Brillouin microscopy measures the mechanical properties of biological samples in a non-contact manner, by measuring inelastic Brillouin scattering due to the interaction between acoustic phonon of materials and illuminated photon. The resultant Brillouin frequency shift is related to the longitudinal modulus of materials, which is one of the elastic moduli and can be considered as the inverse of compressibility. Hence, Brillouin microscopy provides mechanical properties of biological samples in a non-contact manner, and can access to characterize the mechanical properties inside cells, e.g. subcellular organelles, with a high spatial resolution (~ 500 nm).

The frequency shift of the inelastically scattered light, called Brillouin frequency shift, is few GHz, which is 5 orders of magnitude shorter than the frequency of incident light. To capture this tiny frequency shift, we have employed Brillouin spectrometer consisting of a virtually imaged phase array (VIPA) etalon. As the modified Fabry-Perot interferometer, VIPA can convert the temporal frequency shift of light into the angular dispersion in a Brillouin spectrum image with a spectral resolution of ~ 10 MHz. From the measured Brillouin spectrum image, the Brillouin frequency shift can be extracted by calculating the distance between Brillouin peaks and Rayleigh peaks due to elastic scattering.