Acoustic Gain in Solids due to Piezoelectricity, Flexoelectricity, and Electrostriction

Abstract A quantitative discussion of the combined influence of three electromechanical effects: piezoelectricity, flexoelectricity, and electrostriction in solids is provided for acoustic absorption and gain. While piezoelectricity occurs in non‐centrosymmetric materials only, flexoelectricity and electrostriction exist in all materials. Two important new results are demonstrated: 1) the possibility to realize acoustic gain in all materials (centrosymmetric and non‐centrosymmetric) when the acoustic Cherenkov condition is fulfilled, and 2) realization of acoustic gain in the presence of a strong dc electric field, even when the Cherenkov condition is not fulfilled, in the case of strong cross‐coupling between piezoelectricity, flexoelectricity, and electrostriction. A simple analytical expression for the acoustic dispersion relation is derived for the combined effect of piezoelectricity, flexoelectricity, and electrostriction. At lower frequencies, the piezoelectric effect dominates for inversion‐asymmetric materials. At high frequencies (≈>1 MHz) flexoelectricity becomes increasingly important and eventually provides a major mechanism for gain and absorption in barium titanate (BaTiO3). In the presence of strong electric fields (≈>1 MV m−1), electrostriction provides a dominant isolated contribution to absorption/gain in BaTiO3. Strong coupling between the three electromechanical contributions determines the total absorption/gain coefficient.