Time dependent Doppler shifts in high-order harmonic generation in intense laser interactions with solid density plasma and frequency chirped pulses

Abstract

High order harmonic generation from solid targets is a compelling route to generating intense attosecond or even zeptosecond pulses. However, the effects of ion motion on the generation of harmonics have only recently started to be considered. Here, we study the effects of ion motion in harmonics production at ultrahigh laser intensities interacting with solid density plasma. Using particle-in-cell simulations, we find that there is an optimum density for harmonic production that depends on laser intensity, which scales linearly with a 0 with no ion motion but with a reduced scaling if ion motion is included. We derive a scaling for this optimum density with ion motion and also find that the background ion motion induces Doppler red-shifts in the harmonic structures of the reflected pulse. The temporal structure of the Doppler shifts is correlated to the envelope of the incident laser pulse. We demonstrate that by introducing a frequency chirp in the incident pulse we are able to eliminate these Doppler shifts almost completely. V C 2015 AIP Publishing LLC.

Long-wavelength pulse generation via light-sail backscattering

“…Relatedly, the relation between target motion, pulse intensity, and target density profile in the hole boring regime was recently used to infer target properties from the backscattered light and the target acceleration [16]. Furthermore, in studies of high harmonic generation via oscillation of surface electrons [17] to generate intense attosecond or zeptosecond pulses [18,19], maintaining phase locking is critical to harmonic generation [20,21], and the bulk motion of the ions in modifying the backscattered harmonic spectrum must be considered [22,23]). Previous measurements of surface induced Doppler shifts of a pump laser have yielded useful information in experiments [24][25][26].…”