  Optical nanofibers (ONFs) are excellent nanophotonic platforms for various
applications such as optical sensing, quantum and nonlinear optics, due to both
the tight optical confinement and their wide evanescent field in the
sub-wavelength limit. Other remarkable features of these ultrathin fibers are
their surface acoustic properties and their high tensile strength. Here we
investigate Brillouin light scattering in silica-glass tapered optical fibers
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 and remarkable 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 remarkably agrees with our experimental data. These
new regimes open the way to the development of compact tensile strain optical
sensors based on nanofibers.
