Generation of Terahertz Pulses by Laser-Plasma Interactions: From Microjoule to Joule-Class Energy

Terahertz (THz) pulses are very popular because of their numerous applications in security screening, medical imaging, time-domain spectroscopy, and remote detection. Gas plasmas created by two-color femtosecond optical pulses at moderate intensity – i.e. at intensity close to the ionization threshold – supply suitable and undamaged emitters. Electrons are tunnel ionized by an asymmetric light field usually composed of a fundamental wavelength and its second harmonic. The resulting ''photocurrent'' generates an ultrabroadband terahertz pulse which can be useful in, e.g., molecular spectroscopy.

In recent years, however, THz science, generally associated with low photon energies, has seen unprecedented development thanks to the most powerful laser sources capable of delivering Joule-class THz pulses with terawatt peak powers. These pulses will no longer solely be used to probe matter, but also to ionize and transform it. A current challenge in this context is to produce broadband energetic THz pulses by irradiating gas or solid targets at relativistic laser intensities far exceeding 1018 W/cm$^2$. In this regime, the coherent transition radiation (CTR) from electron bunches exiting the rear boundary of a fully ionized target can lead to intense THz emissions characterized by mJ-to-Joule energy yields.

This talk will first review basic properties of µJ THz pulses generated by the two-color-driven photo-ionization of air molecules and will discuss new possibilities for controlling their polarization state. The major part of the talk will then be devoted to mJ-to-J-class THz pulses produced by relativistic plasmas. Using multidimensional particle-in-cell simulations, we will detail the mechanisms underlying THz emissions in gases and solid targets driven by ultraintense laser pulses. Special emphasis will be paid to the THz performances achieved by the CTR process when passing from a wakefield formed in underdense plasmas to solid targets with overcritical density.