Development of extreme-ultraviolet tunable coherent source

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Frequency up-conversion of tunable lasers using the 3rd order nonlineality in gaseous medium, is very useful for laser spectroscopy in the VUV and XUV region. The final target of this project is to extend the tunable range over 72nm where there was no tunable source. We proposed the following method to realize this : 1) two-photon resonant four-wave mixing between tunable ArF and dye lasers, and 2) resonant four-wave mixing between Raman shifted lines of them. Results of the project during these 3years are summarized as follows.1) Efficient resonant third harmonic generation of ArF laser was obtained at 64nm with hydrogen gas jet, when the laser wavelength was tuned on a two-photon resonant line.2) Then, by the mixing of dye laser beam with it, continuously tunable XUV coherent radiation down to 66nm was generated. This is the shortest wavelength record of tunable laser up to now.3) Resonant four-wave mixing between ArF laser and the Raman-shifted lines was generated. However, shorter wavelength generation by ante-Stokes lines has not yet observed. We are developing a short-pulse high -power Ti:sapphire laser to extend the tunable range shorter.