| High-order harmonic generation?HHG?is a high nonlinear process for producing coherent optical radiation in the extreme ultraviolet?XUV?or even in the soft X-ray range when atoms and molecules are exposed to a strong laser field.Therefore,it has a very wide range of applications.First of all,it not only can be used as a low-cost XUV light source,but also it is one of the most effective means of obtaining an optical pulse of attosecond(10-18s)duration.In addition,we can also observe the ultra-fast dynamic behavior inside atoms and molecules.The research of HHG has been concerned by researchers all the time.Especially in recent years,with the rapid development of laser technology,these high-order harmonics driven by complex laser pulses have produced some new interesting features,such as the double circular pulse field drives atoms to produce circularly polarized higher harmonics.It has been found that the polarization of the harmonics produced by the two-color,two-circle laser field is adjustable and can vary arbitrarily from circular to elliptical to linear.We use a beam of elliptical laser to drive the Ar cluster ions to generate higher harmonics and observe their polarization characteristics.In this thesis,we use a two-dimensional multi-well model to simulate the response of an ionization cluster composed of a single atom surrounded by multiple ions to an ultrashort elliptically polarized laser pulse.The standard second-order splitting algorithm is used to solve the two-dimensional time-dependent Schr?dinger equation.In the time evolution of the wave function,we use the fast Fourier transform?FFT?and obtain the initial ground state wave function through the virtual time evolution method.It is found that as the number of ions around the Ar atom increases,the cutoff position of the harmonic spectrum is significantly expanded.For Ar cluster ions,the return electrons can be recombined not only to the parent ion,but also to other ions around the parent ion.Besides,the lower order harmonic intensity of the Ar cluster ions is increased by about two orders of magnitude compared to the single Ar atom case.The generation of low-order harmonics mainly comes from the transition contribution from the bound state to the bound state.For a single Ar atom,the intensity of the below-threshod harmonics and the harmonics near the cutoff position decrease rapidly with the increasing of laser ellipticity.With the increase of the number of Ar ions around the central Ar atom,the ionized electrons can be combined with the non-parent ions in the large ellipticity laser field to emit harmonic photons,resulting in the slower decreasing of the harmonic intensity near the cutoff position as the ellipticity increases.For the below-threshod harmonics,as the number of Ar ions increases,the electrons excited from the parent ions can scatter or be recombined with other surrounding Ar ions,increasing the probability of the excited state transition to the ground state.Thus,strong harmonics can still be generated under the laser field with large ellipticity rate,which provides a simple way to experimentally obtain the HHGs with large ellipticity. |
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