Low-Order Harmonics Emitted from Relativistic Plasma Mirrors Driven by Two-Color and Elliptically Polarized Lasers*

Plasma mirrors are created when high-power light is focused onto the surface of a solid target causing rapid ionization of the target's surface. They have been demonstrated as a practical optical component for numerous applications, including temporal contrast enhancement, spatial mode cleaning, specular reflection, and focusing of high-power lasers. In addition, plasma mirrors driven by relativistic intensities leads to the generation of coherent harmonic radiation with wavelengths spanning from the infrared to the soft x-ray regime. Previous experimental and theoretical works have used spatially and temporally structured light waveforms for controlling the relativistic plasma dynamics leading up to the emission of these harmonics, which in turn allows for controlling the properties of the emitted radiation. In this work, we characterize the low-order harmonics emitted from a plasma mirror that is relativistically driven by a two-color laser and an elliptically polarized laser.  We present measurements of the conversion efficiency and the polarization state of the first three harmonics and compare the results with three-dimensional particle-in-cell simulations.