Investigation of a Solid-State Tuning Behavior in Lithium Niobate

Electric field-based frequency tuning of acoustic resonators at the material level may provide an enabling technology for building complex tunable filters. Tunable acoustic resonators were fabricated in thin plates (h/λ ~ 0.05) of X-cut lithium niobate (LiNbO₃) (90°, 90°, ψ = 170°). LiNbO₃ is known for its large electromechanical coupling (K²) for the shear and symmetric Lamb modes (SH₀: K² = 40%, S₀: K² = 30%) in thin plates and, thus, applicability for low-insertion loss and wideband filter applications. We demonstrate the effect of a dc bias in X-cut LiNbO₃ to shift the resonant frequency by ~0.4% through direct tuning of the resonator material. A nonlinear acoustic computation predicted 0.36% tuning, which was in excellent agreement with the tuning measurement. For X-cut, we predicted electrical tuning of the S₀ mode up to 1.6% and for V-cut the electrical tuning of the SH₀ and S₀ modes was up to 7.0% with K² = 27.1%. The mechanism is based on the nonlinearities that exist in the piezoelectric properties of LiNbO₃. The X-cut SH₀ mode resonators were centered near 335 MHz and achieved a frequency tuning of 6 kHz/V through the application of a dc bias.