Generation of Fourier-limited ultrashort soft X-ray isolated attosecond pulses by two-color chirped lasers without chirp compensation

The inherent attosecond chirp (atto-chirp) in high-order harmonic generation (HHG) is one of the primary restrictions on shortening the duration of attosecond pulses. Although dispersive materials can be used to compensate for the atto-chirp in the extreme ultraviolet region to produce a short isolated attosecond pulse (IAP), it is not feasible for higher-energy X-ray broadband harmonics due to the lack of natural materials with negative group velocity dispersion and low loss in this range. To resolve this issue, we have recently proposed an effective way for generating the chirp-free short IAP in the water window without the need for dispersive materials, although it requires the use of a four-color shaping laser field [ Opt. Lett. 49 , 1571 ( 2024 ) OPLEDP 0146-9592 10.1364/OL.515677 ]. In this work, we further theoretically demonstrate that HHG driven by a two-color chirped laser field can generate Fourier-limited X-ray IAPs with durations as short as ∼30 as and bandwidths exceeding 150 eV with different central photon energies (240–600 eV) without atto-chirp compensation with natural materials, both on single atom and macroscopic levels. It has been discovered that chirp-free IAPs created by such a laser field result from the interference of platform harmonic radiations produced from several ionization events. This work provides a potential scheme for generating ultrashort soft X-ray IAPs without compensating harmonic chirps with dispersive materials to meet the high demand for these radiation sources.