Evanescent CHOTs for the optical generation and detection of ultrahigh frequency SAWs

The use of optical gratings for the generation and detection of ultrahigh frequency narrowband surface acoustic waves is well known. Detection of surface acoustic waves (SAWs) using an optical grating, (without an interferometric set-up) relies on diffraction of light from the grating. Cheap Optical Transducers (CHOTs) for SAWs are optical ultrasonic transducers which are designed to optimise the diffraction from such gratings to yield a local interferometer. They have been demonstrated for typical NonDestructive evaluation (NDE) frequencies (0.5 and 100 MHz). In this paper, we discuss how the CHOTs operate and how their detection mechanism breaks down, when the optical diffraction orders from the grating become evanescent upon moving to multi GHz frequency range. We show that it is possible to design devices with enhanced sensitivity, in this range. This is done by adding a thin "background film" underneath grating fingers, in order to optimise and exploit the interaction between the deformation of the grating and the evanescent fields/ resonances involved in such gratings. The result is a novel operating mechanism, based on an energy balance between the 0-order reflection and resistive heating. These novel transducers are called evanescent wave CHOT (eCHOT). We have used Finite Element Method (FEM) modelling to link the physical displacements caused by the elastic waves with the optical behaviour. We demonstrated that the devices show enhanced sensitivity over a wider range of parameters, than possible with a device designed with conventional CHOT specifications, making the eCHOT an ideal candidate for GHz / nano scale ultrasonics. The eCHOT involves highly resistive thin films and yields thin structures advantageous for ultrahigh frequency ultrasonics. The operating mechanism of the eCHOT suggests that more exotic structures could be built to enhance the sensitivity of these devices.