  We experimentally investigated the optical responses of a superconducting
niobium resonator. It was found that, with increasing radiation power, the
resonance frequency increases monotonically below around 500 mK, decreases
monotonically above around 1 K and exhibits a nonmonotonic behavior at around
700 mK. These observations show that one can operate the irradiated resonator
in three temperature regimes, depending on whether two-level system (TLS)
effects or kinetic inductance effects dominate. Furthermore, we found that the
optical responses at ultra-low temperatures can be qualitatively regarded as a
photon-induced thermalization effect of TLSs, which could be utilized to
achieve thermal sensitive photon detections.
