Analysis of THz generation through the asymmetry of photoelectron angular distributions

We analyze the mechanism of THz generation in a gas medium with intense two-color infrared lasers pulses. The dependence of the amplitude of THz emission on the relative phase between the fundamental color (800 nm) and its second harmonic (400 nm) is shown to be identical to the residual current as well as to the asymmetry of the above-threshold-ionization (ATI) photoelectrons along the left versus the right side of the linear polarization axis of the laser, thus confirming the validity of the semiclassical photocurrent model for the THz emission. We further analyze the even vs odd angular momentum distributions of the ATI electrons. The degree of overlap between the even-parity dominant electrons and the odd-parity dominant electrons within each ATI peak determines the strength of the THz emission, thus favoring the model that THz is generated through free-free transitions in the laser field. A model is also provided to obtain the same phase dependence as the four-wave mixing model.

Figure 1

The time profile of the 18 THz for H atoms in 20-o.c. two-color laser fields for different relative phases $\phi$. The intensity of the fundamental laser field is $1.0\times {10}^{14}\text{W}/{\mathrm{cm}}^2$ and the second-order one is 1% intensity of the fundamental one.

Figure 2

(a) The normalized THz emissions (solid black line), the residual photocurrent (dashed red line), and the asymmetry parameter (dotted blue line) as a function of the relative phase of the fundamental (800 nm) laser field and its second harmonic (400 nm) laser field. The laser parameters are the same as those in Fig. 1. The yields of the ATI electron on the right side ($z>0$, solid black line) and on the left side ($z<0$, dashed red line) are shown in panel (b) for $\phi =0.4\pi$ and in (c) for $\phi =0.9\pi$, where the THz yield in panel (a) is at the minimum and the maximum, respectively.

Figure 3

The ATI spectra of electron with even (solid black line) and odd (dashed red line) integer orbital quantum numbers along the polarization direction for (a) a monochromatic 800-nm laser field and two-color laser fields when (b) $\phi =0.4\pi$, (c) $\phi =0.7\pi$, and (d) $\phi =0.9\pi$. The odd-even mixture degree increases from panel (a) to panel (d) when the THz yield gets stronger. Laser parameters are the same as those in Fig. 1.

Figure 4

(a) The normalized THz emissions (solid black line), the residual photocurrent (dashed red line), and the asymmetry parameter (dotted blue line) as a function of the relative phase of the fundamental laser (800 nm) and its second harmonic (400 nm). The intensity of the fundamental laser is $2.26\times {10}^{14}\text{W}/{\mathrm{cm}}^2$ and the second harmonic intensity is 1% of the fundamental. The total ATI spectra on the right and left of the polarization axis are given in panel (b) for $\phi =0.2\pi$ and in panel (c) for $\phi =0.7\pi$ when the asymmetry of the total ATI spectrum reaches its minimum and maximum, respectively. The corresponding even (solid black line) and odd (dashed line) angular orbital momentum distributions for $\phi =0.2\pi$ and $\phi =0.7\pi$ are shown in panels (d) and (e), respectively.