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 $2{p}^{5}3p$ and $2{p}^{5}3s$ 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\ifmmode\pm\else\textpm\fi{}1.0 ${\mathrm{cm}}^{\ensuremath{-}1}$ for the $J=2 \mathrm{to} 1$ lines at 206.3 and 209.6 \AA{}. We also observed considerable amplification for the same transitions in yttrium at 155.0 and 157.1 \AA{}.