Nonmonotonic increase in laser-driven THz emissions through multiple ionization events

Highly charged states created through multiple ionization of gases are shown to enhance terahertz (THz) generation by intense, single- or two-color femtosecond laser pulses. A one-dimensional, nonpropagating fluid model reveals the main conversion processes up to ${10}^{17}\mathrm{W}{\mathrm{cm}}^{-2}$ laser intensities, namely, ionization-induced photocurrents and plasma current oscillations. For increasing intensities, we demonstrate a cyclic growth in the THz signal associated with the different binding energies of argon and helium. This behavior is confirmed by direct particle-in-cell and unidirectional pulse propagation simulations. Changes in the forward and backward THz spectra owing to multiple ionization are discussed.

Figure 1

(a) Averaged ion charge $Z^{*}$ for $1\text{−}\mu \mathrm{m}$ single-color Gaussian pulses in H, Ar, and He versus $I_0$ for ${\tau }_p=10$ (solid curves) and ${\tau }_p=40$ (dashed curves), calculated from Eq. (21) of the Supplemental Material [22]. Dots refer to analytical thresholds yielded by $W_{\mathrm{MI}}^j\left(E_0\right)={\omega }_0/10$. (b) Solutions of Eqs. (2) and (4) in Ar and He for a $1\text{−}\mu \mathrm{m}$, single-color, ten-cycle-long pulse with $I_0=5\times {10}^{15}\mathrm{W}{\mathrm{cm}}^{-2}$ and $n_a=1.2\times {10}^{18}{\mathrm{cm}}^{-3}$. Legend is specified in the inset. All the electric fields are normalized to their maximum value.

Figure 2

THz field strength vs pump intensity for Ar, He, and H predicted by the fluid-Maxwell model for $n_a=1.2\times {10}^{18}{\mathrm{cm}}^{-3}$: (a) single-color pulses with 10 cycles (solid curves) and 40 cycles (dashed curves); (b) two-color pulses with 10-cycle-long pump; (c) two-color pulses with 40-cycle-long pump. Dotted vertical lines visualize the ionization thresholds in each gas with same color convention. In (a), the predictions of the photocurrent model in Ar are plotted as thin green (light-gray) curves. THz emission is computed for the frequency window $\le 45$ THz.

Figure 3

THz field strength vs pump intensity as predicted by the fluid-Maxwell model (black curve), calder code (red curve linking squares), and uppe3d code (blue curve linking circles): (a) 10-cycle single-color pulse in Ar; (b) 10-cycle (solid curve) and 40-cycle (dashed curve) two-color pulses in He.

Figure 4

On-axis transmitted spectra with mean pulse intensity $I_0=2\times {10}^{16}\mathrm{W}{\mathrm{cm}}^{-2}$ for (a) Ar irradiated by a single-color 10-cycle pulse, (b) He irradiated by a two-color pulse with a 10-cycle pump, and (c) He irradiated by a two-color pulse with a 40-cycle pump. Red dash-dotted curves: 1D calder simulations; blue dashed curves: uppe3d simulations; black solid curves: 1D fluid-Maxwell model. Insets show the time-resolved THz field after propagation over 100 $\mu \mathrm{m}$.

Figure 5

Backward THz spectra for pulse and gas configurations shown in Fig. 4 [blue lower curves (close to $y$ axis)], 4 (black upper curves), and 4 (red middle curves). Solid lines: Fluid-Maxwell model; dashed lines: calder simulations. Inset shows the time-resolved THz field computed from the fluid-Maxwell model (solid) and the PIC code (dashed curves) for a ten-cycle-long two-color pulse in He and a frequency window $\le 45$ THz.