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.