Emergence of Localized Surface Plasmons in Unpatterned Hyperdoped Polycrystalline Silicon

The ability to engineer localized surface plasmon resonances at large scale usually relies on precise nanoscale patterning. Here, we demonstrate that mid-infrared plasmonic responses can instead emerge in unpatterned polysilicon films composed of nanometric (5-50 nm) grains, challenging established design paradigms and eliminating the need for external nanostructuring. Using tailored out-of-equilibrium annealing conditions, we show that hyperdoped polysilicon layers exhibit enhanced light-matter interactions that can be tuned across the mid-infrared range. By combining advanced electron microscopy, infrared spectroscopy and finite-difference time-domain electrodynamic simulations, we demonstrate that these remarkable optical properties originate from naturally formed metal-dielectric interfaces at grain boundaries, which support localized surface plasmon resonances. This work opens up a new field in plasmonics centered on polycrystalline semiconductors, paving the way for cost-effective systems that are fully compatible with microelectronic and photovoltaic technologies, and capable of significantly reshaping light-matter interactions in the infrared range.