Towards 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 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.

Fig. 1. Suspended Lamb-wave resonators (LWRs) with varying thicknesses.

a Image of the prepared lithium niobate-on-insulator (LNOI) chip featuring areas with multiple thickness levels: 67, 107, 165, 230, and 300 nm. The variation in colors in the central region is due to the light reflection of a fluorescent lamp, highlighting the differing thickness levels across the chip. Inset shows the incremental thickness steps of lithium niobate (LN) film. b Image of the chip with fabricated devices. Five regions labeled (i) to (v) are marked. c Cross-sectional schematics of the suspended LWRs with varying LN thicknesses: 67 nm (i), 107 nm (ii), 165 nm (iii), 230 nm (iv), 300 nm (v).

Fig. 2. Electrical responses of nanomechanical resonators.

a Illustration of thickness-shear (TS) mode displacement for varying film thicknesses, all with similar frequencies around 150 GHz, with mode orders indicated. b Reflection (Γ) spectra for devices with thicknesses of 67 nm, 107 nm, 165 nm, 230 nm, and 300 nm, plotted over 110–220 GHz. Mode orders are labeled adjacent to respective resonances. Curves are shifted vertically for clarity. c Smith chart representations of the spectra for devices with thicknesses of 300 nm (i), 230 nm (ii), and 165 nm (iii). Mode orders are labeled adjacent to respective resonances. The origin is marked by a black dot.

Fig. 3. Study of quality factors (Qs) in LN resonators.

a Extracted Qs of mechanical resonances across different mode orders for devices of varying thicknesses. Each curve represents a single device. b Scatter plots displaying Qs at approximately 63 GHz (i) and 168 GHz (ii) for devices of different thicknesses. Each color represents a group of multiple devices with the same thickness.

Fig. 4. Process-induced material damage.

a X-ray reflectometry (XRR) measurements of LN thin films of varying thicknesses (63, 185, and 300 nm). Curves are shifted vertically for clarity. The 63 nm and 185 nm films are thinned from 300 nm using ion milling. The inset illustrates the measurement schematic and the multilayer structure, including the hypothesized damaged layer. b Schematic illustration of the selective wet etching process. A bare LN chip is first thinned using ion milling, and annealed at 200 °C for 24 h. Then half of the chip is masked with photoresist and immersed in 49% hydrofluoric (HF) acid for 3 min. c Thickness maps obtained from optical interferometry for LN chips with initial thicknesses of 70 nm and 180 nm, respectively. A height difference of  ~20 nm is observed between the HF-exposed and photoresist-protected regions, indicating the removal of a damaged surface layer. d Schematic illustration of the damaged surface layer. The scattering length density (SLD) gradually increases from the damaged (piezoelectrically inactive) surface to the crystalline LN. XRR primarily detects the upper region with a sharp SLD contrast (blue dashed line), while HF etching could remove chemically modified material deeper into the film (green dashed line).

Fig. 5. Frequency scaling trend of micro/nanomechanical resonators (published works are dated by their publication dates; this work is dated by its submission date).

The mechanical platforms are categorized as follows: beam^–, thin-film bulk acoustic resonator (FBAR)^–, surface acoustic wave (SAW) resonator^–, optomechanical crystal (OMC)^–, Lamb-wave resonator (LWR)^–,–. The blue dashed reference line marks the THz frequency threshold (300 GHz).