Molecular Rydberg Laser without inversion

Using ab-initio simulations, we demonstrate amplification of XUV radiation in a high-harmonic generation type process in the hydrogen molecular ion. A small fraction of the molecules is pumped with 9 eV photons to a dissociative Rydberg state from which IR-assisted XUV amplification is observed up to 22 eV. We show that starting at IR driving field intensities of 2 × 10 W/cm, the hitherto dominant XUV absorption of the ground state molecules becomes quasi transparent while, due to stabilisation, strong gain from Rydberg states is maintained, thus leading to Rydberg lasing without inversion. We also show that the amplified windows in the XUV range depend on the internuclear distance and may be controlled by proper timing of the pump pulse and the IR driving field.