Efficient soft x-ray lasing was achieved by using plasma waveguide to confine the pump beam. With a 9-mm-long pure
krypton plasma waveguide prepared by using the axicon-ignitor-heater scheme, lasing at 32.8 nm is enhanced by 400
folds. An output level of 8×10<sup>10</sup> photon/shot is reached at an energy conversion efficiency of 2×10<sup>-6</sup>. Seeding the laser
with high-harmonic generation yields small divergence, high coherence, and controlled polarization. Application in digital
holographic microscopy was demonstrated.
We experimentally demonstrate the amplification of optical-field-ionization soft x-ray lasers in an optically preformed plasma waveguide for pure xenon, krypton, and argon gases, respectively. The lasing photon number of Ni-like Kr laser at 32.8 nm generated in waveguide is dramatically enhanced by about three orders of magnitude in comparison to that without plasma waveguide, resulting in a photon number of 8×10<sup>10</sup> and an energy conversion efficiency of 2×10<sup>-6</sup> with a pump pulse of just 235 mJ. In addition to the 46.9 nm main lasing line for Ne-like argon, the 45.1 and 46.5 nm lasing lines are also observed, indicative of the strong enhancement effect and the large gas density in the plasma waveguide.
With this technique multispecies parallel x-ray lasing is also demonstrated in a Kr-Ar mixed-gas waveguide. By seeding
optical-field-ionization plasma with high harmonic signals, 32.8-nm Kr laser output can be further improved to produce brighter and better collimated x-ray laser beams. Comparing with the same laser seeded only with spontaneous emission, seeding with high harmonics yields much smaller divergence, enhanced spatial coherence, and controlled polarization. With the illumination of high-brightness 32.8-nm x-ray laser pulses, single-shot x-ray digital holographic
microscopy with an adjustable field of view and magnification is demonstrated successfully. The ultrashort x-ray pulse duration combined with single-shot capability offers great advantage for flash imaging of delicate samples.
Efficient optical-field-ionization x-ray lasers driven by femtosecond laser pulses have been demonstrated in clustered gas
jets. With a tight focusing configuration, near saturation outputs of Pd-like xenon laser at 41.8 nm and Ni-like krypton
laser at 32.8 nm are generated at low pump energy of 200 mJ. By using the axicon-ignitor-heater scheme to produce a
9-mm-long plasma waveguide in a pure krypton gas jet, the lasing photon number of Ni-like Kr laser at 32.8 nm is
dramatically enhanced by about three orders of magnitude in comparison to that without plasma waveguide, resulting in
a photon number of 8×10<sup>10</sup> and an energy conversion efficiency of
2×10<sup>-6</sup> with a pump pulse of just 235 mJ. Besides for
producing various OFI collisional-excitation x-ray lasers of sufficient photon number for practical applications, an
optically-preformed plasma waveguide may also be a favorable choice for achieving OFI recombination x-ray lasers and
inner-shell x-ray lasers.