High-Order Harmonic Generation from Low-Density Plasma

Abstract

High-order harmonic generation (HHG) of ultra-short laser pulses is an effective method to generate extreme ultraviolet (XUV) radiation and attosecond light pulses. Currently, various techniques are used for HHG, such as the interaction of intense laser with gases (Kazamias et al., 2003), with low-density laser-produced plasmas (Ganeev, 2007), and by reflecting relativistic intensity laser pulses from solid surfaces (Norreys et al., 1996). Such harmonics are promising sources of coherent soft x-ray radiation with various applications, such as XUV nonlinear optics and spectroscopic studies. In particular, the major advantages of HHG are very good spatial quality and femtosecond to attosecond temporal resolution. Single attosecond XUV pulses (with pulse durations of several hundred attoseconds) from HHG has been generated, by irradiating neon gas atoms with intense, few-cycle laser pulse (Drescher et al., 2001), and the current record for the shortest pulse ever generated is 80 attoseconds (Goulielmakis et al., 2008). Furthermore the maximum cut-off energy of harmonics has been extended up to 1.3 keV (corresponding to a wavelength of 0.95 nm) (Seres et al., 2005). This ultra-short nature of harmonics has lead to several applications of pivotal importance, such as controlling the motion of bound electrons in molecular dissociation (Kling et al., 2006), mapping of attosecond electron wave packet motion (Niikura et al., 2005), and nonlinear phenomena in the XUV region (Nabekawa et al., 2005). To extend the harmonic cut-off to shorter wavelengths, one needs to use nonlinear medium with higher ionization potential. This is because generally speaking, the harmonic cut-off energy