Coherent control of flexural vibrations in dual-nanoweb fibre using phase-modulated two-colour CW laser light

摘要

Summary form only given. Optical fields that are spatio-temporally shaped in amplitude and/or phase can be used to coherently control light-matter interactions. An example is the use of femtosecond pulse trains to selectively drive GHz phonons in the core of a photonic crystal fibre (PCF) [1]. These phonons generate, by stimulated Raman-like scattering (SRLS), Stokes and anti-Stokes side-bands within the same optical mode, all spaced by a pump-frequency-independent SRLS shift. Giant SRLS gain was recently measured in a system of two closely spaced and optically-coupled silica “nanowebs” suspended inside an evacuated capillary fibre [2] (Fig. 1(a)). Optical gradient forces give rise to a strong optomechanical nonlinearity, as a result of tight coupling between the optical mode and the lowest-order flexural resonance in each nanoweb (~5.6 MHz). When tuned to this frequency, the beat-note (“optical force”) of two-colour light at ~1550 nm (95% pump and 5% Stokes power) excites a large population of long-lived phonons (lifetime τ). Since the effect of group velocity dispersion is negligible over this small frequency shift, these phonons mediate phase-matched SRLS transitions between many pairs of adjacent side-bands. As a result the beat-note force evolves smoothly along the whole fibre, despite the presence of strong structural non-uniformities [2]. Moreover, since the group velocity of the SRLS phonons is nearly zero, the nanoweb deflection responds synchronously at every position to variations in the launched optical fields.