On the actual spatial resolution of Brillouin Imaging

Abstract

The comprehension of the behavior of complex heterogeneous materials requires the adoption of imaging techniques to analyze spatially resolved properties. With plenty of prospects in biophysical and biomedical fields, Brillouin imaging is an emerging technique able to generate maps with mechanical contrast at microscales. As in any imaging technique a key parameter is the spatial resolution, which depends both on the control of the probe and on the physical mechanism of interaction with the sample. In particular, in Brillouin Spectroscopy the optical scattering volume is only one of the aspects that determine the spatial resolution, the other being the propagation of the detected vibrational modes, an aspect often overlooked in literature. Here, for the first time, by means of theoretical considerations and new experimental data, the importance of the propagating nature of the phonons is clearly disclosed. The unique features of our experimental set-up allow the simultaneous measure of Brillouin and Raman spectra, probing propagating and localized vibrations, respectively. Varying the optical conditions and testing systems with different phonon propagation length provides striking evidence that in Brillouin imaging the phonons contribution to the resolution, inversely related with the optical focusing, can be dominant. Surprisingly, the resolution can even deteriorate when decreasing the scattering volume.

Mechano-chemistry of human femoral diaphysis revealed by correlative Brillouin–Raman microspectroscopy

“…0.27–0.3 cm −1 ). The simultaneous detection of two Brillouin peaks in a single spectrum suggests the coexistence of elastically different structures at micrometric length scale 25 . The longitudinal elastic modulus M of each component can be obtained from the Brillouin frequency shift ν through the relationship: where ρ is the mass density, n the refractive index and λ the wavelength of the laser source.…”

Brillouin microscopy for the evaluation of hair micromechanics and effect of bleaching

“…The curve fitting routine was written in Fortran77 and was based on a damped harmonic oscillator (DHO) model including convolution with an instrumental Gaussian function [20]. Various spurious effects can cause peak broadening, such as illumination/collection geometry, absorption in strongly scattering media, multiple scattering, and the use of high NA objectives (enabling a range of q values) [31, 32]. In this work, a backscattering geometry was employed together with a small NA objective, thus minimising the effect of spurious Brillouin line broadening.…”

Recent progress and current opinions in Brillouin microscopy for life science applications

“…Early work has determined that Brillouin spectroscopy can probe the complete elasticity tensor and mechanical anisotropy of protein fibers (Randall et al 1979;Cusack and Miller 1979;Cusack and Lees 1984;Palombo et al 2014;Edginton et al 2016;Koski et al 2013) and has shown that the longitudinal modulus is a few orders of magnitude higher than the moduli derived from classical biophysical approaches. This discrepancy is thought to arise from the different spatial and temporal scales of the two measurements (at high and low frequencies), as well as from the different types of moduli, i.e., longitudinal modulus vs. shear and Young's moduli that are more widely used in biophysics and assumed to represent the "stiffness" or "rigidity" (Prevedel et al 2019;Edginton et al 2018;Silvia Caponi and Cavalleri 2019;Andriotis et al 2019). However, the contribution of water to both tissue biomechanics and the Brillouin spectrum is a complicating factor.…”