Enhancement of electromechanical coupling coefficient by proton exchange in Z-cut LiNbO3

The electromechanical coupling coefficient K for surface acoustic waves (SAWs) in proton-exchanged Z-cut X-propagation lithium niobate has been experimentally investigated by the method of evaporating a thin metal film and measuring in situ the SAW attenuation. The enhancement in K2 value by as much as two times caused by the proton exchange has been observed. It is attributed to the redistribution of electric potential of the acoustic wave due to formation of a HxLi1−xNbO3 layer at the surface. The change of electromechanical coupling coefficient in samples fabricated under different conditions depends on the acoustic wavelength–layer thickness product kd. The K2 value attains a maximum of about 0.8×10−2 at kd≈0.4, whereas it is equal to 0.46×10−2 in nonexchanged lithium niobate. The increase in electromechanical coupling coefficient can be applied for enhancement of the SAW interdigital transducer efficiency.