Observation of Gigawatt-Class THz Pulses from a Compact Laser-Driven Particle Accelerator

We report the observation of subpicosecond terahertz ($T$-ray) pulses with energies $\ge 460\mu \mathrm{J}$ from a laser-driven ion accelerator, thus rendering the peak power of the source higher even than that of state-of-the-art synchrotrons. Experiments were performed with intense laser pulses (up to $5\times {10}^{19}\mathrm{W}/{\mathrm{cm}}^2$) to irradiate thin metal foil targets. Ion spectra measured simultaneously showed a square law dependence of the $T$-ray yield on particle number. Two-dimensional particle-in-cell simulations show the presence of transient currents at the target rear surface which could be responsible for the strong $T$-ray emission.

Figure 1

Experimental scheme. The laser beam was focused onto a thin metal foil placed at the first focus of the ellipsoidal collection optics. $T$ rays emitted in noncollinear directions are focused at the second focus of the ellipse. Inset shows a typical proton spectrum.

Figure 2

Results of pump probe measurements. (a) Electric field of the THz pulse obtained from the pump-probe measurement. (b) Power spectral density obtained from the electro-optic measurement (blue solid curve) and using a set of calibrated bandpass filters (red dashed curve).

Figure 3

$T$-ray yield as a function of the number of protons measured in a 50 keV energy interval per $10\mu \mathrm{sr}$. Error bars indicate the shot-to-shot fluctuation.

Figure 4

(a) Time-traces of the angle-resolved norm of the Poynting vector recorded by the probes on a $20\mu \mathrm{m}$ hemisphere behind the target. (b) Time-integrated polar plot of the radiation trace [shown in (a)] measured at three semicircles of radii $20\mu \mathrm{m}$ (dotted red curve), $40\mu \mathrm{m}$ (solid green curve), and $60\mu \mathrm{m}$ $\mu$m (dashed blue curve), respectively. The target lies along $0°–180°$ direction and the laser pulse is incident in the $270°–90°$ (red arrow) direction. “Streak” image of current density (c) $J_y$ component (along surface between $67.5$ and $74.5\mu \mathrm{m}$, laser is at $75\mu \mathrm{m}$) and (d) $J_z$ (perpendicular to the target). (e) Current density vectors at $184\mathrm{fs}$. (f) Lineouts of maximum field $E_z$ (solid curve) and current $j_z$ (dashed curve) along the central rear target normal.