Generation of a quasi-chirp-free short isolated attosecond pulse from high-order harmonics by optimized multicolor laser fields

A chirped isolated attosecond pulse (IAP) will be usually produced if several chirped supercontinuum highorder harmonics are coherently superposed. Thus, to generate a broad bandwidth quasi-chirp-free IAP or even short isolated subattosecond pulse, one needs to reduce the attosecond chirp (attochirp) of high-order harmonic pulses. In this paper, we theoretically demonstrate the possibility of generating the quasi-chirp-free IAP from neon atom driven by the optimized four-color laser field, which is synthesized by two fundamental near-infrared lasers and their second-harmonic fields. By increasing the colors in synthesized field from two or three to four, the attochirp of broadband high-order harmonics over 100 eV is significantly reduced, and the generated IAP is shortened from more than 50 attosecond (as) to 37 as, approaching the Fourier-transform-limited pulse duration of 36 as. The classical analysis reveals that this mainly stems from the simultaneous return of electrons ionized at different times to the parent ion, as has been shown from other investigations regarding the necessity of producing attochirp-free high harmonics with multicolor laser fields. Most importantly, such short quasi-chirp-free isolated IAP survives after the macroscopic propagation effects are carried out, under proper gas pressure and laser beam waist. This work provides an efficient method for experiments to generate the quasi-chirp-free IAPs by using the synthesized and optimized four-color laser field.