Observation of Rotational Doppler Effect in Second Harmonic Generation in Reflection Mode

In linear optical regime, one important application of Rotational Doppler Effect (RDE) is to measure the spinning speed of either molecules or macroscopic objects. The RDE can be further magnified using nonlinear harmonic generations. However, the previous measurement of nonlinear RDE in transmission mode, which relies on two beam interferometry, is very challenging due to the critical requirement of optical alignment in both spatial and time domains. Here, the observation of nonlinear RDE of second harmonic generation (SHG) waves from one spinning quartz crystal with silver coating in reflection mode is reported. The utilization of the post‐selection and beating frequency techniques greatly simplify the conventional nonlinear RDE measurement setup.