Modeling terahertz emission from the target rear side during intense laser-solid interactions

Relativistic laser-solid target interaction is a powerful source of terahertz radiation where broadband terahertz radiation is emitted from the front and rear surfaces of the target. Even though several experimental works have reported the generation of subpicosecond duration gigawatt peak power terahertz pulses from the target rear surface, the underlying physical process behind their origin is still an open question. Here we discuss a numerical model that can accurately reproduce several aspects of the experimental results. The model is based on the charged particle dynamics at the target rear surface and the evolution of the charge separation field. We identify the major contributors that are responsible for broadband terahertz emission from the rear surface of the target.

Figure 1

(a) Energy-resolved angular distribution of hot electrons emitted at the rear surface of a 5-$\mu \mathrm{m}$-thick titanium target. A stack of nine radiochromic films (RCFs) with aluminum foils of various thicknesses in between enabled the energy-resolved measurement. The target normal (TN) (${\theta }_{\text{elec}}=0^{\circ }$) and laser-propagation (LP) (${\theta }_{\text{elec}}={45}^{\circ }$) direction are labeled accordingly. (b) The energy spectra of fast electrons in the TN and LP directions measured using the RCF stack (the symbols) and the corresponding temperatures estimated from the Boltzmann fit function (solid lines).

Figure 2

(a) Angular distribution of the proton and ion beams recorded by a CR-39 nuclear tracker placed at the rear side of the target. (b) Typical energy spectra of the dominantly accelerated ions (${\mathrm{H}}^{+}$, ${\mathrm{C}}^{2+}$, ${\mathrm{C}}^{3+}$, and ${\mathrm{C}}^{4+}$) measured by a Thomson parabola.

Figure 3

The diffraction function $D(\omega ,\rho )$ calculated for a transverse boundary with radius $\rho =5$ mm and the coherence function $F\left(\omega \right)$ (right axis) calculated for the electron bunches exiting the target in the TN and LP directions from their corresponding energy spectrum. The table in the inset summarizes the estimated bunch length for the two bunches at the plasma-vacuum boundary, which critically determines the spectrum of the CTR.

Figure 4

Spectrally integrated angular distribution, on the XY plane, of the CTR energy for (a,d) parallel component (${\varepsilon }^{\parallel }$), (b,e) perpendicular component (${\varepsilon }^{\perp }$), and (c,f) the sum of the two polarization components (${\varepsilon }^{\parallel }+{\varepsilon }^{\perp }$) for ${\theta }_{\text{elec}}=0^{\circ }$ (top) and ${45}^{\circ }$ (bottom).

Figure 5

Angular distribution of the energy of different spectral components of the CTR emission by a hot electron bunch leaving the rear surface in (a) the TN (${\theta }_{\text{elec}}=0^{\circ }$) direction and (b) the LP (${\theta }_{\text{elec}}={45}^{\circ }$) direction.

Figure 6

Time-domain representation of the THz pulses produced by the electrons leaving the target rear surface at incident angles $0{}^{\circ }$ and $45{}^{\circ }$.

Figure 7

Angular distribution of the CTR generated in the radiation plane ($\varphi =0^{\circ }$) by hot electron bunches crossing the plasma-vacuum boundary at $0^{\circ }$ and ${45}^{\circ }$.

Figure 8

Spectral and angular distribution of the energy of the SR. Most of the radiation is emitted in the 0.1–$1.5\mathrm{THz}$ spectral range and in the direction normal to the direction of acceleration.

Figure 9

Angular distribution of the SR emitted by the plasma sheath with uncompensated net charge expanding in the $z$-direction.

Figure 10

Measured spectra of CTR pulses (symbols) and the corresponding Gaussian fit functions (solid lines). ${\sigma }_z$ was estimated from ${\sigma }_{\text{rms}}$ using Eq. (16).