Towards terahertz nanomechanics

TL;DR

This paper demonstrates the fabrication of lithium niobate nanomechanical resonators reaching frequencies up to 220 GHz, nearly terahertz, highlighting the potential for high-frequency phononic devices and the challenges of acoustic losses at nanoscale thickness.

Contribution

The study achieves the highest resonant frequencies in lithium niobate Lamb-wave resonators by reducing film thickness to tens of nanometers, approaching terahertz frequencies.

Findings

Resonant frequencies up to 220 GHz were achieved.

Ultrathin films show increased acoustic losses.

Surface defect mitigation is crucial for future terahertz nanomechanics.

Abstract

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 of mechanical motion at such high frequencies present significant challenges, primarily due to the need for device miniaturization to support acoustic waves with nanometer-scale wavelengths. One effective strategy is to aggressively thin down piezoelectric thin films, ideally to a thickness on the order of the acoustic wavelength, which is in the tens of nanometers. In this work, we aggressively 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 as high as 220 GHz, doubling the previous record and approaching the terahertz frequency 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.