Laser-driven amplification of soft X-rays by parametric stimulated emission in neutral gases

We present a new method for parametric amplification of soft-X-ray radiation. The laser-driven amplifier is based on parametric stimulated emission and is seeded with high-order-harmonic radiation generated in the same medium. The exponential increase of the soft-X-ray yield with increasing atomic density is experimentally demonstrated for two different sets of laser parameters. A small-signal gain up to 8 x 10(3) is obtained in both experiments at about 40 eV in argon using 350-fs-long laser pulses and with 6-fs-long ones at about 260 eV in helium, respectively. This new scheme reduces the pumping threshold for lasing with a comparable conversion efficiency into the millijoule level, which is about two orders of magnitude smaller compared with the conventional plasma X-ray lasers. With a simple model, we can estimate the necessary experimental conditions for identifying the spectral range and the magnitude of the maximum gain, which are in reasonable agreement with our measurements.