Abstract
Relativistic electrons moving over a periodic metal grating can lead to an intriguing emission of light, known as Smith-Purcell radiation (SPR), the precursor of the free-electron laser. During the radiation process, the speed of light plays a critical role in determining the emitted angle and frequency spectrum. Inspired by the photonic SPR, here we experimentally demonstrate a photoacoustic version of the Smith-Purcell effect using laser-induced surface shock waves generation. We observe similar acoustic radiation pattern and their associated frequency spectrum in the far-field, perfectly predicted by a universal theory working for both the photonic and acoustic SPR. Moreover, our numerical studies reveal non-constant frequency components due to the supersonic traveling of the shock waves in the near field, greatly contrasting its photonic counterpart. This scheme extends the SPR into the acoustic domain by levitating the wave’s speed limit, paves the way towards coherent acoustic wave generation and microstructure metrology.