  We experimentally study the influence of 1-40 GHz radiation on the resistance
of normal (N) mesoscopic conductors coupled to superconducting (S) loops
(Andreev interferometers). At low RF amplitudes we observe the usual h/2e
superconducting-phase-periodic resistance oscillations as a function of applied
magnetic flux. We find that the oscillations acquire a pi-shift with increasing
RF amplitude, and consistent with this result the resistance at fixed phase is
an oscillating function of the RF amplitude. The results are explained
qualitatively as a consequence of two processes. The first is the modulation of
the phase difference between the N/S interfaces by the RF field, with the
resistance adiabatically following the phase. The second process is the change
in the electron temperature caused by the RF field. From the data the response
time of the Andreev interferometer is estimated to be <40ps. However there are
a number of experimental features which remain unexplained; these include the
drastic difference in the behaviour of the resistance at different phases as a
function of RF frequency and amplitude, and the existence of a "window of
transparency" where heating effects are weak enough to allow for the pi-shift.
A microscopic theory describing the influence of RF radiation on Andreev
interferometers is required.
