Thermal runaway of Si-based metasurfaces under high-power illumination

Si-based dielectric metasurfaces can be used to make compact optical components such as lenses, polarizers, gratings, and highly reflective surfaces.  Si is a particularly attractive material for use in high-power-density applications such as non-linear microscopy, telecommunications, and laser-propelled light sails because of its low absorptive loss at infrared frequencies.  In this work, we assessed the thermal stability of radiatively cooled Si/SiO₂ metasurfaces subjected to high optical fluxes (>1 GW m^-2) of 1550 nm light using a broad-spectrum and temperature-dependent absorption model for Si that we assembled from multiple literature sources.  We found that nonlinear absorption effects at low temperatures in conjunction with a shrinking bandgap and rising free-carrier absorption at high temperatures can result in a thermal runaway effect that ultimately destroys the metasurface.  Furthermore, we find that small hotspots caused by defects or dust can push an otherwise thermally stable metasurface into thermal runaway.  We will discuss in detail the physical phenomena driving this thermal runaway in Si, evaluate the absorption and emission contributions of the SiO₂ layer, and set limits on the maximum optical intensity a Si-based metasurface can survive.