Research On The Cut-off Position Of High-order Harmonics In Two-color Field

When bound electrons in atoms and molecules are irradiated by high-intensity near-infrared laser light,they emit high-frequency radiation outward at an integer multiple of the driving laser frequency,and the emitted light pulses can reach the extreme ultraviolet and soft X-ray band of the electromagnetic spectrum.Since the frequency of the emission is high and limited to integer multiples of the driving field frequency,this process is known as the generation of high harmonics.Given a monochromatic linearly polarized driving field,P.B.Corkum et al.proposed a recognized semi-classical model for this process,also known as the"three-step model",which divides the emission of high harmonics into three main processes:ionization,acceleration and recombination,and formulas for the corresponding cut-off positions are given.However,for the cut-off position of high harmonics in a complex two-color field,people have not give a better method for judging the cut-off frequency.For this reason,this thesis studies the emission process of the high-order harmonics of argon atoms in the two-color field by numerically solving the time-dependent Schrodinger equation and combining the classical trajectory analysis,and focuses on the analysis of the two-color parallel linear polarization and the two-color orthogonal linear polarization field.The cut-off rule of harmonics under the following two aspects:(1)The law of the cut-off position of higher harmonics under the action of two-color parallel linearly polarized laser pulses is studied.Firstly,the high-order harmonic emission of argon atom under the action of two-color linearly polarized laser pulse is calculated by numerically solving the time-dependent Schr(?)dinger equation,and the corresponding cut-off position is found through its harmonic spectrum,and then the harmonic under the classical three-step model is calculated.The maximum kinetic energy was found to be consistent.After that,we systematically studied the relationship between the maximum re-impact kinetic energy of ionized electrons and the parameters of the two-color field under the driving of the two-color field with different field strength ratios and different phase differences by the classical trajectory method with higher computational efficiency,and then summarized the generated high-order The law of the maximum cutoff frequency of harmonics.It is found that under the parallel line polarized bichromatic field,the high-order harmonic cut-off position of the atom can also be given by a simple rule of a simple modified plasmic kinetic energy U_p,namelyE_cutoff(28)3.0U_p+I_p,where the modified plasmic kinetic energy under the bicolor parallel line polarized field Energy (?)andI_p are atomic ionization energy.The relative error between the harmonic cutoff position predicted by this formula and the exact cutoff position of the quantum calculated harmonic spectrum does not exceed 10%.(2)The law of the cut-off position of atomic high-order harmonics under the action of orthogonal two-color linearly polarized laser pulses is studied.Firstly,the high-order harmonic emission of argon atom under the action of orthogonal two-color linearly polarized laser pulse is simulated by numerically solving the time-dependent Schrodinger equation,and its cut-off position is found by harmonic spectrum.The theoretical calculation results show that when the total light intensity of the two-color field is constant,the cut-off frequency of the generated high-order harmonics is regulated by the amplitude ratio and relative phase difference of the two driving fields.In the case of a certain relative phase difference,the higher the amplitude of the low-frequency driving field,the larger the harmonic cutoff energy;in the case of a certain amplitude ratio,the cutoff energy changes periodically with a small amplitude of the relative phase difference,different frequency ratios correspond to different periods.If the frequency ratio is n:1,the period is?/n.However,although the harmonic cutoff position changes with the relative strength and phase difference of the two-color field,it still basically follows E_cutoff=3.0U_p+I_p ,where the total gravitational energy in the quadrature field,(?).The above-mentioned regular formula for the cut-off energy of higher harmonics in the two-color field provides a simple estimation method for pre-judging the spectral width of the harmonic spectrum in experiments,and has certain reference significance.