Brillouin microscopy
Brillouin microscopy recently emerged as a non-contact technique to probe the mechanical response of biological materials.
Brillouin microscopy measures the mechanical response of a material all-optically and without any contact (Scarcelli & Yun, 2008). When light propagates through a sample, it is scattered by thermally-driven acoustic waves (phonons): the scattered light is frequency-shifted by ±νB — the Brillouin shift — around the elastic (Rayleigh) line. This shift is directly related to the longitudinal elastic modulus (through the speed of sound and the density), while the linewidth reflects the viscosity. Measuring νB point by point yields a quantitative, label-free map of the local stiffness (Prevedel et al., 2019).
Brillouin scattering: light exchanges energy with acoustic phonons, giving Stokes / anti-Stokes peaks shifted by ±νB around the elastic line. The shift νB encodes the longitudinal modulus, its width the viscosity.
The main experimental challenge is that the Brillouin signal is extremely weak and sits only a few GHz away from the much stronger elastic line. This calls for a spectrometer that is both high-resolution and high-extinction. We use a double-stage VIPA spectrometer (Zhang et al., 2021): a VIPA is a tilted, partially-reflective etalon that angularly disperses the optical frequencies, turning a tiny GHz shift into a measurable displacement on the camera. Crossing two VIPAs and adding a spatial mask rejects the residual elastic light and provides the extinction needed to recover the Brillouin peaks.
The two-VIPA Brillouin spectrometer: optical layout (left), raw dispersed pattern on the camera (centre) and resulting spectral response showing the obtained attenuation.
As a proof of concept, we image a polystyrene bead immersed in water: the maps of Brillouin shift, linewidth and intensity clearly resolve the mechanical contrast between the stiff bead and its soft surroundings.
Brillouin imaging of a polystyrene bead — maps of the Brillouin shift, linewidth and peak intensity (scale bar 4 µm).
The goal is now to apply this approach to soft connective tissues — vocal cords, cartilage, cornea — to probe their biomechanics in situ. Combined with second-harmonic generation imaging of the collagen network, Brillouin microscopy lets us correlate the architecture of the tissue with its local mechanical properties, linking structure and function at the microscale.