Gan W.

Negative refraction has been verified for sound waves using phononic crystals as focussing lens and shows improvement in resolution limit. In this paper, we apply negative refraction to nondestructive evaluation. An experimental setup for negative refraction is given. The transmission mode is used. This is the transmission of ultrasonic wave from the source through the object and incident onto the negative refraction lens(NRL). The object has to be immersed in water which acts as a couplant. We consider the theoretical approach to a homogeneous isotropic solid bordered by a plane interface in a fluid and we discuss how the different modes of wave propagation are involved in the process. We start with the wave equation for elastic waves in solids with the conditions that there are no external forces(body forces) present on the medium. It also assumes that the medium is lossloss. The expression for the transmitted wave through the object is obtained. This will be the incident wave impinging onto the negative focussing lens(NFL). The multiple scattering theory(MST) is used to calculate the transmmision coefficient through the NFL. We show how a dramatic variation in wave propagation with both frequency and propagation direction leads to novel focussing phenomena associated with large negative refraction. These effects are then used using ultrasonic techniques to image the transmitted wave field. The fields scattered are calculated theoretically using a Fourier imaging technique in which wave propagation through the crystal is accurately described by the 3D equifrequency surfaces predicted from the MST. We analyze the role played by the evanescent waves by including them in the image together with the propagating wave. This shows an improvement in the resolution.