Photonic nanocavities are a key component in many applications because oftheir capability of trapping and storing photons and enhancing interactions oflight with various functional materials and structures. The maximal number ofphotons that can be stored in silicon photonic cavities is limited by thefree-carrier and thermo-optic effects at room temperature. To reduce sucheffects, we performed the first experimental study of optical nonlinearities inultrahigh-Q silicon disk nanocavities at cryogenic temperatures in a superfluidhelium environment. At elevated input power, the cavity transmission spectraexhibit distinct blue-shifted bistability behavior when temperature crosses theliquid helium lambda point. At even lower temperatures, the spectra restore tosymmetric Lorentzian shapes. Under this condition, we obtain a large storedintracavity photon number of about 40,000, which is limited ultimately by thelocal helium phase transition. These new discoveries are explained bytheoretical calculations and numerical simulations.
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