Abstract
A long wavelength optical lattice is generated in a two-level medium bylow-frequency contrapropagating beams. Then a short wave length gap solitongenerated by evanescent boundary instability (supratransmission) undergoes adynamics shown to obey the Newton equation of the parametrically drivenpendulum, hence presenting extremely rich, possibly chaotic, dynamicalbehavior. The theory is sustained by numerical simulations and provides anefficient tool to study soliton trajectories.