Broadband nonlinear phonon-laser frequency comb

Abstract Optical frequency combs have received increasing attention, thanks to their fundamental importance in optical computing, communication, sensing and metrology. Recently, thermal phonon counterparts of frequency combs have shown ~102 comb teeth and ~103 teeth spacing, and limited to the high-frequency range over 100 kHz. In contrast, we report the first truly coherent acoustic frequency comb (AFC), with simultaneous giant enhancements in comb teeth number and teeth spacing, reaching the record teeth number of 10^4, covering six orders of magnitude in frequency; thus enabling a wide range of typical applications as force sensing, deep-sea monitoring or biomedical ultrasonics. In such a single device, two kinds of nonlinear couplings can emerge, i.e., the gain-assisted light-motion coupling and the mechanical wave mixing, the interplay of which can be flexibly tuned by rotating the angle of the membrane. Therefore, a single device could achieve a coherent switch from the multi-color phonon lasing regime to the coherent AFC regime, with almost perfectly uniform comb teeth. This work presents the first example merging phonon lasers and AFCs, and can stimulate more efforts on making and operating high-quality coherent AFC chips for diverse applications, by using phonon lasers already achieved in such diverse systems as membranes, levitated spheres, photonic crystals, semiconductor lattices, and cold ions.