The rich and complex dynamics of light and sound interactions in tiny optical waveguides has recently gained much interest because of their experimental realization in emerging key areas of photonics [1,2]. For instance, we recently demonstrated the generation of a new class of surface acoustic waves in a subwavelength-diameter silica optical fiber taper . In such thin silica wires, boundary conditions induce a strong coupling of shear and longitudinal displacements, resulting in a much richer dynamics of light interaction with hybrid acoustic phonons. Avoided crossings can thus occur due the strong photon-phonon coupling and to the fact that these acoustic modes are no longer orthogonal and thus can interact. This interaction can be sufficiently strong enough for certain acoustic frequencies to be forbidden at some anti-crossing points, giving rise to new characteristics such as Brillouin frequency gap.
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