Enhancement of high harmonic generation by multiple reflection of ultrashort pulses

In recent studies it has been shown that by combining the fundamental laser frequency with its low-order harmonics, significant enhancement of high harmonic intensity can be achieved. Since the laser pulse reflected from a plasma inherently contains low-order harmonics, a second reflection from a fresh plasma surface leads to the increase of spectral intensity. In this work, we have made an extensive study of multiple reflections of a short pulse between two solid density plasma walls at oblique incidence, with the help of 1D and 2D particle-in-cell simulations. A loosely focused pulse is considered that can propagate between two flat foils over consecutive reflections without significant loss of intensity. It is shown that in the weakly relativistic regime the intensity of 10th to 30th harmonics can be amplified by three orders of magnitude.