Quantum Wave-packet Exploration Of High-order Harmonic Generation By A Chirping Effect

People’s understanding of the natural world is getting increasingly deep with the continuous progress of science and technology,the vault of heaven and microscopic particles are also included.In the microcosmos,the electron motion basically maintained at the tiny time scale,which is from dozens to thousands of attosecond(1 as=10~-1818 s),so that the same order of magnitude measurement and control tool are needed to be matched.Owing to short pulse width and high frequency,attosecond pulses make it possible to manipulate and track the ultrafast electron-dynamics processes of atoms,molecules and nano-structures,opening up new doors to the research of the microscopic world.High-order harmonic generation is an effective mean of generating isolated attosecond pulses.When the strong laser pulses interact with atoms or molecules,producing coherent radiation in that the high order nonlinear polarization,which the frequency of radiation wave is an integer multiple of incident laser,this kind of light emission is called harmonic emission.The high-order harmonic with wider platform width and higher emission efficiency is conducive to produce isolated attosecond pulse with broader and better power.The main schemes to produce high-order harmonics are:less cycle pulse,two-color field program,polarization gate program.There are many more perfect combination schemes later,such as:ionization gate scheme,superposition state scheme,chirp compensation technique,phase matching and so on.By Quantum wave package resolving the time-dependent Schrodinger equation for helium atom in a single mid-infrared laser field,we explore the frequency-chirping effect of laser field on high-order harmonic and isolated attosecond pulse generation.The main study is as follows:First,The effect of frequency chirp on high harmonics is studied theoretically.Firstly,a small chirp is added to the frequency of the laser field,the parameter isβt,and the size of the parameter is adjusted continuously.By analyzing and comparing the generated high order harmonic graph,the best chirp parameters are found to prove the favorable influence of frequency chirp on the higher harmonics.Second,The effect of phase chirp on high harmonics is studied theoretically.Firstly,phase modulation is performed on the laser field,and phase chirp is added on the basis of phase modulation to find the best phase and the best phase chirp parameter.The positive response of this phase chirp to high order harmonics is confirmed by the comparison of time-frequency analysis graphs.The results show that when the frequency chirp parameter β=0.3,the High-order harmonics appear two platforms,and the second platform is extremely wide and the width is 782 eV.The second platform superimposed on this continuous spectrum can obtain isolated pulses of 2.2 as with high intensity and very short pulse width.In phase-chirping,the high order harmonic spectra produced byφtaking 0.3π,β=0.3 are best,the second platform is 822 eV,and the isolated nanosecond pulse of 2.7 as is synthesized.The result is that an unprecedented time resolution can be obtained by adding appropriate chirp parameters to the laser field frequency or phase,breaking the new record of solitary nanosecond pulse width.