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Simple Route to Enhancement of Soft X-Ray High Harmonic Generation Sources

Soft x-ray sources based on laser-driven high-harmonic generation (HHG) offer a tabletop alternative to sources generated at synchrotron and free-electron laser facilities [1,2]. In this work we demonstrate two orders of magnitude enhancement in conversion efficiency of HHG soft x-rays. The enhancement occurs on a single particle level and is based upon sub-optical-cycle control and enhancement of the tunnelling ionization rate [3]. Using a simple two-colour synthesizer to drive HHG, here we show that both high enhancement and high HHG flux can be achieved in the soft x-ray (SXR) region. In the presented results, the enhancement spans over a 250 eV bandwidth with pulse energies reaching the pJ level.