Study of a high efficiency optical MEMS transducer for the generation of narrowband laser ultrasound

In this paper we demonstrate an optically powered ultrasonic t ransducer. It has a high efficiency and was designed and fabricated using MEMS (microelectromechanical system) techniques. It can generate narrowband ultrasound from broadband laser excitation. It is a simple two-mask-level MEMS device with a micro-disc seated on a micro-stem. As a laser pulse is incident on the disc centre, the disc is excited into a 'flapping' motion because of the thermomechanical interaction between the absorbing and non-absorbing parts of the disc. This flapping motion is dominated by one of the resonances of the disc, coupling a narrowband longitudinal bulk wave propagating along the axis of the micro-stem into the sample. Experiments with these transducers have shown that narrowband ultrasonic waves with a high SNR (signal to noise ratio) were generated successfully. The device is simple to excite optically and generates higher amplitudes than by normal thermoelastic generation. No physical contact is required to excite the transducer, making it suitable for remote non-contact ultrasonic applications.