Soft X-ray Laser with Low Pumping Energy

Hidetoshi Nakano, Peixiang Lu, Tadashi Nishikawa, and Naoshi Uesugi
Physical Science Laboratory

　Since the first report on laser amplification in soft x-ray regime in 1985, great progress has been achieved in the field of x-ray lasers based on mainly laser facilities with huge size. Recent developments in ultrafast laser technologies opened a new trend toward table-top sized x-ray lasers. Realizing a table-top size x-ray laser is important for practical applications. One of the most promising ways of developing table-top x-ray lasers is based on optical-field induced ionization of atoms in an intense laser field provided by high-power femtosecond laser systems. We have successively demonstrated a gain at 45.2 nm using a transition in boron-like nitrogen ions by an only 25-mJ linearly polarized 100-fs laser pulse.

　The experiments were carried out using a T3(table-top tera-watt)-laser system. The 100-fs, 790-nm linearly polarized laser pulse was passed through a 2-inch-diameter, 5-mm-thick MgF2 widow into a vacuum chamber, and was focused by a 400-mm focal length MgF2 lens into a differentially pumped nitrogen gas cell. The gas cell had pinholes (< 500 μm) at both ends where the laser light passed through. Line-shaped plasma was created along the propagating direction of the laser light. Figure 1 shows the axial emission spectra observed at a fixed gas pressure of 1 Torr and pumping laser energy of 25 mJ for different plasma lengths L. From the figure, the NIII 3s-2p lasing line at 45.2 nm dominates the spectrum at long L. In Fig. 2, line intensity at 45.2 nm is plotted as a function of the length of plasma column. From this figure, we obtained a small signal gain coefficient G and a gain-length product GL as 9.6 cm-1 and 3.84, respectively.

Fig. 1: Axial soft x-ray emission spectra of the line-shaped nitrogen plasmas for different lengths.

Fig. 2: Time-integrated intensity of NIII lines as a function of line-shaped plasma length.

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