A spinning nonlinear crystal with three-fold rotational symmetry is rotated at constant angular frequency Ω around its optical axis. λi and σi, with i = (F, SH), refer to wavelength and helicity of the fundamental (F) and second-harmonic (SH) circularly polarized waves, and δλ (given by equation (3)) refers to tiny wavelength change arising from the rotational Doppler effect. Depicted helices correspond to the spatial distribution of the electric field of the fundamental and second-harmonic wave at a given time (here for σF = −1). Transmitted fundamental wave is not shown for clarity. Adapted from ref. 3, Nature Publishing Group.