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

Cheap Optical Transducers (CHOTs) have been demonstrated for the generation and detection of elastic waves at typical NDE frequencies between 0.5 and 100MHz [1]. In this paper, we discuss how the CHOTs [1] operate and how this breaks down at high frequencies. We also show that it is possible to extend the frequency range upwards towards the multi GHz region using a new evanescent wave CHOT. The "evanescent CHOTs" (eCHOTs) have a different mode of operation from the normal CHOTs. While the CHOTs depend on an energy gain-loss system dominated by the diffracted orders, the eCHOTs depend on a system dominated by resistive heating losses as there are no diffraction orders for the optical wavelength used. We have designed and optimised these transducers using Finite Element Method (FEM) modelling to link the physical displacements caused by the elastic waves with the optical behaviour. We verified the operation mechanism and demonstrated that the devices showed enhanced sensitivity over a wider range of parameters than possible with a device designed for conventional CHOT specifications, making the eCHOT an ideal candidate for GHz / nano scale ultrasonics. The operating mechanism of the eCHOT suggests that more exotic structures could be built to enhance the sensitivity of these devices.