Research On The Generation And Application Of Supercontinus High-order Harmonic

High-order harmonic generation(HHG)is an extremely non-linear optical phenomenon,which occours when a femtosecond laser field interacts with atoms or molecules.So far,HHG has been the main way to generate attosecond pulses.The attosesond pulse offers hyperfine temporal and spatial resolutions,which has been an important tool for probing and controlling the ultrafast electronic dynamics.The generation of attosecond pulses based on HHG has been a hot topic of strong-field physics.On the other hand,HHG contain a wealth of structural and dynamical information of the target matter,which has also been an important mean of strong-field ultrafast detection.Aimming at the generation of supercontinus high-order harmonics and their applications in the generation of ultrashort attosecond pulses and the detection of molecular orbital structures,the main contents and the innovations of our work are as follows:(1)We investigate the generation of broadband supercontinuous high-order harmonics in spatially inhomogeneous laser fields.(i)We propose a multi-cycle spatially inhomogeneous two-color scheme for the generation of broadband(387 to 620 e V)supercontinous high-order harmonics in the water window region.The generated supercontinous high-order harmonics support the generation of a 40-as isolated attosecond pulse(IAP).In addition,this scheme is robust for the generation of the broadband supercontinous high-order harmonics and the IAP against variations of the carrier-envelope phases(CEPs)and the relative phase of the two laser pulses,which makes it possible to generate the ultrashort IAP by a multi-cycle laser and can also relax the requirement of laser conditions for the generation of the ultrashort IAP in the single-shot experiment.(ii)We propose a few-cycle chirped spatially inhomogeneous scheme to generate broadband supercontinous high-order harmonics.We find that the chirped pulse can efficiently control the classical electron trajectories in the inhomogeneous field.Then the harmonic cutoff is extended and the broadband supercontinous high-order harmonics near the cutoff region are generated.The generated supercontinous high-order harmonics can be used to generate an IAP with the duration less than 90 as.Moreover,as the CEP of the laser field varies,the supercontinous high-order harmonics and IAPs can still be produced.This scheme can relax the requirement of CEP stabilization of the laser for generating supercontinous high-order harmonics and IAPs.(2)We investigate the generation of intense circularly polarized(CP)high-order harmonics in femtosecond laser fields.(i)We propose to generate intense CP high-order harmonics from the Rydberg atom by a CP laser field.We find that driven by the CP laser field,the intensity of HHG from the Rydberg atom is enhanced by 4 to 5 orders of magnitude compared to that from the atom in the ground state and is comparable to that in a linearly polarized laser feld.The enhanced high-order harmonics can be used to produce intense IAPs with the ellipticity up to 0.93.Furthermore,the generation of intense CP high-orer harmonics and IAPs is robust against the variation of the CEP of the laser field.This work provides a new route toward generating intense CP high-orer harmonics and IAPs.(ii)We propose a method to produce CP high-order harmonics with tunable helicity from CO molecules by an unidirectionally rotating laser field.We find that when the unidirectionally rotating laser field irradiates the oriented CO molecule,broadband CP supercontinous high-order harmonics can be generated,which support the generation of a 100-as IAP with the ellipticity as high as 0.9 after macroscopic propagation.Moreover,by changing the orientation angle of the CO molecule,the helicity of the generated high-order harmonics as well as the attosecond pulse can be effectively tuned.This scheme enables the selective generation of the high-order harmonics and attosecond pulses with desired helicity.(3)We propose a scheme to image asymmetric molecular orbitals based on HHG in a one-color spatially inhomogeneous laser field.We find that the spatial inversion asymmetry of the inhomogeneous field can efficiently control the electron dynamics.The returning electrons with the energy in a broad range can be forced to recollide with the parent ion from only one direction,which permits reconstruct asymmetric molecular orbitals.With the harmonic spectra generated from the CO molecule oriented at different angles,the highest occupied molecular orbital of CO is satisfactorily reconstructed.This scheme uses the one-color inhomogeneous field which does not require an ultrashort duration and a stable CEP,therefore can extremely relax the requirement of laser conditions.One the other hand,the harmonic cutoff can be extended in the inhomogeneous field,the broadband high-order harmonics are helpful to improve the accuracy of the reconstruction.Furthermore,this method is also applicable to other asymmetric and symmetric molecules.