Study On The Generation Of High-order Harmonics And Isolated Ultrashort Attosecond Pulse By Three-color Combined Laser Fields

When people's cognition of physics reaches microscopic atomic scale,all matters face the same material basis.As a result,the borders between traditional disciplines such as physics,chemistry and biology gradually tend to disappear.The basic structure of matters is the electrons move around the nucleus,so the electron as the object of research will still be the focus in the future.The time of physical processes in the microcosm range from a few attoseconds to a few hundred attoseconds.With the development of attosecond laser technology,people can observe the physical processes in the atomic scale,so attosecond science has become an important research field of ultrafast optics and strong field physics.Attosecond science mainly focuses on the generation,characteristics and applications of attoseconds.In the case of the hydrogen atom,the time an electron moving around the nucleus is about 152 as.If the laser pulse duration in attosecond scale can be obtained,the motion of the electrons can be manipulated and measured precisely,and the attosecond make it possible to explore more subtle physical processes.At present,high-order harmonic generation(HHG)seems to be the most promising way to generate attosecond pulses.The researches of HHG mainly focus on how to improve the conversion efficiency and extend the HHG plateau.The generation of HHG can be explained by the "three-step model".The typical characteristics of HHG are that the intensity drop rapidly at low orders,followed by a broad plateau where all the harmonics have the same strength and appear cut-off.Currently,the usually schemes for generating attosecond pulses by means of HHG are as follows: short-period scheme,polarization gate scheme,two-color field scheme,etc.The scheme of this paper is to synthesize an optimized laser field by three-color laser and drive the atoms to generate HHG,and a supercontinuum spectrum can be obtained,and then an isolated ultrashort attosecond pulse can be obtained by superposition of the supercontinuum spectrum.The major contents are listed as follows:Firstly,we introduced the interaction model between laser and atom,the numerical solving method of the time-dependent Schr?dinger equation(TDSE),and the tool used for analyzing the harmonic characteristics in this paper.That is that the semi-classical model is adopted,and the electromagnetic field is regarded as the classical field,and the process of the interaction between the laser field and the atom is handled by the principle of quantum mechanics.In order to accurately obtain the time-dependent evolution wave function,the time-dependent generalized pseudospectral method(TDGPS)and splitoperator method are adopted.The advantage of the TDGPS is that the calculation amount is small,and the calculation accuracy is excellent,and the accuracy of the results can be ensured.Secondly,according to the semiclassical three-step model,a three-color laser field was synthesized by controlling the time of electron recombination with the parent nucleus and used for driving the hydrogen atom to emit harmonics.We analyzed the dynamic process and the time-frequency characters of the harmonic spectrum and an isolated attosecond pulse with a pulse width of 24 as was obtained.Finally,we extended the optimized three-color laser field from hydrogen atoms system to the helium atom system.The advantage is that the helium has a greater ionization energy and can withstand stronger laser interactions,so that electrons can obtain higher kinetic energy in laser fields,and then emit higher energy photon.Therefore,we may obtain shorter attosecond pulse.In addition,noble gases are usually used as targets in experiments,so that our theory are closer to experiment.The HHG radiation spectrum is obtained by numerical solving the TDSE of helium atom in combined laser field,and the dynamic process of harmonic emission is analyzed.We found that the harmonic spectrum of helium system has some common characteristics with the hydrogen system,and some new characteristics were shown.An isolated attosecond pulse with width of 16 as was obtained by superposition of the supercontinuum of helium derived by the optimized three-color laser field.After further optimization of laser field parameters,an attosecond pulse with width of 10 as was finally obtained.