Efficient Excitation of Gain-Saturated Sub-9-nm-Wavelength Tabletop Soft-X-Ray Lasers and Lasing Down to 7.36 nm

Abstract

We have demonstrated the efficient generation of sub-9-nm-wavelength picosecond laser pulses of microjoule energy at 1-Hz repetition rate with a tabletop laser. Gain-saturated lasing was obtained at ¼ 8:85 nm in nickel-like lanthanum ions excited by collisional electron-impact excitation in a precreated plasma column heated by a picosecond optical laser pulse of 4-J energy. Furthermore, isoelectronic scaling along the lanthanide series resulted in lasing at wavelengths as short as ¼ 7:36 nm. Simulations show that the collisionally broadened atomic transitions in these dense plasmas can support the amplification of subpicosecond soft-x-ray laser pulses. DOI: 10.1103/PhysRevX.1.021023 Subject Areas: Photonics, Plasma PhysicsThe high demand for bright soft-x-ray laser (SXRL) pulses greatly exceeds the beam time available at a few single-user free-electron laser facilities [1,2]. This motivates the development of more compact and widely accessible SXRLs for a broad range of experiments in small laboratory settings. Significant progress has been achieved in the past few years in the development of compact plasma-based soft-x-ray lasers [3][4][5][6][7][8][9][10]. However, repetitive operation of tabletop SXRLs has been limited to wavelengths above 10.9 nm [10]. At shorter wavelengths, the large pump energy required has limited the repetition rate to typically one shot per hour [11][12][13][14][15]. Soft-x-ray lasing at sub-10-nm wavelengths in lanthanide ions was first demonstrated using optical lasers of pump energy of several hundred joules [12,13]. Lasing in nickel-like lanthanum at ¼ 8:85 nm was later obtained by using 18-J pulses from a chirped-pulse-amplification (CPA) laser to achieve transient excitation, but with a gain-length product (g  l ¼ 7:7) that remained insufficient to reach gain saturation [15]. Progress toward saturated lasing in this transition has been recently reported [16,17]. In turn, lasing at ¼ 7:36 nm in nickel-like Sm was initially demonstrated using 130-J pump pulses [12]. Later, gain saturation was reached using a picosecond-duration pump pulse with approximately 40 J of energy added to a prepulse of similar energy [14]. Also recently, the lasing threshold in this line was reached through the use of a total optical pump energy of 36 J [18]. However, the practical realization of highrepetition-rate tabletop lasers at sub-10-nm wavelengths requires obtaining gain-saturated lasing at significantly lower pump energies. We report the generation of gainsaturated picosecond SXRL pulses at ¼ 8:85 nm at 1-Hz repetition rate. The result is obtained by using a picosecond pump pulse with an unprecedentedly low energy of 4 J and a total optical pump energy of 7.5 J, that will make the operation at high-repetition rates possible. Furthermore, using the same pump energy, we observe lasing at wavelengths down to ¼ 7:36 nm in transitions of higher-Z nickel-like lanthanide ions, opening the prospect of practical gain-saturated tabletop lasers at shorter wavelengths. Modeling suggests that these dense-pl...

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