Enhanced two-color high-harmonic generation achieved by adding an extra gas medium

Abstract

imaging [4], interferometry [5], and also for the exploration of atomic and molecular structures [6,7]. This novel XUV source, however, has a significant drawback of low conversion efficiency, and various schemes have been developed to improve it. The employment of a loose focusing geometry to a long gas cell was successful in boosting the harmonic yield to a μJ level in the 40-nm region by increasing the interaction cross section and length [8,9]. Spatial modification to a specific profile, such as flat-top shape, was also successful in expanding the effective harmonic generation volume [10,11]. Methods using waveguide structures have been investigated in order to improve phase matching between a driving laser pulse and generated harmonics [12,13]. These efforts made it possible to apply high-harmonic light source to specific applications such as XUV spectroscopy and microscopy, while it should be enhanced further to improve the applicability.Another approach developed to significantly enhance harmonic efficiency is the use of a synthesized laser field. As a simple synthesized field, a two-color laser field, consisting of a fundamental laser field and its second harmonic (SH), was applied to HHG [14,15]. It significantly improved the harmonic efficiency by generating short-path harmonics from atoms under a strongly ionizing condition, as compared to single-color harmonics generated under a weak ionization condition. We applied the strong harmonic by a two-color laser field to soft X-ray imaging experiments [4], and we found that the intensity of harmonic source should be improved further to enhance the spatial resolution of single-shot microscopy. More recently, twogas schemes were proposed to enhance the harmonic efficiency using a mixed gas medium or two gas cells [16,17]. In order to enhance the harmonic intensity further, the twogas media approach can be employed in two-color HHG scheme. AbstractThe high-harmonic generation in a two-color laser field, consisting of a 30-fs Ti:Sapphire laser pulse and its second harmonic, could be enhanced by installing an extra gas medium of Ar in front of the harmonic generation medium of He. The effects of the extra medium were examined by analyzing harmonic spectra taken under different experimental conditions. The observed characteristics of enhanced harmonic generation were explained in terms of the profile modification of the two-color laser beam. The 34th harmonic at 23.5 nm and 38th harmonic at 21.5 nm were enhanced 2.5 and 2 times, respectively.