Nonadiabatic spectral redshift of high-order harmonics with the help of a VUV pulse

We theoretically investigate the nonadiabatic spectral redshift of high-order harmonics with the help of a VUV pulse. It is found that the nonadiabatic spectral redshift of high-order harmonics can be observed when a weak VUV pulse is properly added in the falling part of the fundamental laser due to the nonadiabatic response of the dipole to rapid change of laser intensity. Further time-frequency analysis shows that the high-order harmonics are mainly generated in the falling part of the fundamental pulse. This is because the VUV pulse enhances the ionization in the falling part of the fundamental pulse by the 1s-2p transition of $\text{H}{e}^{+}$. In addition, this scheme is also used to observe the nonadiabatic spectral blueshift of high-order harmonics by changing the time delay between the fundamental laser and the VUV pulse.

Figure 1

Electric fields of the 400-nm driving pulse (dashed black curve) and the VUV pulse (solid red curve).

Figure 2

The harmonic spectrum produced by exposing $\text{H}{e}^{+}$ to the combined field. Inset: Spectral structure of some harmonics illustrating the presence of redshifted harmonics.

Figure 3

Spectral structure of some harmonics produced by exposing $\text{H}{e}^{+}$ to the combined field. The thick curves and thin curve are the results at $t_{\text{delay}}=-2T_0$ and 0, respectively. VG, velocity gauge; LG, length gauge. Other parameters are the same as in Fig. 2.

Figure 4

Time-frequency distribution of the harmonic spectra generated with (a) $t_{\text{delay}}=-2T_0$ and (b) $t_{\text{delay}}=0$. The same color scale is used in the two pictures.

Figure 5

Spectral structure of the (a) 24th–30th harmonics and (b) 54th–60th harmonics. $t_{\text{delay}}=-2T_0$ (thin solid black curve) and $t_{\text{delay}}=0$ (thick solid pink curve).

Figure 6

Spectral structure of some harmonics produced at different intensities of the VUV pulse. Other parameters are the same as in Fig. 2.

Figure 7

Spectral structure of some harmonics produced at different intensities of the fundamental pulse. Other parameters are the same as in Fig. 2.

Figure 8

Spectral structure of some harmonics produced at $t_{\text{delay}}=2T_0$. Other parameters are the same as in Fig. 2.