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/ \lambda ~\sim ~0.05 ) of X-cut lithium niobate (LiNbO3) (90°, 90°, \psi = 170^{\circ } ). LiNbO3 is known for its large electromechanical coupling ( {K} ^{2} ) for the shear and symmetric Lamb modes (SH0: {K} ^{2} =40 %, S0: {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 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 Y-cut the electrical tuning of the SH0 and S0 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 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.