Two regimes of slow-light losses revealed by adiabatic reduction of group velocity.

The losses in a photonic crystal waveguide were measured with a near-field microscope in the group velocity range of c/7 down to c/200. Our measurements show that the losses scale proportional to v{g};{-2} for group velocities above c/30. Below c/30, the losses are no longer described by the same power-law dependence on v{g} and the modal pattern becomes irregular, indicative of multiple scattering. The findings indicate the existence of two regimes of slow-light losses: one where a perturbative approach describes propagation with fabrication disorder and one where it breaks down.