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We generate elliptically polarized harmonics with tunable handedness by controlling laser ellipticity in relativistic laser-solid interactions. Experiments closely match simulations, enabling multi-color, circularly polarized relativistic-intensity pulses, with up to 10% energy in the second harmonic. 

We experimentally demonstrate enhanced third and fourth harmonic energy using a phase-controlled two-color beam in a multi-pass plasma mirror set-up. Maximum enhancement of 1.6 was measured for on-target intensity of 10^19 W/cm^2 

Sub-optical-cycle dynamics of dense electron bunches in relativistic-intensity laser–solid interactions lead to the emission of high-order harmonics and attosecond light pulses. The capacity of particle-in-cell simulations to accurately model these dynamics is essential for the prediction of emission properties because the attosecond pulse intensity depends on the electron density distribution at the time of emission and on the temporal distribution of individual electron Lorentz-factors in an emitting electron bunch. Here, we show that in one-dimensional collisionless simulations, the peak density of the emitting electron bunch increases with the increase in the spatial resolution of the simulation grid. When collisions are added to the model, the peak electron density becomes independent of the spatial resolution. Collisions are shown to increase the spread of the peaks of Lorentz-factors of emitting electrons in time, especially in the regimes far from optimum generation conditions, thus leading to lower intensities of attosecond pulses as compared to those obtained in collisionless simulations.