Doppler effect tailoring: extra-red shift of structured light.

Doppler effect is a universal phenomenon that describes the frequency shift of waves when interacting with moving targets. Recent studies suggest that the spatial structures of light beams would provide a modification of the group and phase velocities of optical fields, raising the question of how spatial confinement on optical fields affects Doppler shifts of structured beams. In this work, we propose and demonstrate the concept of Doppler effect tailoring, i.e., the transverse structure of optical fields naturally causes an extra red shift on the original Doppler shift, which we call the structure-shearing Doppler effect (SDE). Theoretical analyses suggest that the SDE observed in the experiment is actually a universal effect on both light waves and photons and it is predicted to exist beyond optics in any non-planar electromagnetic and sound waves. The SDE may provide new insights into the Doppler effect for astronomical observations, laser cooling, and light-matter-interaction within hollow waveguides or cavities. Technically, a homodyne-free Doppler velocimeter is developed based on the SDE, facilitating a single probe beam without external reference enabling the system resistant to environmental disturbance. Regarding application prospects, the implementation is of great significance for leveraging structured beams in motion sensing in engineering.