Towards terahertz nanomechanics.

Advancing electromechanical resonators towards terahertz frequencies opens vast bandwidths for phononic signal processing. In quantum phononics, mechanical resonators at these frequencies can remain in their quantum ground state even at kelvin temperatures, obviating the need for millikelvin cooling typically required for GHz resonators. However, electrical actuation and detection at such high frequencies are challenging, primarily due to device miniaturization needed to support acoustic waves with nanometer-scale wavelengths. This requires thinning piezoelectric films to a thickness that matches the acoustic wavelength. In this work, we reduce the thickness of lithium niobate from 300 nm to 67 nm through several stages, and fabricate suspended Lamb-wave resonators at each thickness level. These resonators achieve resonant frequencies of nearly 220 GHz, doubling the previous record and approaching the terahertz threshold. While ultrathin films exhibit a clear advantage in frequency gains, they also experience increased acoustic losses. Our results suggest that future advances in terahertz nanomechanics will critically rely on mitigating surface defects in sub-100 nm thin films.