Polarization control of ultrabroadband supercontinuum generation from midinfrared laser-induced harmonic emission

We present an efficient scheme for generation of an isolated attosecond (as) pulse based on the combination of two well-known methods: polarization gating method plus a method based on using two colors ($\omega +\omega /2$). The first uses a pulse whose ellipticity is small within a very short time interval (called a polarization gating). By adding a second pulse whose frequency is a half-frequency (midinfrared) of the first pulse, we extend significantly the cutoff frequency, as well as the width of the supercontinuum harmonic spectrum. The high-order harmonic generation is calculated using the strong-field approximation theory. As a result, an isolated 42-as pulse is generated directly by superposing the hundreds of harmonics. To explain the mechanism of the ultrashort attosecond generation, we perform the semiclassical three-step model simulation and wavelet time-frequency transform of the harmonic spectra.

Figure 1

(a) The superposition of the $x$-direction polarized pulse and a midinfrared laser pulse (green solid line), the $x$-direction polarized pulse alone (red dashed dot line), and the $y$-direction polarized pulse alone (blue dashed dot line, $E_{py}=0$). (c) Same as (a), for $E_{py}\ne 0$. The corresponding HHG spectra are presented in (b) and (d), respectively.

Figure 2

(a) Two-cycle combined laser field of the $x$-direction polarized pulse and a midinfrared laser pulse. (b) Return energy as a function of the ionization time $t_i$ and emission time $t_r$. The parameters used are the same as those in Fig. 1.

Figure 3

Wavelet time-frequency profile of the HHG spectra of He atoms driven by the combined laser field with (a) $E_{py}=0$, (b) $E_{py}\ne 0$. The parameters used are the same as those in Fig. 1.

Figure 4

Attosecond pulse generation by superposing the harmonics from (a) 460th $\sim$ 540th harmonic order, (b) 540th $\sim$ 620th harmonic order, and (c) 620th $\sim$ 700th harmonic order.