Sub GV/cm terahertz radiation from relativistic laser-solid interactions via coherent transition radiation

Abstract

Broadband terahertz (THz) radiation with extremely high peak power, generated by the interaction of a femtosecond laser with a thin solid target, has been investigated via particle-in-cell simulations. The spatial (angular) and temporal profiles of the THz radiation reveal that it is caused by the coherent transition radiation emitted when laser-produced hot electrons pass through the front or rear surface of the target. Dependence of the THz radiation on laser and target parameters is studied; it is shown to have a strong correlation with hot electron production. The THz radiation conversion efficiency can be as high as a few times 10^{-3}. This radiation is not only a potentially high power THz source, but may also be used as a unique diagnostic of hot electron generation and transport in relativistic laser-solid interactions.

Aligned copper nanorod arrays for highly efficient generation of intense ultra-broadband THz pulses

“…The electron energy in the y -direction ( E ky ) contributes more to THz emission in specular reflection direction (120°) than the electron energy in the x -direction ( E kx ). This is consistent with the theory of coherent transition radiation that the radiation is mostly emitted in a large angle or nearly perpendicular to the moving direction of the electron264041 when the electrons are of moderate energy. The average kinetic energy of hot electrons is normally at the order of 100 keV under the laser conditions.…”

“…Figure 3(b) is a snapshot of the spatial distribution of the magnetic field, which is time-averaged over a laser period in order to filter out the high frequency components. Both backward and forward radiations are emitted from the target front and back sides, respectively, which can be attributed to coherent transition radiation generated by hot electrons produced in the laser-plasma interaction2640. For polished planar targets, the THz radiation is the weakest around the specular reflection direction and strongest along the target surface direction, typical for transition radiations.…”

Structured targets for advanced laser-driven sources

“…The experimental observation of 400μJ per pulse broadband THz emission from solid foils irradiated at I 10 W cm 19 2 has been reported [6,66], with a peak power of the emitted radiation approaching 1 GW [6]. THz emission from the rear of irradiated targets is attributed to coherent transition radiation of hot electrons [12,65,66,70,71], transient surface currents [6] and time varying TNSA charge separation field at the rear side of the target [6]. In these scenarios the intensity of THz emission has been directly correlated with the laser absorption.…”

“…Conventional THz sources with high energy per pulse (in the range of 100 J m ) are based on large electron accelerators [62,63]. Recently, sources based on ultra-intense laser-solid interaction have been proposed as a possible path to generate THz pulses with energies similar to those that can be obtained with particle accelerators, but with a compact set-up [6,12,[64][65][66][67][68][69]. The experimental observation of 400μJ per pulse broadband THz emission from solid foils irradiated at I 10 W cm 19 2 has been reported [6,66], with a peak power of the emitted radiation approaching 1 GW [6].…”

High-energy coherent terahertz radiation emitted by wide-angle electron beams from a laser-wakefield accelerator

“…This produces emission of conical THz radiation with conversion efficiencies as high as 10 −3 , although saturation tends to occur at high laser intensities [18]. Laser-solid interactions at high intensities can produce THz radiation through several mechanisms, which typically involve the excitation of surface currents on solid targets [21][22][23][24][25], embedded plasma dipoles [26] and also coherent transition radiation (CTR) at boundaries and interfaces [27][28][29]. These methods can lead to high energies (in the sub-millijoule range) and high peak electric fields (1-100sMV cm −1 ), which can be scaled with the laser intensity [24,25].…”