Super-continuum Spectra Of High-order Harmonic Generation And Isolated Attosecond Pulse Generation In Intense Laser Pulse

As the development of laser technology, the research on the interactionbetween the laser pulses and atoms, molecular etc. has attracted extensive attention.The generation of the attosecond pulse opens a new area of the microscopic world:attosecond science. Because of the short width and the big frequency, the attosecondpulse could probe the electronic dynamics even control the electron, which is verybenefit to understand the microscopic world. Currently, the High-order HarmonicGeneration （HHG） is the most promising way to generate attosecond （as） pulses inexperiment. HHG whose frequency is an integral multiple of that of the incidentlaser field is radiated is caused by the highly nonlinear electronic response. TheHHG spectrum has a plateau structure that can’t be understood by the traditionaryperturbation theory. Attosecond pulse can be generated by superimposing abandwidth of the harmonics in the plateau area because of its character, such as theintensity of the harmonics generation is close, the phase is pretty much the same, thefrequency interval is equal and so on. In this thesis, we theoretically investigate thehigh-order harmonic generation and attosecond pulse generation with differentschemes. Detailed research work mainly includes three parts as follows:First, we investigate the HHG and isolated attosecond pulse （IAP） by the scheme of combined filed with add a static electric field. We investigate the HHGand （IAP） generation form the Helium atom by adding a static electric field to thepolarization gating scheme （PG）, which is combining by a left-and a right-circularlypolarized laser field. We find that, for the case of the ratio of the static field is zero,two peaks A1and B1with both long and short quantum path are contributed to theHHG; for the case of α=0.1, the harmonic spectrum presents a two-plateaustructure and the time-frequency analysis shows that the peak A2is suppressed andthe long path of the peak B2is weaken, so that only the short path of the B2iscontributed to the second plateau and the quantum interference is suppressed whenthe static field is added. The single attosecond pulse with a duration of80as isobtained by superposing the supercontinuum area of the second plateau50eV （160eV-210eV）.We also theoretically investigate the HHG and IAP generation by combing astatic field with an orthogonally polarized two-color laser field which is synthesizedby12.5fs,2000nm pulse in y-component and a weaker12fs,800nm pulse inx-component. We found that the plateau of the harmonic spectrum is broaden whena static field is added in the y-component. We investigate the emission time of theharmonics in terms of the time-frequency analysis to illustrate the physicalmechanism of HHG. We also calculate the ionization rate using ADK model tointerpret the variation of harmonic intensity for different static electric field strength.For the case of field strength ratio α=0.1between the static field and the laserfield of the y-component, the supercontinuum harmonic plateau is formed from220eV to420eV, and an IAP with a duration of about75as is obtained by superposinga bandwidth of50eV （300eV-350eV）, and the attosecond pulse is nearly linearpolarized.Second, we theoretically investigate the quantum path control and IAPgeneration by using a left and a right-circularly polarized Gaussian pulse with aproper time-delay. For the case of the carrier phase and the frequencies of the twopulses are equal, i.e. a PG scheme. For the case of the the carrier phase or thefrequencies of the two pulses are different, the ‘gating’ structure of the combined laser field disappears, but the numerical results indicate that no “gating” structurecombined laser pulse can obtain a smoother harmonic spectrum than the PG scheme.From the time-frequency analysis we found that the long path of the peakB₂isweaker for the case of no “gating” structure combined laser pulse than for the caseof the PG scheme. The HHG can be well explained by the semiclassical three-stepmodelm, and an transverse velocity is considered for the initial time to compensatethe transverse displacement. Change the wavelength of the right-circularly polarizedpulse, we found that for the case ofλ₂=1600nm, quantum path was controlledand only the short path of the peakA₄contributed to the HHG. By superposing abandwidth of50eV in the plateau area, we can obtain an IAP with the duration ofsub-100as. We also investigated both the left and a right-circularly polarized are10fs,1600nm, and found that for the case of φ₁=0，φ₂=0.5π, a super-continuumplateau was obtained from180to570eV, which contains the whole “waterwindow” spectrum. By superposing a arbitrary bandwidth of50eV in the plateauarea, a tunable “water window” attosecond pulse with the duration of75as would beobtained. By Lewenstein theory, we reappeared the results form the Split-Operatormethod, and found it is equivalent for this model.Third, we theoretically investigate the HHG and IAP generation from atwo-dimensional model of Ar⁺cluster in a synthesized two-color laser pulse. Ascheme to generate an IAP is proposed by using a4fs800nm fundamental field incombination with a wavelength adjustable weaker pulse. By changing the number ofthe surrounding Ar ions we investigated the HHG, and found that the bigger numberof the surrounding Ar ions, the higher cutoff energy, and the intensity of the HHGwas also enhanced. From the time-frequency analysis, we found that the quantumpath is not a long quantum path and a short quantum path for a peak as an atom, butmany paths in one energy peak, which is because the many cores around the centerAr atom. Take the number of the surrounding Ar ions is24as an example, wechange the parameters of the pulse, and found for the case ofλ₂=1200nm, a super-continuum HHG spectrum is obtained with the cutoff220eV. By superposinga bandwidth of45eV （175eV-220eV）, we obtain an IAP with a duration of90as,which accompanied with three weak satellite pulses. For the case of4fs800nmfundamental field with a intensity ofI₁=3×10¹⁴/cm², combination with a10fs1200nm pulse with the intensity I₂=×10¹⁴W/cm², a super-continuumHHG spectrum with a band-width of100eV is obtained, and an almost linearlypolarized IAP with the duration of45as is obtained straightforwardly bysuperposing a bandwidth of100eV near the cutoff of high-order harmonicgeneration.