Theory Study On The Origins Of Below-threshold Harmonics In Strong Laser-atom Interactions

The investigations of HHG have never been stopped since people first ob-served HHG spectrum in the experiment in 1980s.Most previous studies have concentrated on the harmonic generation beyond the ionization threshold.Less attention has been paid to the below-threshold harmonics(BTHs).In recent years as it provides a potential approach for producing vacuum-ultraviolet(VU-V)frequency combs,the generation of below-threshold harmonics(BTHs)has attracted a lot of attention.Unlike the higher-order harmonics can be described by three step model,the origin of BTHs is very complex.However,there are emerging evidences that rescattering trajectories(such as the long trajectory,the short one and multiple returns)relating to this model also contribute to BTHs.However,as these mechanisms mingling with each other,it still does not have a clear and indisputable explains about this question.Therefore,a quantitative analysis on the contributions of different rescattering trajectories to BTHs with varied laser parameters are highly desired.The explanation of above questions constitute the main purpose of this the-sis,which is divided into five chapters,as following:In the 1 st chapter,we briefly review the development of the laser technol-ogy and introduces the interesting non-perturbative phenomena of strong field physics.At the same time,We introduce the research significance and advances in the high-order harmonic generation.The second chapter details the theoretical methods in studying the high-order harmonic generation:the semiclassical rescattering model and Lewestein model.The third chapter,first we briefly introduces several numerical solution of the time-dependent Schrodinger equation(TDSE):the Floquet method,direct integration method and time-evolution method.We give a detail explanations about the split-operator pseudospectral method,which is used in this thesis.At the same time,we show the wavelet method of the wave function.In the forth chapter,we tracking origins of below-threshold harmonics with a trajectory-resolved full-quantum approach.With numerical solution of the time dependent Schrodinger equation(TDSE)for a three dimensional model He atom,we study the generation mechanism of BTHs at varied laser intensities and wavelengths.Furthermore,a scheme is developed based on TDSE simulations with the help of a semiclassical model including the atomic potential,which al-lows us to "pick out" the contributions of desired trajectories(such as the short trajectory,the long one,or multiple returns)to BTHs.At the same time,the con-tributions of other rescattering trajectories are well excluded.We also plot the time energy distribution of the rescattering wave packet and use a full-quantum energy analysis.Our simulations show that multiple returns,especially the third return,contribute substantially to BTHs at high laser intensities as the long tra-jectory plays a trivial role.At low laser intensities,the main contributions to BTHs,however,come from multiphoton-related processes occurring near the core.The important role of the interference between the different contributions in BTHs is also revealed.