  In a superconductor absorption of photons with an energy below the
superconducting gap leads to redistribution of quasiparticles over energy and
thus induces a strong non-equilibrium quasiparticle energy distribution. We
have measured the electrodynamic response, quality factor and resonant
frequency, of a superconducting aluminium microwave resonator as a function of
microwave power and temperature. Below 200 mK, both the quality factor and
resonant frequency decrease with increasing microwave power, consistent with
the creation of excess quasiparticles due to microwave absorption.
Counterintuitively, above 200 mK, the quality factor and resonant frequency
increase with increasing power. We demonstrate that the effect can only be
understood by a non-thermal quasiparticle distribution.
