Overview on HHG High-Flux Sources

High order laser harmonic generation is a relatively recent and promising source of intense coherent radiation in the extreme ultraviolet and soft X-ray domains. The process, due to the highly non linear interaction between an intense and ultrashort laser pulse and a gas medium, has a relatively low efficiency at the single atom level. However, progresses in laser technology and a better understanding of the macroscopic construction of the harmonic field have enabled high flux sources. In this chapter, we will review the last developments that allow the publication of the first demonstrations of photon demanding experiments in solid state physics and non linear optics. Control and shaping of the fundamental laser field have led to an increase of the conversion efficiency of several orders of magnitude. Microjoule level emissions have been obtained on high power lasers thanks to phase matching and quasi phase matching schemes. We will also mention very high repetition rate laser systems that produce photon flux per second of the same order of magnitude than those obtained with lower repetition rate and higher energy level.