Fast laser generated Rayleigh wave scanning microscope

A fast non-contact Rayleigh wave scanning microscope is demonstrated. This is capable of scan rates of up to a maximum of 1000 measurements per second with typical speeds in excess of 40 measurements per second 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 determines the upper limit of the scanning rate. The generating laser illumination is delivered and controlled by a computer generated hologram. The generating laser produces around 30 pulses at 82 MHz and additional harmonics at 164 MHz, 246 MHz and above. The microscope can operate at these harmonics provided the spatial bandwidth of the optics and the temporal bandwidth of the electronics are suitable. The ultrasound is detected with a specialized knife-edge detector. This technique is very successful on isotropic materials. At present imaging and interpreting images taken using this technique can be difficult on anisotropic materials because the anisotropy can distort the Rayleigh wavefront leading to signal loss. Rayleigh wave amplitude images are demonstrated on silicon nitride at 82 MHz and 164 MHz and aluminum at 82 MHz.