Fig. 1

Schematic of the interaction between front and probe. a Schematic representation of an intraband indirect photonic transition in a slow light silicon PhC waveguide. The solid curve represents the dispersion band of waveguide mode in the ground state (with refractive index \(n_{{\mathrm{si}}}\)), while the dashed curve indicates the switched state with refractive index \(n_{{\mathrm{si}}} + \Delta n_{{\mathrm{FC}}}\). We use the PhC band with positive group velocity to represent the dispersion relationship. The orange line represents the phase continuity line with a slope equal to the group velocity of the pump pulse set at the knee of the dispersion band (orange dot). The red and blue dots indicate the initial (slow mode) and final (fast mode) states of a probe wave, respectively. b Schematic of the experiment. A pump pulse generates FCs in the silicon by TPA and, consequently, induces a change of refractive index which propagates with the velocity of the pump pulse. The region with the red colour gradient corresponds to the rising edge of the front. The orange arrow indicates the velocity of the pump pulse, while red, green and blue arrows indicate the velocities of a wave packet of the probe at different times, respectively