Optimized Two-Color Laser Pulses For Enhanced High-Harmonic Efficiency

High-order harmonic generation(HHG) is a nonlinear response of atoms and molecules to intense laser fields. HHG is one of the most important research of the strong-field physics due to its potential application, such as the generation of extreme ultraviolet(XUV) light source and even soft X radiation source, the synthesis of attosecond pulses and the detection of ultrafast processes. High-order harmonic have been successfully applied to many research fields, for example attosecond science, diffraction imaging, magnetic resonance scattering. At present, the restriction on harmonic for its practical application is that conversion efficiency is very low. This work focus on how to improve the conversion efficiency of high-order harmonics by optimizing the waveform of two-color laser field. The details are as follows.First, the waveform of a two-color laser field is determined by using a genetic algorithm to optimize the maximum high-order harmonic yield. The harmonic intensities(conversion efficiencies) generated by the optimized laser field can be increased by at least one order without increasing total laser power, for the lasers with wavelengths of 800 nm to 3000 nm.Second, we compared the wavelength scaling of single-atom and macroscopic harmonic yield on the basis of previous work. For the single atom, the harmonic yield scales as-5? ~ 8-?, which agrees with the previous results. While for the macroscopic high harmonic generation, its efficiency falls more dramatically with increasing wavelength, with a very unfavorable-8? ~ 13-? scaling. Our simulations also show that it is difficult to obtain a better wavelength scaling of harmonic yield in a two-color than in the single-color fields. The present simulations will have an enormous impact on the future application of HHG in the laboratory.