Microwave-to-optical conversion using lithium niobate thin-film acoustic resonators

Acoustic or mechanical resonators have emerged as a promising means to mediate efficient microwave-to-optical conversion. Here, we demonstrate conversion of microwaves up to 4.5 GHz in frequency to 1500 nm wavelength light using optomechanical interactions on suspended thin-film lithium niobate. Our method uses an interdigital transducer that drives a freestanding 100 μm-long thin-film acoustic resonator to modulate light traveling in a Mach–Zehnder interferometer or racetrack cavity. The strong microwave-to-acoustic coupling offered by the transducer in conjunction with the strong photoelastic, piezoelectric, and electro-optic effects of lithium niobate allows us to achieve a half-wave voltage of Vπ = 4.6 V and Vπ = 0.77 V for the Mach–Zehnder interferometer and racetrack resonator, respectively. The acousto-optic racetrack cavity exhibits an optomechanical single-photon coupling strength of 1.1 kHz. To highlight the versatility of our system, we also demonstrate a microwave photonic link with unitary gain, which refers to a 0 dB microwave power transmission over an optical channel. Our integrated nanophotonic platform, which leverages the compelling properties of lithium niobate, could help enable efficient conversion between microwave and optical fields.