  By measuring the Josephson emission of a diffusive Superconductor-Normal
metal-Superconductor (SNS) junction at a finite temperature we reveal a
non-trivial sensitivity of the supercurrent to microwave irradiation. We
demonstrate that the harmonic content of the current-phase relation is modified
due to the energy redistribution of quasiparticles in the normal wire induced
by the electromagnetic field. The distortion originates from the
phase-dependent out-of-equilibrium distribution function which is strongly
affected by the ac-response of the spectral supercurrent. For phases close to
$\pi$, transitions accross the Andreev gap are dynamically favored leading to a
supercurrent reduction. This finding is supported by a comparison with the
quasiclassical Green's function theory of superconductivity in diffusive SNS
junctions under microwave irradiation.

Mesoscale and Nanoscale Physics