Optically Induced Indirect Photonic Transitions in a Slow Light Photonic Crystal Waveguide

We demonstrate indirect photonic transitions in a silicon slow light photonic crystal waveguide. The transitions are driven by an optically generated refractive index front that moves along the waveguide and interacts with a signal pulse copropagating in the structure. We experimentally confirm a theoretical model which indicates that the ratio of the frequency and wave vector shifts associated with the indirect photonic transition is identical to the propagation velocity of the refractive index front. The physical origin of the transitions achieved here is fundamentally different than in previously proposed refractive index modulation concepts with fixed temporal and spatial modulation frequencies; as here, the interaction with the refractive index front results in a Doppler-like signal frequency and wave vector shift. Consequently, the bandwidth over which perfect mode frequency and wave vector matching is achieved is not intrinsically limited by the shape of the photonic bands, and tuning of the indirect photonic transitions is possible without any need for geometrical modifications of the structure. Our device is genuinely nonreciprocal, as it provides different frequency shifts for co- and counterpropagating signal and index fronts.

Figure 1

(a)–(b) Schematic of the experiment. A switching pulse with high peak power generates free carriers in the silicon by two-photon absorption and, consequently, induces a change of refractive index which propagates with the velocity of the switch. (a) The switching pulse is faster than the signal, corresponding to Fig. 2. (b) The signal pulse is faster than the switch, corresponding to Figs. 2 and 2. (c) Transmission and (d) group index measured on the fabricated photonic crystal waveguide.

Figure 2

Schematic representation of transitions, measured relative center wavelength, and selected signal spectra in the different configurations: (a),(b),(g) $n_{g,\text{signal}}=29$, $n_{g,\text{switch}}=11$, correspond to Fig. 1; (c),(d),(h) $n_{g,\text{signal}}=11$, $n_{g,\text{switch}}=32$, correspond to Fig. 1; (e),(f),(i) $n_{g,\text{signal}}=26$, $n_{g,\text{switch}}=32$, correspond to Fig. 1.