Fast, all-optical Rayleigh wave microscope: Imaging on isotropic and anisotropic materials

A fast, non-contact Rayleigh wave scanning microscope is demonstrated, whic h is capable of scan rates of up to a maximum of 1000 measurements/s with t ypical speeds in up to 250 measurements/s on real samples. The system uses a mode-locked, Q-switched Nd:YAG laser operating at a mode-locked frequency of 82 MHz and a Q-switch frequency of 1 kHz. The Q-switch frequency determ ines the upper limit of the scanning rate. The generating laser illuminatio n is delivered and controlled by a computer-generated hologram (CGH). The g enerating laser produces around 30 pulses at 82 MHz and additional harmonic s at 164 and 246 MHz and above. The microscope can operate at these harmoni cs provided the spatial bandwidth of the optics and the temporal bandwidth of the electronics are suitable. The ultrasound is detected with a speciali zed knife-edge detector. The microscope has been developed for imaging on isotropic materials. Despi te this, the system can be used on anisotropic materials, but imaging and i nterpreting images can be difficult. The anisotropy and grain structure of the material can distort the Rayleigh wavefront, leading to signal loss. A model has been developed to simulate polycrystalline-anisotropic materials; this is discussed along with possible solutions that would overcome the pr oblems associated with anisotropy. Rayleigh wave amplitude images are demonstrated on silicon nitride at 82 an d 164 MHz and on polycrystalline aluminium at 82 MHz.