Abstract
The reflection of light from moving boundaries is of interest both fundamentally and for applications in frequency conversion, but typically requires high pump power. By using a dispersion-engineered silicon photonic crystal waveguide, we are able to achieve a propagating free carrier front with only a moderate on-chip peak power of 6 W in a 6 ps-long pump pulse. We employ an intraband indirect photonic transition of a co-propagating probe, whereby the probe practically escapes from the front in the forward direction. This forward reflection has up to 35% efficiency and it is accompanied by a strong frequency upshift, which significantly exceeds that expected from the refractive index change and which is a function of group velocity, waveguide dispersion and pump power. Pump, probe and shifted probe all are around 1.5 µm wavelength which opens new possibilities for “on-chip” frequency manipulation and all-optical switching in optical telecommunications.
Highlights
Reflection of electromagnetic waves from moving boundaries has been of interest for many years due to their potential for frequency conversion
We investigate the reflectivity via the interaction of a continuous wave (CW) probe wave copropagating with the plasma front inside a 400 μm-long slow light waveguide
After injection of the pump pulse into the Photonic crystal (PhC) waveguide, the pulse modifies the optical properties of the waveguide by generating free carrier (FC) via two photon absorption (TPA)
Summary
Reflection of electromagnetic waves from moving boundaries has been of interest for many years due to their potential for frequency conversion. Slow light PhC waveguides allow for tailoring the photonic bands with regions of different group velocities and dispersion[18,19] This way, an ionization front can be generated by a pump moving with a group velocity that is different to the group velocity of the probe[20,21]. The portion of CW light interacting with the front can be determined from the group velocities of the pump and probe and the length of the slow light waveguide This way the energy at the shifted frequency can be directly compared to the energy in this portion of CW light resulting in the conversion efficiency. It will accelerate and bounce a slow probe wave packet propagating in front of it, yet it will transmit a fast probe wave packet approaching from behind
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