Spatiotemporal visualization of the terahertz emission during high-power laser-matter interaction

Single-cycle pulses with multimillion volts per centimeter field strengths and spectra in the terahertz (THz) band have attracted great interest due to their ability to coherently manipulate molecular orientations and electron spins resonantly and nonresonantly. The tremendous progress made in the development of compact and powerful terahertz sources have identified intense laser-thin foil interaction as a potential candidate for high-power broadband terahertz radiation. They are micrometers in size and deliver radially polarized terahertz pulses with millijoule energy and gigawatt peak power. Although several works have been carried out to investigate the terahertz generation process, their origin and angular distribution are still debated. We present here an indisputable study on their spatiotemporal characteristics and elaborate the underlying physical processes via recording the three-dimensional beam profile along with transient dynamics. These results are substructured with the quantitative visualization of the charge particle spectra.

Figure 1

$3\text{D}$ angular profile of the terahertz radiation emitted from the target rear surface. Inset shows the geometry of the collection angles. FE denotes the emission in the forward direction and NE corresponds to noncollinear direction. Laser pulse is incident onto the thin metal foil target at ${45}^{\circ }$. Terahertz emission from the rear surface of the target is asymmetric and multiple peaks present in the laser propagation direction. In the Supplemental Material video a ${360}^{\circ }$ perspective of the angular emission can be seen [33].

Figure 2

(a) The polarization of the terahertz radiation measured using a wire-grid polarizer. (b) Transient structure of the terahertz radiation obtained from the electro-optic (EO) measurement for a single shot. Noncollinear (NE) or wide-angle emission (light-green area of Fig. 1 inset) and forward emission (FE; blue area marked in Fig. 1 inset) using balanced detection scheme. Multiple pulses are present both in the wide-angle and forward emission with similar pulse durations and comparable temporal delays. (c) Spectral intensity of the terahertz pulses.

Figure 3

(a) Schematic of the particle detection scheme. (b) Angular distribution of the electron beam detected using gafchromic films with different thickness aluminum filters. The hole represents the target normal direction. Clearly, low energy background emission is dominated by energetic electrons emitted in the laser propagation direction. (c) Energy spectra of the electrons measured along $0^{\circ }$ (target normal) and ${45}^{\circ }$ (laser propagation direction). (d) Proton and ion beam angular distribution recorded by the $\mathrm{CR}39$ plastic detector.

Figure 4

(a) Angular intensity distributions of coherent transition radiation (CTR) emission estimated for the electrons exiting the target rear surface (red line) and sheath radiation emitted by the plasma expansion (green line). Solid blue line is the sum of the CTR and sheath radiation emission estimated numerically. Red squares and green circles represent the experimental data taken from Fig. 1. (b) The corresponding spectral distributions of the radiation calculated from the electron distribution taken from Fig. 3. CTR $0^{\circ }$, solid red line; CTR ${45}^{\circ }$, dotted red line; and solid green line, the sheath radiation emitted by the expanding plasma sheath formed in the target normal direction at the rear surface.