Multiphoton Process Observed in the Interaction of Microwave Fields with the Tunneling between Superconductor Films

Calculations are made which explain qualitatively the multiphoton-assisted electron tunneling recently observed in superconducting diodes by Dayem and Martin. It seems to us that the microwave field is much too weak to cause any nonlinearities in the conduction current in the superconductors. Thus, the interaction does not cause transitions between electron states with different wave numbers. Rather, the energies of the electrons are varied adiabatically by the microwave fields. This gives rise to effective changes in the density of states versus energy which are dramatically illustrated in the tunneling current.Calculations are performed for three different possible forms of the field interaction. Qualitatively, the theory fits the experimental observations very well, but, as in the somewhat similar case of phonon-assisted tunneling, the largest postulated interaction seems about an order of magnitude too small to explain the observations on a quantitative basis.