High‐Order Harmonic Generation and Attosecond Light Pulses: An Introduction

Abstract

This chapter describes how the understanding of high-order harmonic generation (HHG) and the generation of attosecond pulses progressed since the first experiments, which were performed at the end of the 1980s. The text describes how the ideas arose, as perceived by the author of this chapter. It is neither a detailed and comprehensive review nor a documented historical background. Early Work, 1987-1993Somewhat surprisingly, HHG was not discovered in the nonlinear optics community, but by scientists interested in the response of atoms to intense laser fields. In nonlinear optics, the general (and quite reasonable) understanding was that highorder processes are less probable than low-order processes. Consequently, on a not so successful route toward vacuum ultraviolet light, researchers used ultraviolet radiation in order to start with VUV photons. At the same time, a lot of progress in laser technology allowed scientists to focus radiation at very high intensities, reaching 10 13 10 15 W/cm 2 . Atoms, when exposed to such intense radiation, get ionized by absorbing several photons. Above-threshold-ionization and multiple-ionization were observed as unexpected results of such experiments.One experimental idea was to detect fluorescence emitted by excited atoms or ions that were produced during strong field excitation. To this end, the pressure of sample atoms in the interaction chamber had to be increased: gas jets were installed, and photon detectors were placed along the laser propagation axis. First results for HHG from VUV radiation were obtained by Rhodes and his co-workers, using a 248-nm excimer laser with high laser intensity [1]. Strong fluorescence was seen from Ar, Kr and Xe ions, together with high harmonics up to the 17th harmonic in Ne. Shortly afterward, high-order harmonics were observed in Xe, Kr and Ar using a 30-ps Nd:YAG laser (1064 nm) [2]. The HHG spectra exhibited a characteristic behavior, with a rapid intensity decrease for the first orders, a plateau from the seventh harmonic up to a very high order (e.g., the 29th in Ar) and an abrupt cutoff.Attosecond and XUV Physics, First Edition. Edited by Thomas Schultz and Marc Vrakking.