Simulating the mechanisms producing intense terahertz waves in relativistic plasmas

Intense terahertz waves – which have applications in medicine, security and molecular spectroscopy – are difficult to produce by conventional optoelectronic means. While the energy of terahertz pulses generated by non-relativistic laser sources is usually limited to a few microjoules, it can rise to the millijoule level by irradiating solid targets at relativistic laser intensities.

Déchard et al. used particle-in-cell numerical simulations to elucidate the various conversion mechanisms accounting for the emission of picosecond-long terahertz pulses from thin solid foils, as reported from recent experiments. The results reveal that the complex dynamics of the laser-driven hot electrons, associated surface shielding currents and target deformation trigger distinct terahertz radiation bursts.

The simulations involved sub-micron foil targets illuminated by ultraintense (∼10²⁰ W cm⁻²), femtosecond laser pulses. The authors demonstrate that first to arise is the coherent transition radiation generated by the forward-accelerated electrons. As the target surface expands and becomes increasingly curved, secondary terahertz waves originate at its extremities in an antenna-like fashion due to electron currents moving along the surface. The radiation emerging at later times may be due to ion acceleration, as experiments suggest. Those various emissions accumulate to form a picosecond-long terahertz waveform, the amplitude of which depends on the foil thickness: Thinner targets expand faster, and hence strengthen the terahertz signal.

The researchers believe that improving the performance of laser-driven terahertz sources in terms of their spectra, energy and beam directivity is a topic full of potential discoveries. They now plan to investigate the impact of laser polarization or an external magnetic field on the radiation yield.

Source: “Terahertz emission from submicron solid targets irradiated by ultraintense femtosecond laser pulses,” by Jérémy Déchard, Xavier Davoine, Laurent Gremillet, and Luc Bergé, Physics of Plasmas (2020). The article can be accessed at http://doi.org/10.1063/5.0013415 .