Introduction

When the radiation pulse of an intense laser propagates in gas, the atoms of the gas emit odd harmonics of incident radiation. If λ is the incident wavelength, the outgoing radiation consists in a superposition of λ, λ/3, λ/5,…, components. The experimental environment of high harmonic generation is similar to that described for OFI soft X-ray lasers in Section 7.6. The only important difference is that OFI lasers requires circularly polarized pump in order to strongly heat the free electrons produced by interaction with laser field, while high harmonic generation (HHG) is produced by linearly (or elliptically) polarized radiation which generates a low electron energy distribution (cf. Fig. 7.93, Section 7.6). Whereas OFI laser emission is a collective nonlinear response of gas to an intense pump beam, harmonic emission is a coherent nonlinear response of individual atoms [1]. The possibility to go from OFI laser to harmonic emission by a simple π/2 rotation of a λ/4 thin plate has been experimentally demonstrated [2].