Picosecond acoustic pulses are tiny in extent, typically less than 0.1 μm long. Because of this, until now we have not been able to measure their shape as they move. We presented a tomographic method for recording a longitudinal picosecond acoustic pulse as it travels through a transparent solid into a journal paper 'Tomographic reconstruction of picosecond acoustic strain propagation,' M. Tomoda, O. Matsuda, O. B. Wright and R. Li Voti, Appl. Phys. Lett. 90, 041114 (2007). Here we present an upgraded method and results.
We use a specially shaped sample in the form of a 10 mm diameter glass hemisphere, with the flat surface thinly coated with aluminum. Picosecond acoustic pulses generated by infrared laser pulses in the aluminum then travel into the glass, where we detect them with blue laser pulses. See the figure.
The schematics of the picosecond acoustic tomography, and angular dependence of reflectance change and reconstructed ultrasonic pulse distribution at each time
By rotating the hemisphere and making measurements at different angles we build up a databank of results that can be analysed by a computer algorithm. The result is that we can extract the shape of the acoustic pulse as it moves through the glass. See the animation below.
Click the image to see a 254 kB animation of the picosecond acoustic pulse in the glass.
Three graphs are displayed in animation. The one labeled "Theory" is a simulation of the ultrasonic pulse waveform at each time. The one labeled "Reconstruct" is a reconstruction of the spatial distribution of the ultrasonic pulse from the angle dependency of the reflectivity change under the same conditions as the experiment, using this method based on the waveform of Theory in an ideal situation with absolutely no noise, etc. The one labeled "Experiment" is the experimental result of this method.
The frequency of this acoustic pulse is about 20 GHz. You can also make out a second, smaller acoustic pulse later in the animation. This is an acoustic echo generated at the interface between the glass and the aluminum film, traveling once throught the aluminum film and then propagating back to the glass hemisphere.
This method should prove useful for understanding the shape of picosecond acoustic pulses and how they change when travelling. We hope that we can ultimately follow even shorter acoustic pulses by the use of shorter optical wavelengths. For more details see 'Tomographic reconstruction of picosecond acoustic strain pulses using automated angle-scan probing with visibule light,' M. Tomoda, H. Matsuo, O. Matsuda, R. Li Voti and O. B. Wright, Photoacoustics. 34, 100567 (2023).