  Photonic nanocavities are a key component in many applications because of
their capability of trapping and storing photons and enhancing interactions of
light with various functional materials and structures. The maximal number of
photons that can be stored in silicon photonic cavities is limited by the
free-carrier and thermo-optic effects at room temperature. To reduce such
effects, we performed the first experimental study of optical nonlinearities in
ultrahigh-Q silicon disk nanocavities at cryogenic temperatures in a superfluid
helium environment. At elevated input power, the cavity transmission spectra
exhibit distinct blue-shifted bistability behavior when temperature crosses the
liquid helium lambda point. At even lower temperatures, the spectra restore to
symmetric Lorentzian shapes. Under this condition, we obtain a large stored
intracavity photon number of about 40,000, which is limited ultimately by the
local helium phase transition. These new discoveries are explained by
theoretical calculations and numerical simulations.
