The effect of nonlinearity on adiabatic evolution of light

We investigate the effect of nonlinearity in a system described by an adiabatically evolving Hamiltonian. Experiments are conducted in a three-core waveguide structure that is adiabatically varying with distance, in analogy to the STIRAP process in atomic physics. In the linear regime, the system exhibits an adiabatic power transfer between two waveguides which are not directly coupled, with negligible power recorded in the intermediate coupling waveguide. In the presence of nonlinearity the behavior of this configuration is drastically altered and the adiabatic light passage is found to critically depend on the excitation power. We show how this effect is related to the destruction of the dark state formed in the STIRAP configuration.