Tightly trapped acoustic phonons in photonic crystal fibres as highly nonlinear artificial Raman oscillators

Interactions between light and hypersonic waves can be enhanced by tight field confinement, as shown in periodically structured materials(1), microcavities(2), micromechanical resonators(3) and photonic crystal fibres(4-6) (PCFs). There are many examples of weak sound-light interactions, for example, guided acoustic-wave Brillouin scattering in conventional optical fibres(7). 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 experiments(8). 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 degrees to the fibre axis, the acoustic waves form optical-phonon-like modes with a flat dispersion curve and a distinct cutoff frequency Omega(a). This ensures automatic phase-matching to the guided optical mode so that, on pumping with a dual-frequency laser source tuned to Omega(a), multiple optical side bands are generated, spaced by Omega(a). 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.