High Harmonic Generation From Gases Driven By Bichromatic Counterrotating Circularly Polarized Laser Fields

When the intense femtosecond laser fields is focused on the gases,high harmonics can be gen-erated,whose frequencies are integer multiples of the pump light frequency.High harmonics gener-ation(HHG)can be used to produce attosecond pulses in extreme ultraviolet and soft X-ray regions and has developed into the important tool of attosecond science.Circularly polarized high harmon-ics(CPHH)and circularly polarized attosecond pulses(CPAP)have very important applications in detecting chiral interactions and electronic dynamics in magnetic materials on the sub-femtosecond time scale.Therefore,the generation of CPHH in broad bandwidth with high intensity have been the focus of research in strong field optics in recent years.Researchers have proposed many schemes for generating CPHH and CPAP.Among them,the use of bichromatic counterrotating circularly po-larized(BCCP)laser fields as the pump light is one of ingenious methods.Furthermore,bicircular high harmonic spectroscopy also has many advantages in sutdying the symmetry of the system and the electronic structure of atoms and molecules.Compared to atoms,molecules have more complex sturctures and various symmetries.Therefore,there are some unique physical phenomena in the in-teraction between molecules and strong laser fields,such as orientation dependence of HHG,chiral HHG and so on.A large number of studies have shown that high harmomics from molecules can be used to accomplish a tomographic reconstruction of the molecular orbitals,studying the chirality of chiral molecules and the ultrafast processes in molecular systems.Considering the advantages of the BCCP laser fields as the pump light and the applications of the molecular high harmonics,in this dissertation,we study the HHG from polyatomic molecules driven by BCCP laser fields.Meanwhile,based on the BCCP laser fields,we proposed that a synthesized field consisting of the circularly polarized and an intense laser field can provide a momentum gate for electrons.The main contents of the paper are:1.Generating highly elliptically polarized attosecond pulses with degenerate current-carrying orbitals of molecules driven by the BCCP laser fields.When the molecules with degenerate current-carrying orbitals interact with the BCCP laser fields,there is interference effect between the high harmonics from these orbitals.The destructive interference exits in harmonic emission with the se-lected helicity and the constructive interference exits in harmonic emission with the other helicity,which results in a strong suppression of harmonic emission with the selected helicity and an en-hancement of the intensity of harmonic emission with the other helicity.The analysis shows that this interference effect is related to the noncylindrical symmetry of the molecular current-carrying orbitals and therefore depends on the relative orientation between the molecule and the driving laser fields.Our results show that this interference can be controlled by adjusting the relative intensity ratio of the BCCP laer fields and highly elliptically polarized attosecond pulses can be generated by using this interference.Furethermore,the ellipticities of these attosecond pulses can be tuned by controlling the molecular orientation.2.Probing the chirality of axial chiral molecules with HHG spectra.HHG from three typical molecules(3-chloropropa-1,2-dien-1-ol,propadiene,and 2,3-pentadiene)is simulated with time de-pendent density functional theory and strong field approximation.By comparing the high harmonic spectra driven by the fields with different helicities,we found that HHG from three-dimensional ori-ented axial chiral molecules exhibit obvious circular dichroism.However,the circular dichroism of HHG from achiral molecule is much trivial.If the axial chiral molecule belongs to the C₁point group,even in the one-dimensional orientation,the circular dichroism of its harmonics still exists.However,if the axial chiral molecule belongs to the C₂point group,the high harmonics do not exhibit circular dichroism when when the moleucle is one-dimentional orientated.3.Momentum gate for electrons with a circularly polarized control field.We propose a method for selecting the recollision electrons with specific transverse momentum at the exit of tunneling.The strategy is based on the manipulation of the consequent acceleration and recollision processes by adding a circularly polarized control field(CPCF)to an intense laser field.The scheme is illustrated by the example of tricircular laser fields,i.e.adding a CPCF to the BCCP field,and the physical mechanism of the momentum gate is analyzed in detail in the rotating frame.