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 (LiNbO 3 ) (90°, 90°, ψ = 170°). LiNbO 3 is known for its large electromechanical coupling (K 2 ) for the shear and symmetric Lamb modes (SH 0 : K 2 = 40%, S 0 : K 2 = 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 LiNbO 3 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 S 0 mode up to 1.6% and for V-cut the electrical tuning of the SH 0 and S 0 modes was up to 7.0% with K 2 = 27.1%. The mechanism is based on the nonlinearities that exist in the piezoelectric properties of LiNbO 3 . The X-cut SH 0 mode resonators were centered near 335 MHz and achieved a frequency tuning of 6 kHz/V through the application of a dc bias.