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Reflection at relativistically moving plasma mirrors is a well-known approachfor frequency conversion as an alternative to nonlinear techniques. A key issuewith plasma mirrors is the need for a high carrier concentration, of order10^21 cm^-3, to achieve an appreciable reflectivity. To generate such highcarrier concentrations, short laser pulses with extreme power densities of theorder >10^15 W/cm^2 are required. Here, we introduce a novel waveguide-basedmethod for generating relativistically moving plasma mirrors that requires muchlower pump powers and much less carrier concentration. Specifically, we achievean experimental demonstration of 35% reflection for a carrier concentration of5*10^17/cm^3 generated by a power density of only 1.2*10^9 W/cm^2. Both theplasma mirror and the signal are confined and propagating within a solid statesilicon slow light photonic crystal waveguide. This extraordinary effect onlybecomes possible because we exploit an indirect intraband optical transition ina dispersion engineered slow light waveguide, where the incident light cannotcouple to other states beyond the moving front and has to reflect from it. Themoving free carrier (FC) plasma mirror is generated by two photon absorption of6 ps long pump pulse with a peak power of 6.2 W. The reflection wasdemonstrated by the interaction of a continuous wave (CW) probe waveco-propagating with the relativistic FC plasma mirror inside a 400 micro-meterlong slow light waveguide. Upon interaction with the FC plasma mirror, theprobe wave packets, which initially propagate slower than the plasma mirror,are bounced and accelerated, finally escaping from the front in forwarddirection. The forward reflection of the probe wave packets are accompanied bya frequency upshift. The reflection efficiency is estimated for the part of theCW probe interacting with the pump pulse.

Highly effective relativistic free carrier plasma mirror confined within a silicon slow light photonic crystal waveguide