Chip-scale optically driven phononic frequency comb with 1-70 GHz span

A phononic frequency comb consists of equally spaced components in the mechanical frequency domain and holds promise for numerous applications. Yet, prior demonstrations have been limited in spectral range due to the inherently low mechanical frequencies. In this work, we report a phononic comb with a record span from 1 to 70 GHz. This result is achieved by harnessing the strong mechanical nonlinearity of a $2.5$-$\mu$m-radius silicon carbide microdisk, which supports a radial breathing mode at $1.655$ GHz with a mechanical quality factor of 13,500. With just 1 mW of dropped optical power, radiation pressure from a continuous-wave pump drives strong phonon lasing, generating 42 phase-locked harmonics with $1.655$ GHz spacing. The combination of such broad bandwidth, low phase noise (-132 dBc/Hz at 1 MHz offset frequency) and frequency stability ($<10^{-7}$ at 1 second of averaging time) positions this ultracompact phononic comb as a powerful platform for diverse applications.