Sudden spontaneous acceleration and deceleration of gap-acoustic solitons

Gap-acoustic solitons (GASs) are stable pulses that exist in nonlinear Bragg waveguides. They are a mathematical generalization of gap solitons, in which the model includes the dependence of the refractive index on the material density. We derive unified dynamical equations for gap solitons along with Brillouin scattering, which also results from the dependence of the refractive index on the material density. We find accurate values of the coefficients for fused silica. The analysis of the GAS conserved quantities-Hamiltonian, momentum, photon energy (or number of photons), and material mass-shows dramatic differences compared to the model neglecting the dependence of the refractive index on the material density. In particular, subsonic GASs in fused silica have far more momentum at low velocities than at high velocities. The dependence of the GAS momentum on velocity due to acoustic effects is dramatic up to approximately 1% of the speed of light. These momentum-connected effects mean that instability of a slow GAS may make it suddenly accelerate to high speeds, and also that an unstable high-speed GAS can abruptly decelerate to close to zero velocity. The predictions are confirmed by a direct numerical simulation.