Generation and applications of intense terahertz fields produced by multi-color laser pulses interacting with gas plasmas

Terahertz (THz) pulses are very popular because of their numerous applications, for example in security. Located between microwaves and optical waves in the electromagnetic (EM) spectrum, their spectral domain can be exploited for molecular spectroscopy using the THz waves emitted from plasmas created by two-color femtosecond laser pulses ionizing air. Down-conversion of broadband optical spectra in the plasma produces intense EM radiation suitable for the identification of suspect materials, even remotely. The physical mechanisms involved to create terahertz radiation by laser-matter interaction, such as photocurrents, are here reviewed. The new potentialities offered by ultrafast intense lasers allow the acquisition of many spectral signatures within 20-50 THz broad bandwidths associated to high-field single-cycle THz pulses. We report new features obtained in the framework of the French project ALTESSE, the main objective of which consisted in testing the efficiency of a laser-based terahertz time-domain spectroscopy of various materials, using the air-biased coherent detection (ABCD) method. The last part of this presentation will discuss new perspectives in the production of ultra-intense terahertz pulses from laser-wakefield accelerators in relativistic plasmas.