Modulated transfer process forced at phonon frequencies in optical heterodyne spectroscopy

A simple spectroscopic technique is used to detect low-frequency acoustical phonons. Resonantly enhanced modulated transfer processes in optical heterodyne spectroscopy are accomplished from a frequency shifted modulated beam to an unmodulated beam in a nonlinear medium having low-frequency induced phonons. The resonance condition is achieved when the phonon frequency is set equal to the beat frequency of the two beams.