Enhancement of picosecond ultrasonic signals through the use of an optical cavity

We report an experiment in which an optical cavity is employed to enhance picosecond ultrasonic signals. The optical cavity is a Fabry-Perot interferometer formed by a distributed Bragg reflector (DBR) and the sample to be studied. By adjusting the thickness of the air gap between the DBR and the sample, the optical cavity can be tuned to be most sensitive to the sound-induced surface displacement of the sample. It is demonstrated that in the presence of the optical cavity, the efficiency of both the generation and detection of sound is improved. We furthermore show that this technique provides an efficient method of detecting sound in materials in which the piezo-optic coefficients are close to zero.