Tracking the spectral evolution of slow light en route

This paper reports on experimental investigations of the evolution of the spectral density of an ultrashort laser pulse as it propagates through a composite sample consisting of a slow-light photonic crystal (PhC) waveguide and polymer access waveguides. A novel method of near-field scanning optical microscopy was used to measure the spectrum at different locations, and for different powers, only inside the PhC waveguide. This is in contrast to traditional transmission measurements where the evolution of the spectrum due to the entire system is observed.