Two-Color Soft X-Ray Lasing in a High-Density Nickel-like Krypton Plasma

We report evidence of strong lasing on the $4p\text{−}4s$ transition at 62.7 nm in nickel-like krypton occurring simultaneously with the usual $4d\text{−}4p$ lasing at 32.8 nm. The gain dynamics of both transitions were experimentally and numerically investigated and found comparable. The two-color amplifier was seeded by the same harmonic pulse, therefore producing a short-duration coherent two-color soft x-ray laser pulse. Both transitions offer similar prospects of pulse energy and duration and could lead to the delivery of intense and ultrashort two-color coherent soft x-ray pulses with a controllable delay.

Figure 1

Calculated gain of the Kr ix $4p\text{−}4s$ and $4d\text{−}4p$ transitions for a Kr density of ${10}^{19}{\mathrm{cm}}^{-3}$. Inset: Kr ix levels and lasing transitions.

Figure 2

(a) Raw end-on spectrum of the Ni-like Kr amplifier pumped by a 0.6 J pulse. The satellite lines come from diffraction by the mesh supporting the grating. (b) Variations of the 32.8 and 62.7 nm emissions with the pump energy. Each measurement results from an average over 5 shots. The dashed curves are fits by a power function.

Figure 3

Gain dynamics of the $4d\text{−}4p$ and $4p\text{−}4s$ laser transitions: intensity of both seeded lines while scanning the seeding delay. The electron density was $8\times {10}^{19}{\mathrm{cm}}^{-3}$.

Figure 4

Collisional-radiative calculations for an initial krypton density of $n_0={10}^{19}{\mathrm{cm}}^{-3}$ of the different populations of (a) all involved energy levels in the lasing transitions and (b) ground state and sum of all the excited states. Panels (c) and (d) show the contributions of ground state and excited states in the collisional population rate of the involved $4p$ and $4d$ levels, respectively.

Figure 5

(a) Spectrum of the two-color ASE and seeded emission and (b) far-field profile of the beam in identical conditions.