Thermo-optical bistability with Si nanocrystals in a whispering gallery mode resonator

We report on the observation of optical bistability in an integrated planar microresonator with embedded silicon nanocrystals (Si-ncs). The phenomenon originates from the thermo-optical modulation of the silica-embedded (Si-ncs) refractive index, which in turn alters the spectral position of the resonator mode. The estimated thermo-optical coefficient of the Si nanocrystalline material, dn∕dT ≈ 2.92 × 10−5 K−1, is an order of magnitude lower than that of bulk silicon. Both time-resolved pump-and-probe experiments and numerical simulations confirm that the silica host is responsible for the heat dissipation from the resonator. Moreover, a negligible Q-factor degradation at pump powers as high as 100 mW, along with the absence of a fast component in time-resolved measurements, confirm the minute contribution from excited carriers effects. These observations, combined with the already published large third-order nonlinearities of Si-ncs (an order of magnitude larger than in bulk Si), make this system an outstanding candidate for low-power on-chip nonlinear comb generation.