Brillouin Microscopy

In line with our investigation of the mechanical properties of biological systems, our group employs Brillouin microscopy to evaluate the viscoelastic properties of biological samples through photo-acoustic interactions at the [GHz] timescale.

This technique involves focusing an incident laser beam on a region of interest, where inherent local density variations, arising from microscopic statistical fluctuations, generate propagating pressure waves, or acoustic waves. The incident light probes these acoustic waves and undergoes scattering. While most of this scattering is elastic (Rayleigh scattering), a small fraction is inelastic (Brillouin scattering), resulting from energy exchange and a corresponding frequency shift. The local speed of sound, which can be approximated from the detected frequency shift, is directly proportional to the storage modulus, serving as a proxy for the material’s elasticity. Additionally, the linewidth of the detected Brillouin peak in the spectrum is proportional to the damping or attenuation of the propagating sound wave, relating to the loss modulus and serving as a proxy for the material's viscosity. By mapping both the frequency shift and linewidth, we can visualize the mechanical properties of the probed region.

Our group utilizes Brillouin microscopy to investigate the viscoelastic properties of various biological systems, including but not limited to, cells and sub-cellular compartments (1), as well as the central nervous system (CNS) tissue of zebrafish larvae in vivo to study mechanical changes following spinal cord injury (2, 3).

Furthermore, with knowledge of the local density distribution and refractive index of the sample, it is possible to extract the absolute values of the storage and loss moduli. Motivated by these capabilities, we have developed a microscopy setup that combines Brillouin microscopy with optical diffraction tomography (ODT) and fluorescence microscopy. This integrated setup allows for the correlative analysis of Brillouin frequency shift and mass density, with specificity to fluorescent-labelled structures.