Acoustic transmission across the interface in impulsive stimulated Brillouin microscopy

Brillouin microscopy, an emerging mechanical imaging technology, has made rapid development in recent years. The Brillouin imaging signal is not only determined by acoustic waves in the optical focus volume but also by acoustic waves outside the focus volume. Here, we study how acoustic propagation across the polydimethylsiloxane (PDMS)–ethanol interface affects the heterodyne impulsive stimulated Brillouin scattering (ISBS) signal. When the acoustic direction is perpendicular to the interface, a frequency component corresponding to the PDMS Brillouin shift appears in the signal although the probe focus is in the ethanol. The transmitted wave from PDMS appears when the acoustic wave propagates to the probe focus. To discuss the acoustic contribution of spatial resolution and the acoustic mode propagation distance, it is necessary to consider the acoustic properties of the medium. The influence of transmitted waves on the interference signal is observed in about 100 μm (phonon mean free path in ethanol). Since ISBS analyzes the signal in the time domain, it is possible to distinguish the transmitted wave from the local signal in a single pixel. Different spatial resolutions are obtained by the time-domain method and frequency-domain method. Using mechanical information outside the optical focus volume, ISBS has the potential to image with fewer pixels, which is more flexible and faster than point-by-point scanning.