  The spectral evolution and spatial distribution of backscattered Brillouin
signals is experimentally investigated in sub-wavelength silica microfibers.
The Brillouin spectrum evolution reveals the different dynamics of the various
peaks, offering evidence of backscattering signals induced by acoustic waves
with phase velocity greater than that of the longitudinal wave. The spatial
distribution is found to have significant influence on the response of
Brillouin scattering under tensile load, with hybrid acoustic modes providing a
smaller response under axial strain. This insight into interactions between
optical and hybrid acoustic modes at sub-wavelength confinements could help
understand ultrasonic waves in tapered waveguides, and have potential
applications in optical sensing and detection.
