We report here recent work on an optical-field ionized (OFI), high-order harmonic-seeded EUV laser. The amplifying medium is a plasma of nickel-like krypton obtained by optical field ionization focusing a 1 J, 30 fs, circularly- polarized, infrared pulse into a krypton-filled gas cell or krypton gas jet. The lasing transition is the 3d94p (J=0) −→ 3d94p (J=1) transition of Ni-like krypton ions at 32.8 nm and is pumped by collisions with hot electrons.
The polarization of the HH-seeded EUV laser beam was studied using an analyzer composed of three grazing incidence EUV multilayer mirrors able to spin under vacuum. For linear polarization, the Malus law has been recovered while in the case of a circularly-polarized seed, the EUV signal is insensitive to the rotation of the analyzer, bearing testimony to circularly polarized.
The gain dynamics was probed by seeding the amplifier with a high-order harmonic pulse at different delays. The gain duration monotonically decreased from 7 ps to an unprecedented shortness of 450 fs FWHM as the amplification peak rose from 150 to 1,200 with an increase of the plasma density from 3 × 1018 cm−3 up to 1.2 × 1020 cm−3. The integrated energy of the EUV laser pulse was also measured, and found to be around 2 μJ. It is to be noted that in the ASE mode, longer amplifiers were achieved (up to 3 cm), yielding EUV outputs up to 14 μJ.
Stéphane Sebban, "Toward compact and ultra-intense laser driven soft x-ray lasers (Conference Presentation)," Proc. SPIE 10243, X-ray Lasers and Coherent X-ray Sources: Development and Applications, 1024302 (Presented at SPIE Optics + Optoelectronics: April 24, 2017; Published: 16 June 2017); https://doi.org/10.1117/12.2266771.5464857847001.
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