Simulations on the single attosecond pulse generation by optimized multicolor laser field

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The attosecond (as) pulses are of great significance to observe and control the ultrafast electron dynamics in atoms, molecules and condensed matter. By utilizing some sub-cycle gating methods, i.e., amplitude gating, polarization gating, and double optical gating, isolated attosecond pulses (IAP) have been generated experimentally from high-order harmonics generation (HHG) by the interaction of intense femtosecond laser fields and gas targets. At present, about 50-as IAPs are now available only in few laboratories in the world. Thus, searching for the ways to generate more short IAPs is still desirable and important for the strong field physics and attosecend science. The color gating is another potential technique generating ultrashort IAP, which could generate continuum high-order harmonics without using sub-cycle laser pulses. On the theoretical side, many schemes with two- and three-color combination field have been proposed to generate short IAP, and some works propose that in this way the IAP with width less than 10 as may be generated. However, these investigations are performed only on the single-atom level, and the real experimental conditions and strength of IAPs are not considered in their simulations. In this work, we perform a numerical experiment on the generation of strong IAP from HHG driven by multicolor driving fields. Unlike the previous studies, the macroscopic effects are included in the present simulations. Considering the detection range of spectrograph, the centering energy of IAP is confined as ~200 eV. To obtain single attosecond pulse with short width, a genetic algorithm is used to optimize waveforms of two- and three-color excitation laser pulses, which is determined by intensity, wavelength and carrier envelope phase for each component. We choose the fundamental pulse as Ti:sapphire laser field with the wavelength of 800 nm to ensure the intensity of the generated attosecond pulse, and the second or third component is not confined as the commensurate harmonics of fundamental pulse. In order to possess an acceptable computation time, the model of strong field approximation is adopted in the simulations of HHG. Our simulations show that by coherent synthesizing two or three multi-cycle pulses (the full width at half maximum of each laser field is 6 fs) we obtain sub-optical-cycle laser pulse, which can easily generate supercontinuum high-order harmonic spectra with wide bandwidth. Different from the single-color driving field, which produces the attosecond pulse train, the single attosecond pulses are generated from optimized laser fields. The width of IAP decreases with the increase of intensity of driving laser field. On the single atom level, our simulations show that about 50 as and 40 as IAP can be obtained by superimposing appropriate harmonics generated by optimized two- and three-color laser pulses, respectively. More importantly, after considering the propagation effects, about 50 as IAPs are obtained with spherical spatial filtering of harmonics in the far field, under reasonable conditions, i.e., the length and position of gas jet, gas pressure. With great advent of the ultrafast lasers technology, such as coherent waveform synthesizing, optical parametric amplification and optical parametric chirped-pulse amplification, this work gives helpful guidance for intense short IAP generation in the experiment.