The role of resonance and bandgaps in high $k_\textrm{eff}^2$ transducers

Abstract

Bandgaps formed in a piezoelectric transducer with large coupling, $k_\textrm{eff}^2$, qualitatively modify its electrical response. This regime in which electrical loading strongly couples forward and backward waves occurs in thin-film lithium niobate which has recently become available and amenable to nanopatterning. In this work, we study how resonance and bandgaps modify the design and performance of transducers and delay lines in thin-film lithium niobate. These films are an attractive platform for GHz frequency applications in low-power RF analog signal processing, optomechanics, and quantum devices due to their high coupling, low loss, excellent optical properties, and compatibility with superconducting quantum circuits. We demonstrate aluminum IDTs in this platform for horizontal shear (SH) waves between $1.2$ and $3.3$ GHz and longitudinal waves between $2.1$ and $5.4$ GHz. For the SH waves, we measure a piezoelectric coupling coefficient of $13\%$ and $6.0$ dB/mm propagation losses in delay lines up to $1.2$ mm with a $300$ ns delay in air at room temperature. Reflections from electrical loading when $k_\textrm{eff}^2$ is large lead to a departure from the impulse response model widely used to model surface acoustic wave devices. Finite element method models and an experimental finger-pair sweep are used to characterize the role of resonance in these transducers, illuminating the physics behind the anomalously large motional admittances of these small-footprint IDTs.