双圆场中原子的高次谐波发射
发布时间：2019-05-10 07:17
【摘要】：原子、分子的高次谐波发射(HHG)是一个高度非线性的过程,即原子、分子在强红外线偏振激光脉冲作用下产生极紫外(XUV)甚至软X射线波段的相干光辐射。这种辐射可作为低成本XUV光源,也是当前获得阿秒(10-18s)持续时间光脉冲的最有效手段,也可作为复杂的分子结构及动力学过程的探测手段等。因此关于谐波特性的研究一直是强场领域的热门课题。尤其是近几年,随着飞秒激光技术的进步,各种先进的时间、空间、偏振等整形技术的出现,使得这些复杂激光脉冲驱动下的高次谐波产生一些新的特性。如双圆脉冲场驱动原子可以产生圆偏振的高次谐波。本文通过精确数值求解双圆驱动脉冲与原子相互作用的含时薛定谔方程,模拟原子在双圆场驱动下的单原子响应过程。在理论上系统地研究了原子在双圆场驱动下高次谐波的产生机理、谐波阶次的禁戒规律、谐波谱的截止规则、谐波场的偏振特点,以及双圆场中两个圆偏振脉冲的相对强度比、相对相位差、旋转方向等对高次谐波的影响。理论研究表明,由两个共面反向旋转、频率比为1:2的圆极化脉冲组成的双圆场,在这样的双圆场驱动下,原子产生的高次谐波频率为低频驱动场频率的()qq=±3,2,113(42)倍,3q次谐波不存在。这一结论可以由量子跃迁选择定则给出解释,即原子的基态电子吸收多个驱动光光子跃迁到高能连续态,再由高能连续态释放一个谐波光子跃迁回基态,在该过程中遵循角动量守恒。由经典三步模型角度来分析,基态电子在驱动场作用下发生隧穿电离,之后的三分之一低频周期时间内,在驱动场和母核库仑场共同作用下,获得一定动能并再次返回到母离子附近并与之复合发射高能谐波光子。最大光子能量约为pp2.12/3.17IU+。另外,各个单一阶次的谐波场均是圆偏振的,左旋圆偏振和右旋圆偏振间隔分布,这也通过量子跃迁给出了明确解释。我们还发现,对应于一段较宽频带的谐波场,表现出在特定方向线偏振,偏振方向每三分之一低频周期旋转120度。这一点可以从经典三步模型给出的贡献于谐波发射的电离电子的运动给出解释。基于上述双圆场下原子高次谐波的机理、特点和规律,我们发现可以通过双圆场的旋转方向来控制特定频率谐波场的左旋或右旋方向;也可以通过两个圆偏振驱动场的相对相位差,调控一定频带的谐波场的极化方向;最后我们还发现通过两个圆偏振驱动场的强度比的调控,可以增强或抑制特定阶次谐波的相对强度达一个数量级以上。这篇双圆场驱动下原子高次谐波的系统研究工作,深化了人们对复杂光场驱动下原子高次谐波的新的特性和规律的认识。为人们通过驱动激光的整形调控手段,更加有效地操控和利用强场高次谐波积累了经验。
[Abstract]:The high-order harmonic emission (HHG) of atoms and molecules is a highly nonlinear process, in which atoms and molecules produce ultra-ultraviolet (XUV) and even coherent light radiation in soft X-ray band under the action of intense infrared polarization laser pulses. This radiation can be used as a low-cost XUV light source. It is also the most effective means to obtain the attosecond (10? 18s) continuous-time light pulse. It can also be used to detect the complex molecular structure and the kinetic process, and so on. Therefore, the study of harmonic characteristics has always been a hot topic in the field of strong field. Especially in recent years, with the progress of femtosecond laser technology and the emergence of various advanced time, space, polarization and other shaping techniques, these complex laser pulse-driven high-order harmonics produce some new characteristics. For example, the driving atom of double circular pulse field can produce high order harmonics of circular polarization. In this paper, the time-dependent Schrodinger equation of the interaction between double circular driving pulse and atom is solved accurately, and the single atomic response process driven by double circular field is simulated. The generation mechanism of high order harmonics, the forbidden law of harmonic order, the cut-off rule of harmonic spectrum, the polarization characteristics of harmonic field and the relative intensity ratio of two circular polarization pulse in double circular field are systematically studied theoretically. The influence of relative phase difference and rotation direction on high order harmonics. The theoretical study shows that the double circular field is composed of two coplanar reverse rotation and the frequency ratio is 1:2. Driven by such a double circular field, the high order harmonic frequency produced by the atom is () qq= 卤3, 2 of the low frequency driving field frequency. 113 (42) times, 3Q harmonics do not exist. This conclusion can be explained by the selection rule of quantum transition, that is, the electron of the ground state of the atom absorbs a number of driven photons to the high energy continuous state, and then releases a harmonic photon transition back to the ground state from the high energy continuous state. In this process, the conservation of angular momentum is followed. From the point of view of the classical three-step model, the ground state electrons are tunneling ionized under the action of the driving field, and then under the interaction of the driving field and the parent nuclear Coulomb field in the 1/3 low frequency periodic time thereafter. A certain kinetic energy is obtained and then returned to the vicinity of the parent ion and combined with it to emit high-energy harmonic photons. The maximum photonic energy is about pp2.12/3.17IU. In addition, the harmonic fields of each single order are circularly polarized, and the distribution of left-handed circular polarization and right-handed circular polarization interval is given, which is also explained by quantum transition. We also find that the harmonic field corresponding to a wide band is polarized in a specific direction, and the polarization direction rotates 120 degrees per 1/3 low frequency period. This can be explained by the motion of ionized electrons contributed to harmonic emission by the classical three-step model. Based on the mechanism, characteristics and law of atomic high order harmonics in the double circular field, we find that the left or right rotation direction of the harmonic field at a specific frequency can be controlled by the rotation direction of the double circular field. The polarization direction of harmonic field in a certain frequency band can also be controlled by the relative phase difference of two circular polarization driving fields. Finally, it is found that the relative intensity of specific order harmonics can be enhanced or suppressed by adjusting the intensity ratio of two circular polarization driving fields. In this paper, the systematic research work of atomic high order harmonics driven by double circular field deepens the understanding of the new characteristics and laws of atomic high order harmonics driven by complex light field. It accumulates experience for people to control and utilize high order harmonics in strong field more effectively by shaping and controlling laser driving.
【学位授予单位】：吉林大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：O562
本文编号：2473448
[Abstract]:The high-order harmonic emission (HHG) of atoms and molecules is a highly nonlinear process, in which atoms and molecules produce ultra-ultraviolet (XUV) and even coherent light radiation in soft X-ray band under the action of intense infrared polarization laser pulses. This radiation can be used as a low-cost XUV light source. It is also the most effective means to obtain the attosecond (10? 18s) continuous-time light pulse. It can also be used to detect the complex molecular structure and the kinetic process, and so on. Therefore, the study of harmonic characteristics has always been a hot topic in the field of strong field. Especially in recent years, with the progress of femtosecond laser technology and the emergence of various advanced time, space, polarization and other shaping techniques, these complex laser pulse-driven high-order harmonics produce some new characteristics. For example, the driving atom of double circular pulse field can produce high order harmonics of circular polarization. In this paper, the time-dependent Schrodinger equation of the interaction between double circular driving pulse and atom is solved accurately, and the single atomic response process driven by double circular field is simulated. The generation mechanism of high order harmonics, the forbidden law of harmonic order, the cut-off rule of harmonic spectrum, the polarization characteristics of harmonic field and the relative intensity ratio of two circular polarization pulse in double circular field are systematically studied theoretically. The influence of relative phase difference and rotation direction on high order harmonics. The theoretical study shows that the double circular field is composed of two coplanar reverse rotation and the frequency ratio is 1:2. Driven by such a double circular field, the high order harmonic frequency produced by the atom is () qq= 卤3, 2 of the low frequency driving field frequency. 113 (42) times, 3Q harmonics do not exist. This conclusion can be explained by the selection rule of quantum transition, that is, the electron of the ground state of the atom absorbs a number of driven photons to the high energy continuous state, and then releases a harmonic photon transition back to the ground state from the high energy continuous state. In this process, the conservation of angular momentum is followed. From the point of view of the classical three-step model, the ground state electrons are tunneling ionized under the action of the driving field, and then under the interaction of the driving field and the parent nuclear Coulomb field in the 1/3 low frequency periodic time thereafter. A certain kinetic energy is obtained and then returned to the vicinity of the parent ion and combined with it to emit high-energy harmonic photons. The maximum photonic energy is about pp2.12/3.17IU. In addition, the harmonic fields of each single order are circularly polarized, and the distribution of left-handed circular polarization and right-handed circular polarization interval is given, which is also explained by quantum transition. We also find that the harmonic field corresponding to a wide band is polarized in a specific direction, and the polarization direction rotates 120 degrees per 1/3 low frequency period. This can be explained by the motion of ionized electrons contributed to harmonic emission by the classical three-step model. Based on the mechanism, characteristics and law of atomic high order harmonics in the double circular field, we find that the left or right rotation direction of the harmonic field at a specific frequency can be controlled by the rotation direction of the double circular field. The polarization direction of harmonic field in a certain frequency band can also be controlled by the relative phase difference of two circular polarization driving fields. Finally, it is found that the relative intensity of specific order harmonics can be enhanced or suppressed by adjusting the intensity ratio of two circular polarization driving fields. In this paper, the systematic research work of atomic high order harmonics driven by double circular field deepens the understanding of the new characteristics and laws of atomic high order harmonics driven by complex light field. It accumulates experience for people to control and utilize high order harmonics in strong field more effectively by shaping and controlling laser driving.
【学位授予单位】：吉林大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：O562
【参考文献】
相关期刊论文 前1条
1 张杰;强场物理—— 一门崭新的学科[J];物理;1997年11期
，本文编号：2473448
本文链接：https://www.wllwen.com/shoufeilunwen/benkebiyelunwen/2473448.html
