High-order Harmonic Generation From Atoms And Molecules Irradiated By Spatially Inhomogeneous Field

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.