Temperature bandgaps and thermal dopants arising from photothermal nonlinearities in high-Q silicon metasurfaces

Strong light-matter interactions in silicon metasurfaces give rise to photothermal nonlinearities. While the effect of this strong coupling on light has been extensively studied, its impact on matter remains largely unexplored. Here, we investigate photothermal energy harvesting using strong light-matter interactions in high-quality-factor metasurfaces. First, we show that maximum metasurface temperatures can differ by $90^\circ C$ based on the spectral excitation pathway: direct excitation of the metasurface from its nominal equilibrium by a single wavelength results in lower metasurface temperatures compared to continuous spectral scans. Investigating the temporal dynamics of this thermo-optic nonlinearity, we show an initial linear rise to a critical temperature over $~10s$ of seconds, followed by a rapid rise to the high temperature steady state in $~33ms$ around the bistable transition point. This linear rise time increases sharply around the bistable transition point. Most significantly, we show emergence of a Q-factor-dependent temperature band splitting and energy gap of around $60^\circ C$ between the critical temperature and the high temperature steady state of the metasurface under single wavelength illumination. This forbidden equilibrium temperature band signifies that the high temperature steady state of the metasurface resonance after transitioning across the laser wavelength is further away from its nominal resonance than its linewidth. Interestingly, hyperspectrally accumulated nonlinearities from prior wavelengths effectively dope the metasurface with optical states within this bandgap. These thermal nonlinearities are tunable using the geometry and material of the metasurface. Collectively, these results show how high-Q metasurfaces enable strongly localized, path-dependent photothermal heating to drive site-selective on-chip catalytic and biochemical transformations.