteristics. Our results open highly promising perspectives for integrated all-silicon-based plasmonic devices for instance for chemical or biological sensing or for thermal imaging.<br/><br/>^{1 D.}Li, Ning, C. Z. All-semiconductor active plasmonic system in mid-infrared wavelengths. Opt. Express 2011, 19, 14594.<br/>² T. Taliercio, P. Biagioni, P. Semiconductor infrared plasmonics, Nanophotonics 2019, 8, 949−990.<br/>³ N. Chery, et al. Study of recrystallization and activation processes in thin and highly doped<br/>silicon-on-insulator layers by nanosecond laser thermal annealing. Journal of Applied<br/>Physics 2022, 131, 065301.<br/>⁴J. M. Poumirol et al. Hyper-Doped Silicon Nanoantennas and Metasurfaces for Tunable Infrared Plasmonics. ACS Photonics 2021, 8, 1<br/>393-1399.<br/>⁵ C. Majorel, et al. Theory of plasmonic properties of hyper-doped silicon nanostructures. Opt. Commun. 2019, 453, 124336.