Picosecond Ultrasonic Measurements Using an Optical Cavity

In this method we place a distributed Bragg reflector (DBR) on the film sample to be studied so that an optical cavity is formed in the space between the sample and the DBR. We can choose any cavity spacing from 400nm to a few microns to perform the picosecond ultrasonic measurement. And we chose a cavity spacing where pump light pulse is at resonance and is most sensitive to the change of reflectivity of the probe light pulse. With this method we achieved significant signal enhancment as shown in figure 1.

Figure 1. Measured change of reflectivity of the same Al film when the cavity is not used in (a) and used in (b) with cavity spacing of 2005nm. The wavelength of both the pump and the probe light is 800nm.

With this method we were able to extract more information in the picosecond ultrasonic measurements, such the actual shape of the returning acoustic pulse. We were also able to perform measurements on materials with very small piezo-optic coefficients.

For more details, please refer to our publication: Picosecond Ultrasonic Measurements Using an Optical Cavity, J. Appl. Phys. 105, 083516 (2009)