Abstract
Interactions between light and hypersonic waves can be enhanced by tight field confinement, as shown in periodically structured materials1, microcavities2, micromechanical resonators3 and photonic crystal fibres4,5,6 (PCFs). There are many examples of weak sound–light interactions, for example, guided acoustic-wave Brillouin scattering in conventional optical fibres7. This forward-scattering effect results from the interaction of core-guided light with acoustic resonances of the entire fibre cross-section, and is viewed as a noise source in quantum-optics experiments8. Here, we report the observation of strongly nonlinear forward scattering of laser light by gigahertz acoustic vibrations, tightly trapped together in the small core of a silica–air PCF. Bouncing to and fro across the core at close to 90∘ to the fibre axis, the acoustic waves form optical-phonon-like modes with a flat dispersion curve and a distinct cutoff frequency Ωa. This ensures automatic phase-matching to the guided optical mode so that, on pumping with a dual-frequency laser source tuned to Ωa, multiple optical side bands are generated, spaced by Ωa. The number of strong side bands in this Raman-like process increases with pump power. The results point to a new class of designable nonlinear optical device with applications in, for example, pulse synthesis, frequency comb generation for telecommunications and fibre laser mode-locking.
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