Observation of photon-assisted noise in a diffusive normal metal-superconductor junction

We report measurements of nonequilibrium noise in a diffusive normal metal- superconductor (N-S) junction in the presence of both de bias and, high-fre quency ac excitation. We find that the shot noise of a diffusive N-S juncti on is doubled compared to a normal diffusive conductor. Under ac excitation of frequency nu the shot noise develops features at bias voltages \ V \ = h nu/(2e), which bear all the hallmarks of a photon-assisted process. Obser vation of these features provides clear evidence that the effective charge of the current carriers is 2e, due to Andreev reflection.