Reflection at relativistically moving plasma mirrors is a well-known approach for frequency conversion as an alternative to nonlinear techniques. A key issue with plasma mirrors is the need for a high carrier concentration, of order 10^21 cm^-3, to achieve an appreciable reflectivity. To generate such high carrier concentrations, short laser pulses with extreme power densities of the order >10^15 W/cm^2 are required. Here, we introduce a novel waveguide-based method for generating relativistically moving plasma mirrors that requires much lower pump powers and much less carrier concentration. Specifically, we achieve an experimental demonstration of 35% reflection for a carrier concentration of 5*10^17/cm^3 generated by a power density of only 1.2*10^9 W/cm^2. Both the plasma mirror and the signal are confined and propagating within a solid state silicon slow light photonic crystal waveguide. This extraordinary effect only becomes possible because we exploit an indirect intraband optical transition in a dispersion engineered slow light waveguide, where the incident light cannot couple to other states beyond the moving front and has to reflect from it. The moving free carrier (FC) plasma mirror is generated by two photon absorption of 6 ps long pump pulse with a peak power of 6.2 W. The reflection was demonstrated by the interaction of a continuous wave (CW) probe wave co-propagating with the relativistic FC plasma mirror inside a 400 micro-meter long slow light waveguide. Upon interaction with the FC plasma mirror, the probe wave packets, which initially propagate slower than the plasma mirror, are bounced and accelerated, finally escaping from the front in forward direction. The forward reflection of the probe wave packets are accompanied by a frequency upshift. The reflection efficiency is estimated for the part of the CW probe interacting with the pump pulse.
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