Non-degenerate two-wave mixing in the subwavelength periodic motion regime

We experimentally study non-degenerate two-wave mixing where reflection from a moving mirror is used to create frequency shifts and show that both the subwavelength motion of the mirror and periodic reversal of the intensity grating direction significantly affect the frequency dependence of the two-wave mixing signals detected both in ruby and colloidal media. It is shown that the Doppler shifts are suppressed when the reflecting moving mirror excursions are small compared to a wavelength and that the periodic reversal of the mirror motion results in a response that does not produce the expected maximum at the grating relaxation time. An approximate theoretical model agrees well with the results of the experiments and elucidates the transition from the observed response to the previously described idealized Doppler shift models. •Doppler shifts are suppressed for subwavelength displacements.•Conventional two-wave mixing theory does not apply to subwavelength periodic motion.•Subwavelength two-wave mixing is analogous to Dicke narrowing.