Isolation of Weak Four-Wave Mixing Signal Components in Reflection Experiments

Nonlinear spectroscopic experiments were performed on a quantum well in a reflective pump-probe geometry, detecting weak four-wave mixing signals copropagating with the excitation pulses. By demodulating the nonlinear response at unconventional frequencies, we were able to isolate individual terms contributing to the nonlinear signal. Signal components linear in the electric field amplitudes of excitation pulses exhibit unusual properties compared with more familiar signals that are linear in excitation pulse intensities. The isolated signal terms may be used to perform multidimensional spectroscopic measurements to completely characterize the phase and amplitude of the nonlinear response.