Improving The Efficiency Of High-Order Harmonic Generation By Using Atomic Autoionizing And Excited States

The interaction between intense laser field with atoms and molecules releases high-energy photons,i.e.,high-order harmonics,whose frequency is high order than the laser frequency.The high-order harmonic generation(HHG)not only can become a preferred way for the generation of attosecond pulses,but also can be used to generate the coherent sources of radiation in XUV and X-ray areas,in addition,which can explore the microstructure of atoms and molecules.At present,how to improve the conversion efficiency of harmonics is a major research topic.The paper opens out it's works on the how to use the atoms autoionizing and excited states to improve conversion efficiency of harmonics.The main work is as follows:Firstly,we study the influence of the laser pulse envelope on the high-order harmonic by using the atomic resonance model potential when the resonant transition from the ground state and the autoionizing state(AIS)4d105s21s0?4d95s25p(2D)1p1 of In was taken place.The influence of the two-color field on high-order harmonic and the generation efficiency of high-order harmonic of In in less periodicity laser pulses was also explored.The results show that the transition energy between the ground state and the autoionizing state is close to the energy of the 13th-order harmonic in the 800 nm laser pulse,thus,a resonance peak with several orders of magnitude higher than the other order harmonic waves is appeared.The trapezoidal enveloping laser field produces more discrete harmonics than the Gaussian enveloping laser field,mainly due to the interference effect of the pulse.Secondly,based on the exactly model potential,a scheme for the generation of controllable ground state and lower excited state(n<3)by laser pulse driving He atom is proposed.The corresponding laser parameters are obtained by solving the three-dimensional time-dependent Schrodinger equation.Our simulations show that for the 2s,2p and 3p states,the populations at the end of laser pulse can reach 100%,while for states of 3s and 3d,the maximum population are 0.8 and 0.86,respectively,due to the very close energy levels.High-intensity harmonics can be generated by using these excited atoms.