  We present the experimental realization of ordered arrays of hyper-doped
silicon nanodisks, which exhibit a localized surface plasmon resonance. The
plasmon is widely tunable in a spectral window between 2 and 5 $\mu$m by
adjusting the free carrier concentration between 10$^{20}$ and 10$^{21}$
cm$^{-3}$. We show that strong infrared light absorption can be achieved with
all-silicon plasmonic metasurfaces employing nano-structures with dimensions as
low as 100\,nm in diameter and 23 nm in height. Our numerical simulations show
an excellent agreement with the experimental data and provide physical insights
on the impact of the nanostructure shape as well as of near-field effects on
the optical properties of the metasurface. Our results open highly promising
perspectives for integrated all-silicon-based plasmonic devices for instance
for chemical or biological sensing or for thermal imaging.
