Polarization Modulation And Properties Of High Harmonics

When an intense femtosecond laser pulse interacts with atoms and molecules,high order harmonics are radiated.High order harmonic generation(HHG)is a highly non-linear and non-perturbative process.So far,HHG has been the most promising way to obtain attosecond extreme ultraviolet and soft x-ray pulses.Due to the attosecond temporal and ?ngstr?m spatial resolutions of the attosecond pulse base on HHG,it provides possibility to uncover new fundamental understanding of dynamics in atoms,molecules,and materials.With the rapid development of laser technology,people have entered the research field of attosecond science,and have made unremitting endeavor to achieve even shorter and even stronger attosecond pulses.Polarization is a basic property of the light wave.Comparing to the linearly polarized laser pulses,the circularly or elliptically polarized XUV and soft x-ray provide an additional degree of freedom and greatly extend the range of applications.They are very important for investigation of the magnetic and chiral properties under the interaction between attosecond pulse and targets.Therefore,generation of the circularly polarized attesecond pulse becomes urgent.Aiming at high-efficient generation of the circularly and elliptically polarized attosecond pulses and controlling their polarization properties in frequency and temporal domain,the main contents and the innovations of our work are as follows:(1)We propose a method to generate near-circularly polarized single attosecond pulse based on the interaction of aligned nitrogen molecules and spatially inhomogeneous laser fields.The intensity of the enhanced laser field increases exponentially with the position.The electrons can obtain more energy in the spatially inhomogeneous laser fields and the cutoff position in the harmonic spectra can be extended.The spatially inhomogeneous can modify the trajectories of the electrons suppressing some recombination of electrons.A supercontinuum with wide bandwidth in the XUV can be generated.In the supercontinuum,one can obtain a near-circularly polarized single attosecond pulse with pulse duration 155 as by synthesising a range of harmonics.For different target molecules,one can achieve the same results based on our method.(2)We propose and theoretically demonstrated a method to generate attosecond XUV pulses with tunable ellipticity by using a rotating bichromatic counterrotating circularly polarized laser field(BCCP).By adjusting the relative phases between the two circularly polarized components,the BCCP driving laser field can rotate on the polarization plane.When the BCCP laser pulse with different relative phase interacts with the aligned non-spherical targets,the high harmonics with tunable ellipticity and helicity will be generated.The tunability of the harmonic polarization is based on the dependence of the harmonic process on the angle between the driving field and the molecular axis.One can obtain the attosecond pulses with arbitrary ellpticity and helicity only just by adjusting the relative phases.The relative phase can be easily adjusted in an experiment.Such a property makes our scheme extremely attractive for practical implementation.(3)We investigate the polarization properties of high harmonics generated with BCCP laser field.When the BCCP laser field interacts with the 2p orbital of Ne atom,it is found that the helicity of the elliptically polarized harmonic emission is reversed at particular harmonic orders.Based on the time-frequency analysis and the classical three-step model,the correspondence between the positions of helicity reversions and the classical trajectories of continuum electrons is established.Our study performs a detailed analysis of high harmonics in terms of electron trajectories and depicts a clear and intuitive physical picture of the HHG process in BCCP laser fields.(4)We demonstrate a scheme to produce coherent extreme ultraviolet and soft x-ray radiation with subpetahertz temporal helicity modulation based on the electronic dynamics of current carrying states.When a linearly polarized laser pulse interacts with current carrying state,the electronic possibility distribution and electronic angular momentum of the orbitals oscillates periodically in femtosecond scale.This intriguing and undescribed phenomenon is qualitatively interpreted under adiabatic approximation and attributed to the different energy shifts of the instantaneous eigenstates.The properties of wavepackets can be reflected in HHG.The wavepackets with positive angular moment is more likely to generate radiation with positive helicity and vice versa.So we can obtain the attosecond pulse with ultrafast helicity modulation in temporal domain.The frequency of the helicity modulation reaches sub-PHz and can be continuously adjusted by the intensity of the driving field.