The propagation speed of ultrasonic waves is faster in hard materials and slower in soft materials. By propagating an ultrasonic pulse through a transparent material and measuring its position at each moment using probe light, information regarding the elasticity of the material can be obtained. However, let us assume that the physical constants of the material do not change abruptly or significantly, and that the reflections of ultrasound and light within the material are negligible.
A picosecond ultrasonic pulse typically has a spatial width of less than 0.1 ƒÊm, making it smaller than the wavelength of visible light, and it slightly reflects light, acting somewhat like a semitransparent mirror. The weak light reflected by this ultrasonic pulse interferes with light reflected from other parts, such as the surface of the transparent material. As the ultrasonic pulse propagates, the conditions for this interference change, causing the samplefs light reflectance to oscillate very weakly and at high speed.
When the refractive index of the sample is known and its variation with respect to changes in the speed of sound in the sample is small, the instantaneous frequency of these reflectance oscillations can be used to determine the sound speed. In our research on 3D animal-cell imaging with picosecond ultrasonics, we applied this assumption to investigate the elasticity and attenuation of ultrasound within cells.
In cases where both the change in the speed of sound and the change in the refractive index of light must be considered for the sample, how should we proceed? We have developed two measurement methods to address this challenge.
One approach is to combine reflectance measurements from multiple angles. When light is directed at an angle onto the sample, refraction at the surface allows the separation of information regarding both sound speed and refractive index. However, the optical setup for directing light at various angles and detecting the reflected light can become quite complex. To address this issue, we use a high numerical aperture objective lens —typically used to focus light to a tight spot— to instead direct light at an angle. For more details see 'Time-domain Brillouin imaging of sound velocity and refractive index using automated angle scanning,' M. Tomoda, A. Kubota, O. Matsuda, Y. Sugawara, and O. B. Wright, Photoacoustics. 31, 100486 (2023).
Conceptual diagram of the angle scanning (the left figure) and graph of reflectance change with probe light displacement on the objective lens and delay time (the right figure)
The other method involves directing light from the side of a specially shaped sample to counteract the effects of refractive index changes. Although this approach imposes significant limitations on the types of samples that can be used, it allows for a more accurate determination of the sound speed distribution, as the only unknown variable is the speed of sound. For more details see 'Sound velocity mapping from GHz Brillouin oscillations in transparent materials by optical incidence from the side of the sample,' M. Tomoda, A. Toda, O. Matsuda, V. E. Gusev, and O. B. Wright, Photoacoustics. 30, 100459 (2023).
Schematics of optical incidence from the side of the sample (left) and measured reflectance change (right)