Indirect transitions of a signal interacting with a moving refractive index front

Abstract

The dynamic manipulation of light can be achieved b y the interaction of a signal pulse propagating thr ough or reflected from a refractive index front. Both the frequency a nd the wave vector of the signal are changed in thi s case, which is generally referred to as an indirect transition. We have developed a theory to describe such transitio ns in integrated photonic crystal waveguides. Through indirect trans itions, the following effects can be envisaged: lar ge frequency shifts and light stopping and order of magnitude pulse compression and broadening without center frequency sh ift. All effects can be potentially realized with a refractive index modulation as small as 0.001. For the experimental realization, we have used slow light photonic crystal waveguides in silicon. The refractive index front was obtained by free carriers generation with a switching pulse co-propagating with the signal i n the same slow light waveguide. The group velocities of the signal and the front could be varied arbitrarily by choos ing the right frequencies of the signal and switching pulses. The indirect transition was unambiguously demonstrated by considering two situations: a) the front overtaking the signal and b) the signal overtaking the front. In both cas es, a blue shift of the signal frequency was observed. This blue shift can only be explained by the occurrence of the expected indirect transition and not by a direct transition without w ave vector variation.