The translational Doppler e ect of electromagnetic and sound waves has been successfully applied in measurements of the speed and direction of vehicles, astronomical objects and blood flow in human bodies [1][2][3][4][5][6][7][8] , and for the Global Positioning System. The Doppler e ect plays a key role for some important quantum phenomena such as the broadened emission spectra of atoms 9 and has benefited cooling and trapping of atoms with laser light [10][11][12] . Despite numerous successful applications of the translational Doppler e ect, it fails to measure the rotation frequency of a spinning object when the probing wave propagates along its rotation axis. This constraint was circumvented by deploying the angular momentum of electromagnetic waves
13-the so-called rotational Doppler e ect. Here, we report on the demonstration of rotational Doppler shift in nonlinear optics. The Doppler frequency shift is determined for the second harmonic generation of a circularly polarized beam passing through a spinning nonlinear optical crystal with three-fold rotational symmetry. We find that the second harmonic generation signal with circular polarization opposite to that of the fundamental beam experiences a Doppler shift of three times the rotation frequency of the optical crystal. This demonstration is of fundamental significance in nonlinear optics, as it provides us with insight into the interaction of light with moving media in the nonlinear optical regime.A beam of light with spin angular momentum (SAM) σ (σ = ±1), which corresponds to either left or right circular polarization (LCP and RCP) states respectively, flips its spin when it passes through a rotating half-wave plate (HWP). In this process, the circularly polarized light applies a torque on the HWP (refs 14,15) and in return experiences a frequency shift 16 . For circularly polarized light propagating along the normal axis of a rotating HPW at angular frequency Ω, it was predicted and experimentally confirmed that the transmitted circularly polarized light with the opposite sense has a frequency shift of ±2Ω (Fig. 1a), where the + and − signs correspond to the rotation direction of the HWP, being the opposite or same as that of the circularly polarized incident light [16][17][18][19] . In recent years, the rotational Doppler effect has been successfully used for probing and controlling the rotation of molecules 20 , and studying rotating quantum systems 21,22 . Recently, the observation of the rotational Doppler effect has been extended to light with orbital angular momentum [23][24][25][26][27][28] , which has shown the capability of remotely measuring the rotational frequency of a spinning object, such as air turbulence, rotating astronomical bodies and so on 27 . Compared with the rapid development and applications of the rotational Doppler effect in linear optics, it has received much less attention in the nonlinear optical regime since it was first predicted for second harmonic generation (SHG) processes more than four decades ago 29 . So far, t...