Anisotropic elastic properties of synthetic and natural fibers determined by micro-Brillouin light spectroscopy

Synthetic and natural fibers with diameters in the range of a few tens of micrometers can be routinely fabricated. Because of the intricate microstructure of the fibers, however, their elastic properties remain poorly understood. In this study, we employ micro-Brillouin light spectroscopy (micro-BLS) to explore direction-dependent acoustic phonon propagation in amorphous E-glass, synthetic silk, polyamide 11 (PA11), and flax fibers. The technique is.non-invasive and non-destructive, and is an alternative to static mechanical tests. The observable frequency shifts of laser light resulting from Brillouin scattering from hypersonic acoustic phonons of the fiber are in a few 10-GHz range. We determine the full elastic tensors and the optical anisotropy, assuming only transversely isotropic symmetry at the optical wavelength scale. The obtained elastic constants are compared with values reported in the literature for similar materials.