Speed of sound measurement and mapping in transparent materials by impulsive stimulated Brillouin microscopy

Impulsive stimulated Brillouin scattering (ISBS) is a variant of stimulated Brillouin scattering, which can overcome the shortcomings of the long acquisition time of traditional Brillouin microscopy. We introduce the difference between ISBS and other Brillouin microscopies in calculating longitudinal modulus. The Brillouin frequency shift obtained by ISBS is only related to the system parameters and the speed of sound (SOS) in the sample, not to the refractive index. Non-contact SOS measurement of homogeneous samples is an important application of Brillouin scattering, used in the early study of Brillouin spectroscopy and the mechanical properties of liquids. However, the measurement requires prior knowledge of the sample refractive index, which limits the measurement of the unknown refractive index sample. Here, we propose a method to measure the SOS based on ISBS, which in principle avoids the need for refractive index parameters. The SOS of several liquids are measured and compared with the standard values. The mean relative standard deviation is 1.13%. Moreover, we measure the SOS of a mixture of ethanol and water to demonstrate an application of measuring SOS without refractive index information. We also demonstrate the high spatial resolution of ISBS with a methanol-filled PDMS sample.