High-order Harmonic Generation In Atoms Induced By A Bicircular Field

High harmonic generation(HHG)in atoms and molecules is a highly nonlinear process that up-converts intense infrared laser field into the extreme ultraviolet(XUV)and soft x-ray radiation.The emitted light can be used to track quantum dynamics underlying the nonlinear response,or as a tabletop source of bright,coherent,ultra-short pulses.This radiation can be used as a low-cost XUV light source,and is currently the most effective means of obtaining a pulses of attosecond(10-18s)duration.It can also be used as a detection method for the sophisticated molecular structure and the dynamic processes.Therefore,the study of the harmonic characteristics has been an important research topic in strong fields.Especially in recent years,with the development of the femtosecond laser technology and the emergence of a variety of advanced time,space,polarization and other shaping technology,some new characteristics of the high harmonics driven by these complex laser pulse are found,such as the atoms in the bicircular field produce the circularly polarized high harmonics.In this paper,we simulate the single atomic response process under the bicircular field by numerically solving the time-dependent Schr?dinger equation of the atom in the bicircular driven pulse.In theory,we systematically studied the generation mechanism of the harmonics,the forbidden rules of the harmonics order,the cutoff rules of the harmonic spectra,the polarization characteristics of the harmonic field,and the relative intensity ratio,the relative phase difference,the rotation direction and so on of the two circular polarization pulse of the bicircular field.The theoretical study shows that a bicircular field consists of two coplanar counter-rotating circularly polarized fields and a frequency ratio of 1:2,the atom high harmonic frequency is((42))qq=±3,2,113 times of that of the low frequency driving field,and the 3qth order harmonic does not exist.This conclusion can be explained by the quantum transition selection rule,that is,the atomic electron in the ground state absorbs multiple photons of the driven light,and transits to the high-energy continuous state,and then release a harmonic photon to transit to the ground state,which follows the angular momentum conservation.From the perspective of the classical three-step model,the ground-state electrons undergo tunneling ionization in the driving field,and in the third-order low-frequency period it obtains the kinetic energy under the driving field and the coulomb field of the parent nucleus and returns to the parent ion again and recombine with it,then emits the high-energy harmonic photons.The maximum of the photon energy is about pp2.12/3.17 IU +.In addition,each of the harmonics field of single order are circularly polarized,and the left circularly polarized and the right circularly polarized are interevent time distribution,which is also given by a clear explanation with the quantum transitions.We also found that the harmonic field corresponding to a wider band shows a linear polarization in a particular direction,and the polarization direction rotates by 120 degrees every third of the low frequency period.This can explained with the ionized electron action contribute to the harmonic emission given by the classical three-step model.Based on the mechanism,characteristics and regularity of atomic harmonics in the double circle field,we found that the left or right direction of the harmonic field can be controlled by the rotation direction of the bicircular field.The relative phase difference can be used to control the polarization direction of the harmonic field in a certain frequency band.Finally,we also found that the relative intensity of the specific order harmonic can be increased or inhibited to more than an order of magnitude by adjusting the intensity ratio of two circularly polarized driving fields.This research of the atomic harmonic system driven by the bicircular field deepens the understanding of the new characteristics and laws of the atomic high harmonics driven by the complex light field.It accumulates the experience of controlling and using the strong field harmonics more effective by the shaping and controlling means of driving the laser.