Local probing of higher-order dispersion in photonic crystal waveguides

One of the exciting features of photonic crystals is that light may travel at very low group velocities at specific optical frequencies. In photonic crystal waveguides, such low group velocities are also possible. We have studied the propagation of femtosecond pulses in these waveguides, with a phase-sensitive and time-resolved near-field microscope. With this microscope, we visualized the pulses as they propagate in the waveguide. Moreover, we monitored the pulse envelope in time, at various positions in the structure, in order to study the pulse dispersion. We found, that the pulses indeed propagate more slowly as we approached the Brillouin zone boundary by decreasing the optical frequency. As the group velocity is reduced, a broadening of the pulses was observed. On top of that, the pulses exhibited a strong asymmetric distortion as the propagation distance increased. This asymmetric broadening can be attributed to an increase in higher-order dispersion. Our findings show, that if the most commonly used slow light regime in photonic crystals is to be exploited, great care has to be taken about higher-order dispersion, even at moderate group delays