Spectral control of high harmonics from relativistic plasmas using bicircular fields

We introduce two-color counterrotating circularly polarized laser fields as a way to spectrally control high harmonic generation (HHG) from relativistic plasma mirrors. Through particle-in-cell simulations, we show that only a selected group of harmonic orders can appear owing to the symmetry of the laser fields and the related conservation laws. By adjusting the intensity ratio of the two driving field components, we demonstrate the overall HHG efficiency, the relative intensity of allowed neighboring harmonic orders, and that the polarization state of the harmonic source can be tuned. The HHG efficiency of this scheme can be as high as that driven by a linearly polarized laser field.

Figure 1

(a) Electric field waveform of the incident laser pulses propagating along the $x$ axis, composed of two-color (fundamental frequency and its second harmonic) counterrotating circularly polarized fields. (b) The corresponding high harmonic spectrum from surface plasmas.

Figure 2

Electric field waveform of the (a) 10th and (b) 11th harmonic shown in Fig. 1. In order to see the helicity clearly, only parts of the waveform are shown. The rotation direction is defined as seen by a wave receiver, and the harmonic waves propagate along the $-x$ axis.

Figure 3

Polarization plane projections of the laser electric fields generated with intensity ratios (a) $I_{2\omega }/I_{\omega }=0.0$, (b) $I_{2\omega }/I_{\omega }=0.1$, (c) $I_{2\omega }/I_{\omega }=0.5$, and (d) $I_{2\omega }/I_{\omega }=2.0$. The total intensity is kept constant ($I_{\omega }+I_{2\omega }=4.2\times {10}^{21}$ W/${\mathrm{cm}}^2$). (e)–(h) The generated harmonic spectra corresponding to the lasers of panels (a)–(d).

Figure 4

High harmonic spectrum for the case of (a) oblique incidence with incidence angle $\theta ={45}^{\circ }$ and (b) plasma scale length of $L_s=0.15{\lambda }_0$. The other simulation parameters are the same as in Fig. 1.