Generation of Bright Isolated Attosecond Soft X-Ray Pulses Driven by Multi-Cycle Mid-Infrared Lasers*

Advances in the understanding of macroscopic phase-matching of high harmonic generation (HHG) driven by mid-IR lasers have made it possible to generate bright, coherent, high harmonic x-ray beams in the UV to keV with attosecond or even zeptosecond bandwidths. We perform advanced theoretical analysis, corroborated with experimental results, to unveil the characteristics of HHG soft x-ray \textit{as} pulses. We show that when mid-IR lasers are used to drive HHG, the conditions for optimal soft x-ray generation naturally coincide with the generation of bright isolated \textit{as} pulses that are also shorter in duration if compressed. In addition, in contrast to \textit{as} pulse generation in the EUV, multi-cycle driving laser pulses are more suitable for generating bright isolated soft x-ray bursts.