Demonstration of a Soft X-Ray Amplifier

We report observations of amplified spontaneous emission at soft x-ray wavelengths. An optical laser ionized thin foils of selenium to produce a population inversion of the $2p^{5}3p$ and $2p^{5}3s$ levels of the neonlike ion. Using three time-resolved, spectroscopic measurements we demonstrated gain-length products up to 6.5 and gain coefficients of 5.5±1.0 ${\mathrm{cm}}^{-1\mathrm{}}$ for the $J=2\mathrm{} \mathrm{to} 1$ lines at 206.3 and 209.6 Å. We also observed considerable amplification for the same transitions in yttrium at 155.0 and 157.1 Å.