Ultrabroadband phonon laser frequency comb

Optical frequency combs have received increasing attention, thanks to their fundamental importance in optical computing, communications, and metrology. Recently, acoustic frequency combs have been achieved, with several hundred comb teeth and a high-frequency range of over 100 kHz. Here, we report how to achieve an acoustic frequency comb with phonon lasers, featuring not only the record tooth number of 6000 and a tunable tooth spacing from 10 Hz to 100 kHz but also the ultrabroad bandwidth, spanning six orders of magnitude. In this device, two kinds of nonlinear couplings can emerge, i.e., gain-assisted light-motion coupling and cascaded four-wave mixing. Their interplay can be flexibly tuned by driving different vibration modes of the membrane. Therefore, a switch between a multi-color phonon laser and a phonon laser frequency comb is achievable. We provide the first step toward developing various phonon laser combs for potential applications in underwater sensing, flaw detection, and biomedical ultrasonics, with the help of phonon lasers, which have already been achieved in a wide range of systems such as membranes, photonic crystals, semiconductor lattices, cold ions, and levitated objects.