Optical nanofibers (ONFs) represent versatile nanophotonic platforms for important photonic applications such as optical sensing and quantum and nonlinear optics. The attractiveness of ONFs arises from the tight optical confinement, their wide evanescent field in the subwavelength limit, their surface acoustic properties, and their high tensile strength. Here we investigate Brillouin light scattering in silica-glass ONFs under high tensile strain and show that the fundamental properties of elastic waves dramatically change due to elastic anisotropy and nonlinear elasticity for strain larger than 2%. This yields to unexpected Brillouin strain coefficients for all Brillouin resonances including surface and hybrid waves, followed by a nonlinear evolution at high tensile strength. We further provide a complete theoretical analysis based on third-order nonlinear elasticity of silica that agrees well with our experimental data. These new regimes open the way to the development of compact tensile strain optical sensors based on nanofibers.
