Interference of nonlinear response components in degenerate four-photon spectroscopy of ultrathin metal films

It is shown that a specific dip in the spectral dependence of the nonlinear response of ultrathin metal films observed by the method of degenerate four-photon spectroscopy is caused by a change in the phases of interfering components of the electron nonlinear susceptibility. This allows one to measure the time scale of subpicosecond intraband processes using picosecond light pulses.