Abstract

As is well known, the interaction of a laser field with an atomic gas leads to generation of odd harmonics of the fundamental frequency. When the incident laser is intense and of short pulse duration, high order odd harmonics can be created. This has been extensively studied with excimer lasers (248 nm), with Nd-YAG or Nd-Glass lasers and recently with visible dye lasers [1-3]. The intensities of these harmonics in general are found to be approximately constant, up to a very high order. This "plateau" behavior is characteristic of a strong (non-perturbative) laser-atom interaction, as discussed in several theoretical papers [4,5]. The width of the plateau increases with the laser intensity, up to the intensity at which the medium becomes ionized. For example, we have observed up to the 53rd harmonic of a 1ps 1053 nm Nd-Glass laser at an intensity of 5×10¹⁴ W.cm^-2 (The photon energy of 62 eV). The observation of harmonics of higher order than 100 of a 800 nm 150 fs Ti:S laser has been reported by a group at Stanford University (which means a photon energy of up to 200 eV) [6]. This radiation emission in the XUV domain has unique properties of coherence and brightness. Its pulse width follows that of the pump field and can therefore be very short. The investigation of these processes is worthwhile not only for its basic interest as a probe of the behavior of atoms in strong laser fields but also because it might lead to short pulse, bright, compact and possibly tunable sources of coherent radiation in the XUV range.

© 1992 The Author(s)