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Third-harmonic Generation Of Ultrafast Laser Pulses In A Hollow Waveguide

Taro Sekikawa, Takahito Kumazaki, Yohe Kobayashi, Yasuo Nabekawa, Shuntaro Watanabe, Institute for Solid-state Physics, University of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo 116, JAPAN Recent development of the high peakpower pulse laser makes it possible to generate the high-order harmonics in the extreme ultraviolet (XUV) region and provides us the opportunity to investigate the atomic physics under the high electric field of the laser light. In the present paper, we will report the generation of the quasi-continuum of the XUV light between 25 and 40 nm using the high peak-power Ti:sapphire (Tis) laser system.' The quasi-continuum of the XUV light was generated by focusing the 30-fs, 25-mJ pulse of the linearly polarized TIS laser to the rare gases with the 7-mm interaction length. The wavelength of the fundamental light was 800 nm. The gas pressure was changed from 1 to 17 Torr. The generated XUV light was detected two-dimensionally by a chargecoupled device (CCD) camera after passing through a monochromator. Figure 1 shows the CCD images of the XUV light between 25 and 35 nm at 1 and 17 Torr. The used rare gas was Xe and He. In the case of He, only the discrete high-order harmonics were observed. On the other hand, the spots of the harmonics generated in Xe are dispersed widely along the horizontal direction with increase in the pressure. Figure 2 shows the spectrum of the harmonics between 24 and 42 nm in Xe. The spectral width of the harmonic becomes broader with pressure, leading to the generation of the quasi-continuum of the XUV light at 17 Torr. The broadening of the spectral widths of the harmonics are qualitatively explained by the creation of a free-electron plasma as follows: The laser pulse ionizes the gas and forms the free-electron plasma. Since the ionization rate depends on the pulse shape, the free-electron density changes largely during the pulse