Loss mechanisms of microwave frequency acoustic waves in thin film lithium niobate

Abstract

Thin-film lithium niobate (TFLN) has emerged as a versatile platform for phononic and photonic devices with applications ranging from classical signal processing to quantum technologies. However, acoustic loss fundamentally limits the performance of acoustic devices on TFLN platforms, yet its physical origin remains insufficiently understood. Here, we systematically investigate acoustic propagation loss in various TFLN platforms, including lithium niobate on insulator (LNOI), lithium niobate on sapphire (LNOS), suspended LN thin films, and bulk LN at gigahertz frequencies over temperatures ranging from 4 K to above room temperature. Using a delay-line method, we extract frequency- and temperature-dependent losses for Rayleigh, shear-horizontal, and Lamb modes. We observe an anomalous non-monotonic temperature dependence in LNOI that closely resembles acoustic loss in amorphous materials, indicating a dominant loss channel associated with the buried oxide layer at low temperatures. At elevated temperatures, the loss converges to the Akhiezer damping governed by phonon-phonon interactions. High-resolution electron microscopy further reveals nanoscale interfacial crystal impurities that may contribute to the increased acoustic loss in TFLN platforms relative to bulk LN. These results elucidate the acoustic loss mechanisms in TFLN and provide guidelines for designing low-loss acoustic devices.

Figures (5)

• Figure 1: Device design and measurement principle. (a). An optical microscope image of an IDT pair fabricated on x-cut LNOI substrate. Two slightly chirped IDTs are fabricated along the LN $y$-axis with a delay length $L$. (b) Schematic illustration of acoustic propagation loss measurement scheme using a delay line. (c). Representative S-parameter measurement results of the device. Region I and II correspond to the excitation of Rayleigh and shear-horizontal acoustic modes, respectively. (d). Finite-element-method (FEM) simulations showing the displacement field distributions of the two acoustic modes.
• Figure 2: Acoustic loss measurement using a delay-line method. (a). Time-domain signals obtained by taking the inverse Fourier transform of the $S_{12}$ response within the 1.82-1.87 GHz frequency range. Each trace corresponds to an IDT pair with a different delay length $L$, varying from $400~\mu \mathrm{m}$ (bottom) to $800~\mu \mathrm{m}$ (top), with vertical offsets applied for clarity. The shaded wave packets indicate acoustic pulses arriving at the receiving IDT via direct transmission or multiple round-trip reflections. Cross and circle markers denote the peak amplitudes of the first and the second arrival wave packets, respectively. Measurements are performed at room temperature. (b). Acoustic group velocity extracted from a linear fit of the delay time of the first wave packet as a function of the corresponding delay length. (c). A 6.97 dB/mm propagation loss extracted from a linear fit of the measured amplitude ratio between the first and the second wave packets at different delay lengths $L$. (d). Frequency dependence of acoustic propagation loss, fitted with a quadratic function of frequency. The shaded regions in this figure and the following indicate the 95% confidence interval.
• Figure 3: Temperature dependence of acoustic loss in TFLN platforms. (a) Acoustic propagation loss of different acoustic modes measured in LNOI and LNOS in the temperature range from 4 to 400 K. (b) The inverse quality factors extracted from measurements on the LNOI sample. (c) Zoom-in view of the acoustic propagation loss versus temperature in LNOS with a linear fit, where T is the temperature (K).
• Figure 4: Temperature-dependent acoustic propagation loss of (a). Fundamental Rayleigh mode of the bulk LN and (b). Fundamental asymmetric Lamb (A0) mode of the suspended LN. The inset shows the FEM simulation of the A0 mode profile of a 300 nm thick LN with 100 nm Au electrodes on top.
• Figure 5: Cross-sectional HAADF-STEM images of the TFLN sample along the $<0001>$ zone axis, showing (a) Lattice continuity at the bonding interface of LNOI; (b) Atomic lattice periodicity of lithium niobate on LNOI; (c) Lattice continuity at the bonding interface of LNOS.