The Generation Of High-order Harmonic And Attosecond Pulses From The Interaction Of Atom With Inhomogeneous Fields

Light-matter interaction is always the one of the most important research subjectsin Physics. Recently, with the rapid development of the laser technology, the pulsewidth of laser fields was shortened gradually while the laser intensity was increasedgreatly, and at the same time, the research of light-mater interaction has expanded intothe domain of strong-field physics. The application of strong laser pulse has greatlyenriched the people’s recognition and knowledge about the light-matter interactionand has led to the discoveries of a series of novel strong-field phenomena. These newstrong-field phenomena challenged the traditional photo-physics theory and broughtgreat difficulty to the experimental study, but on the other hand, they also promotedthe development of modern laser technology. Among these new strong-fieldphenomena, the high-order harmonic generation (HHG) from atom and molecule hasattracted great attention. In the HHG, the frequency intervals between every twoadjacent harmonics are all the same, and the power spectrum of harmonics has anextensive plateau. The two characters of the HHG provide us the good methods togenerate the extreme ultraviolet coherent radiations and also the atto-second lightpulse. The realization of atto-second pulse will unseal the gate of the recognition of the ultra-fast dynamics in atom and molecule. Now, the most studies of HHG mainlyfocus on how to enhance the conversion efficiency of HHG and how to extend thecutoff of harmonic spectrum.Most recently, the experiment found that the HHG can be realized by exploitingthe local field enhancement induced by resonant plasma within a metallicnanostructure. The experimental and theoretical studies have demonstrated that theinhomogeneous field induced by plasma can decrease the necessary laser intensity forHHG effectively and can also enhance the conversion efficiency and extend the cutoffof HHG. Due to these good characters, the inhomogeneous field will become a newtool to generate shorter and stronger atto-second pulse. In addition, the unique spatialdistributions of the inhomogeneous fields can affect the re-scattering process greatly,which will bring us the new understandings of the physical mechanisms of the HHGand stimulate people to study new methods of generating isolated atto-second pulse.In this thesis, by theoretical calculations we carry out the research work on the HHGwithin inhomogeneous fields. There are two main parts in the thesis.First, we theoretically study high-order harmonics (HHG) and isolatedatto-second pulse generation from a one-dimensional (1D) model of He+ion in achirped double-color of inhomogeneous laser fields. Studies have shown that,high-order harmonics cutoff is significant extended and we get a smoothsuper-continuum in a chirped double-color of inhomogeneous laser fields. And we geta atto-second pulse with width48as. The semi-classical three-step model is applied toillustrate the physical mechanism of HHG. We investigate time-frequency profile ofthe time-dependent dipole, and with the classical electron trajectories were compared,which shows that, the short quantum path is enhanced, and the long quantum pathdisappears in spatially inhomogeneous fields. We discussed the influence ofdelay-time of chirped double-color inhomogeneous laser fields on HHG. We foundthat by appropriate adjustments of the value of the delay-time, we can get a smoothand large extended super-continuum. When the delay-time is1.6p w1, the HHG hasbeen the greatest extension. By synthesizing the high-order harmonics from600 orders to680orders, an atto-second pulse with duration32as can be obtained.Second, by solving one-dimensional time-dependant Schr dinger equation, westudy the HHG of Helium under the inhomogeneous fields driven by the few-cyclelaser fields. We calculate the HHG spectra in every atom position within a smallspatial range around the center of the gap of the nanostructure. When the laserintensity is I=5×1014W/cm2, the pulse duration is4cycles and the CEP is f=0, wefind that in the positive region of the coordinate axis, the cutoff of the HHG increaseswith the increasing of the distance between the atom and the center of the gap, and atthe same time, the continuum part of the HHG spectrum extends to both the lowerharmonic orders and the higher harmonic orders. Finally, at the atom position9a.u., asmooth continuum with bandwidth125orders occurs on the HHG spectrum. However,in the negative region of the coordinate axis, with the increasing of the distancebetween the atom and the center of the gap, the cutoff of the HHG decreases and thebandwidth of the continuum shorten as well. These results show that the spatialposition of atom in the nanostructure gap can have a great influence on the HHGprocess. In addition, we analyze the physical mechanism of HHG in different atompositions by applying the wavelet transformation and semi-classical three-step model.