Robust generation of relativistic mirrors from laser wakefields for enhanced laser backscattering

By adopting an up-ramp density profile, we propose to generate relativistic electron mirrors from laser-driven underdense plasma waves, which are insensitive to finite thermal temperature in certain range. Along the density ramp, premature plasma wave-breaking due to thermal effects is shown to be well mitigated, and overcritical dense electron sheets can pile up when approaching the end of the up-ramp. The consequent mirror speed can be stably driven to the group velocity of the laser propagating in a corresponding homogeneous plasma. Such a mirror is suitable for coherent backscattering of a counter-propagating probe pulse with increased stability and enhanced efficiency as compared with that produced in homogeneous plasma. Mirror reflectivity as high as a few percent for the field amplitude is identified through multi-dimensional particle-in-cell simulations.