A Silicon Broadband Traveling-wave Acousto-optic Modulator

Optical modulators are pivotal components in modern communication and computing systems. Modulators with a broadband optical window can facilitate massive wavelength multiplexing and unbounded information capacity. Conventional realizations of modulators depend on Pockels electro-optics or piezo-optomechanics, setting the requirements for material properties. Here, we demonstrate 6-gigahertz acousto-optic modulation of telecom-band photons in a broadband photonic/phononic crystal waveguide on monolithic silicon-on-insulator platform. The modulating acoustic wave is transduced via electrostatic actuation of a nanomechanical resonator and subsequently coupled to traveling wave co-propagating with the optical input. Free of piezoelectric materials, our acousto-optic modulator features simple fabrication, practical scalability, and high modulation efficiency at cryogenic temperatures (V_πL = 2.8 V·cm). Our work marks an important milestone towards silicon integrated photonics for large-scale frequency-domain optical computing and high-rate quantum transduction.