Nonlinear Thz Radiation Investigated By Quantum Simulations

Terahertz（THz）:between microwave and infrared light,the frequency:0.1～10THz,wavelength:0.03～3 mm.THz can be generated by the interaction of external enhanced laser field(～10¹³W/cm²and above)with atoms or molecules.THz generated by ultrashort laser pulses interacting with air has been widely studied.There are two important processes in the generation of THz:first,electrons outside atomic or molecular nuclei,driven by an external laser field,get rid of the Coulomb bound of nuclei and are ionized.Second,ionized electrons move in space.That is,the electrons outside the atomic or molecular nucleus will break away from the Coulomb bound of the nucleus under the action of the external laser field,and tunnel ionization will occur.At the same time,the ionized electrons will move in space at a certain speed to form a current,and at the same time emit light waves in THz frequency band.The generation process of THz is similar to the three-step model in higher harmonics.Especially in the first two steps of the higher harmonic three-step model,electrons are separated and electrons move in space.THz has always been a hot topic,but improving the yield of THz has always been a hot issue that people pay more attention to.Many problems of THz emission from ultra-intense ultra-short laser and some target materials have been studied,but there are still many problems to be further studied,discovered and solved.People have never stopped exploring the generation process of THz.We solved the Time-dependent Schr（?）dinger equation（TDSE）,using the semiclassical photocurrent model（PC）,two different model methods to carry out our theoretical research work.Our research work mainly includes the following aspects:（1）We have developed a new method to simulate THz based on TDSE.We separate the wave function.When the wave function evolves in space,we divide the spatial wave function into two parts,the internal spatial wave function and the external spatial wave function.In this way,we can extract the absorbed wave function.According to the classical photocurrent model,the wave function that really contributes to THz is the absorbed wave function.Due to the high resolution requirement of our spectrum,our simulation time is very long,but our computer memory is limited.In order to solve this problem,we convert the external spatial wave function from length specification to velocity specification and propagate in momentum space,which greatly reduces our calculation amount,improves the calculation efficiency and,most importantly,improves the accuracy of THz simulation.（2）We studied the quantum effect and the influence of the electron wave function of the first excited state on the THz generation process.At the same time,we also studied the interference effect of electron wave packet on THz generation process.The classical photocurrent model does not consider the contribution of the first excited state,but only the contribution of the ground state electrons.This is equivalent to a two-level system in which electrons directly transition from the ground state to the continuous state,which is very unscientific and imprecise.To solve this problem,we adopted two different model methods and qualitatively reached the same conclusion.（3）We can improve THz yield by introducing a suitable electrostatic field into the driving laser field.It is generally believed that THz cannot be generated by monochromatic field.In the experiment,two-color field is mostly used,and THz is generated by the combined field of fundamental light and its second harmonic.We can also generate THz by introducing an electrostatic field into a monochromatic field.Of course,we can improve THz yield and have more choices for the wavelength and phase of the second laser beam by introducing a reasonable electrostatic field into a two-color field system.At the same time,with the increase of electrostatic field,THz yield will also increase.（4）The strong field interacts with the target material to radiate THz,and the target material we choose is gas（nitrogen）.