【作者】 王俊；

【导师】 丁大军； 杨玉军；

【作者基本信息】 吉林大学 ， 原子分子物理， 2015， 博士

【摘要】 强激光与原子分子相互作用一直是人们研究的前沿和重点。在人们的不断探索和研究中,发现了一系列新奇的强场现象。在这些强场现象中,原子分子的高次谐波发射受到了人们广泛且持续的关注。高次谐波发射谱存在一个平台结构,随着频率增加该平台的发射效率几乎不变,该现象是一种高阶的非线性效应。高次谐波具有的平台结构特点使其成为产生超短阿秒光脉冲和极紫外相干辐射光源的绝佳途径。利用阿秒光脉冲与原子分子相互作用可以在阿秒的时间尺度和埃的空间尺度实现对物质结构和动力学过程的探测和操控。阿秒光脉冲的实现打开了原子分子内部电子超快动力学研究的大门,具有重大科学意义和应用价值。目前由于高次谐波强度较低,使得由此产生的阿秒光脉冲也较弱,限制了其在泵浦探测实验中探测电子超快运动过程的应用。为了改变这一现状,人们正在努力提出新的改进方案提高高次谐波发射的转换效率,缩短产生的超短光脉冲的脉宽。人们在实验上发现使用强激光与金属纳米材料作用可以产生局域化的表面等离激元。该方法可以有效地使局域空间中的电场强度得到增强,从而形成空间非均匀场。通过实验和理论的研究,人们发现利用这种空间非均匀场分布特性会对电离电子的运动轨迹进行非常强烈的调制,进而影响高次谐波的发射。不仅可以有效的降低产生高次谐波所需要的初始入射激光脉冲的强度,还可以大幅度的展宽高次谐波的频谱平台,并提高高次谐波的发射效率。而且,空间非均匀场方案可以进一步提高人们对高次谐波发射机制的认知,从而激发人们发展新的方案来调控高次谐波发射强度和实现超短孤立阿秒脉冲。本论文从理论上对等离激元产生的空间非均匀场下氢原子和氢分子离子的高次谐波产生进行了研究。由于谐波的发射是由电离电子返回母体离子产生,对电离电子运动的准确刻画尤为重要,而电离电子的运动是由空间非均匀电场决定,因此在理论计算中首先需要给出和实验结果相一致的电场分布。根据实验的测量和时域有限差分法数值计算的结果,我们提出一个新的指数衰减模型势函数,并将该势函数和人们普遍使用的线性模型势函数相比较,发现在靠近金属纳米结构位置二者相差较大,在远离金属纳米结构位置处二者的势函数很接近。通过对两种势函数作用下的原子的高次谐波进行系统的研究,发现当靶原子的位置远离金属纳米结构和入射激光电场较弱的条件下,两种模型势函数给出的计算结果一致。而对于靶原子的位置靠近纳米结构和较强入射激光强度时,由于空间电场的差别导致电子运动行为的不同,二者的谐波发射强度差别较大。利用这一新的模型势,同时考虑到金属纳米材料的损伤阈值对入射激光光强的限制,将通常使用的与纳米材料相互作用的激光波长从近红外替换为中红外,从而延长了单个光学周期内激光与纳米材料的作用时间,降低了对金属纳米材料的损伤阈值,这样使得入射激光的强度可以从1011W/cm2提高到1012W/cm2。我们进一步研究了中红外激光脉冲辐照纳米材料产生的空间非均匀电场与原子相互作用及谐波发射过程,发现在空间非均匀电场作用下电离电子的运动轨迹明显发生了改变,使得从不同时刻电离的电子可以在相同的时间返回到母离子,进而极大地减小了高次谐波发射的固有啁啾,有利于产生阿秒光脉冲。通过对靶原子位置的优化,得到无啁啾的宽频带连续谐波光谱,并利用这一谐波发射得到接近傅立叶变换极限的宽度为127阿秒的孤立阿秒脉冲。该方案为实验上减小谐波发射啁啾获得更短的孤立阿秒脉冲提供一条新的有效途径。此外,我们分别利用空间均匀和非均匀的中红外脉冲电场辐照固定核间距的氢分子离子,数值模拟了两种情况下高次谐波发射过程。对于空间均匀电场条件下,发现在谐波发射谱中存在一个高效率的谐波发射平台,其转化效率远高于通常的电离复合产生的谐波效率。该谐波发射平台的截止能量等于入射激光电场峰值振幅和分子中两个原子核间距的乘积。利用分区的小波变换分析发现这部分谐波产生的机制是:一个原子势阱的束缚态波包隧穿两阱间的势垒,与相邻的原子复合发射出高能光子。由于该高效谐波发射的截止位置由激光电场的峰值强度决定,我们将该谐波发射用于表征空间非均匀电场的强度。通过改变分子离子靶在空间非均匀场的不同位置模拟其谐波发射过程,发现随着靶离子的位置远离纳米结构,高强度谐波发射谱的截止位置逐渐减小。通过定量的比较,可以利用高效谐波发射谱的截止位置准确确定出空间非均匀场在各个空间位置的电场增强幅值。更多还原

【Abstract】 The study on interactions of intense laser fields with atoms and molecules has always been a subject of frontier and significance. A series of novel strong field phenomena have been found during people’s continuous exploration and study. Among all these strong field phenomena, high-order harmonic generation(HHG) from atom or molecule system has drawn extensive and continuous attentions. High-order harmonic spectrum has a typical plateau structure. As the frequency increases, the emission efficiency of the plateau hardly changes, and this is a higher order nonlinear effect. The typical feature of plateau structure of HHG spectrum makes it a superexcellent way to generate ultrashort attosecond light pulses and coherent XUV radiations. Utilizing attosecond laser pulses interacting with atom or molecule, probe and control over material structures and dynamic processes on time resolution of attosecond and spatial resolution of angstrom can be realized. The realization of attosecond laser pulses has opened the door to study ultrafast electron dynamics in atom and molecule, which is of great scientific significance and application value. By far, because of the low intensity of HHG, the attosecond laser pulses obtained from HHG are also quite weak, which limits its use in pump-probe experiments to probe ultrafast electron dynamics. In order to change this current situation, people make great efforts to propose new plans to improve the conversion efficiency of HHG emission and shorten the pulse width of the attosecond laser pulse generated.People experimentally found that localized surface plasmon can be generated through interaction between metal nanomaterials and intense laser field. This method can efficiently increase the electric field intensity in local space, thus creates a spatially inhomogeneous field. Through theoretical and experimental studies, it is found that utilizing the distribution character of this kind of spatially inhomogeneous field to generate HHG will strongly modulate the motion trajectory of the ionized electron and influence the emission of high-order harmonics. This further can effectively reduces the needed initial driving laser intensity to generate high-order harmonics, widely widens the harmonic plateau, and increases the emission efficiency of HHG. Meanwhile, this spatially inhomogeneous field scheme can further improves people’s cognition on emission mechanism of HHG, which will motivate people to develop new methods to adjust and control the emission intensity of high-order harmonics and realize the generation of ultrashort isolated attosecond laser pulse.In this dissertation, we theoretically study HHG from atomic hydrogen and molecular hydrogen ion interacting with the spatially inhomogeneous field created by plasmon. As the harmonic emission is generated through ionized electron recombining with the parent ion, accurate description of the ionized electron dynamics is particularly important. The ionized electron dynamics are determined by the spatially inhomogeneous electric field, so that we firstly need to get the consistent electric field distribution as that of the experiments in our theoretical calculation. According to experimental measurements and finite-difference time-domain numerical calculation results, we proposed a new exponential decay model potential formulation, and compared this potential function with the widely used linear model potential function. It is found that these two model potentials are quite different from each other near the metal nanostructure, while they approach each other at positions far from the metal nanostructure. Through systematic theoretical study on atomic HHG with these two potential functions, we discovered that when the target atom is far from the metal nanostructure as well as that the electric field intensity of the incident laser pulse is low, consistent results can be achieved. On the contrary, when the target atom is close to the metal nanostructure and the electric field intensity of the incident laser pulse is high, because of different electron motions resulted from differences between these two electric fields, harmonic emission intensities under these two cases will differ a lot.Utilizing this new model potential and considering the limits on the incident laser intensity set by the damage threshold of the metal nanostructure, we changed the wavelength of the laser pulses interacting with the nanostructure from the commonly used near-infrared ones to mid-infrared ones. With this change, the action time of the laser on the nanomaterial in one optical cycle is extended, and the damage threshold of the metal nanomaterial is reduced. Thus, the intensity of the incident laser pulse can be increased from 11 210 W/cm to 12 210 W/cm. Then we study the HHG process from interaction between atom and spatially inhomogeneous field induced from nanomaterial irradiated by mid-infrared laser pulses. It is found that under impacts of this spatially inhomogeneous field, the motion trajectory of the ionized electron has been significantly changed, which makes electron ionizing at different instants return at the same time to the parent ion. This extremely decreases the inherent chirp of the high-order harmonic emission spectrum, which is good for HHG. Through careful optimization of the electron’s position in the spatially inhomogeneous field, a chirp-free continuous harmonic spectrum with broad band width can be achieved. Utilizing this chirp-free harmonic spectrum, we can generate isolated attosecond laser pulse with FWHM of 127 as which approaches the Fourier-transform limit. This scheme provides a new effective way to decrease the chirp of harmonic emission spectrum and generate isolated attosecond pulse with shorter pulse width in experiment.Besides, we use respectively the spatially homogeneous and inhomogeneous mid-IR laser pulses to irradiate molecular hydrogen ion with fixed nuclear separation, and numerically simulate High-order harmonic emission processes in both cases. For the case of spatially homogeneous electric field, it is found that there exists a high efficiency harmonic emission plateau in the harmonic spectrum, and the conversion efficiency of it can be much larger than that of the conventional harmonic spectrum created by ionization-recombination. The cutoff energy of this harmonic plateau is equal to the product of the peak field amplitude of the incident laser field and the nuclear separation of the two atomic nucleuses in the molecular hydrogen ion. Through divisional wavelet transform, we figured out the generation mechanism of this part of harmonic. That is the bound state electron wave-packet in one atomic potential well tunnels the potential barrier between potential wells and recombines with the neighboring atomic nucleus with a high energy photon emitted. As the cutoff energy of this high efficiency harmonic emission is determined by the peak field strength of the incident laser field, we thus adopt this harmonic spectrum for the use of electric field intensity distribution characterizations of the inhomogeneous field. By changing the position of the molecular hydrogen ion in the spatially inhomogeneous field, we mimic the harmonic emission process. It is found that as the target ion’s position becomes farther from the nanostructure, the cutoff energy of the high intensity harmonic emission spectrum decreases. Through quantitative comparison, the cutoff energy of this harmonic spectrum can be used to accurately determine the electric field enhancement amplitude at each position in the spatially inhomogeneous field.更多还原