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펨토초 레이저를 이용한 자외선, 극자외선 극초단 펄스 발생에 관한 연구

펨토초 레이저를 이용한 자외선, 극자외선 극초단 펄스 발생에 관한 연구
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Femto-attoscience has increased our understanding of ultrafast dynamics in physics and chemistry over the past decade, and it is still great challenge in basic science. An ultrashort pulse is essential tool for the extreme time resolution. This thesis is research about an ultrashort UV, XUV pulse generation. This thesis consist of three chapters
first chapter is the experimental study of a few cycle femtosecond UV pulse generation using few cycle NIR laser, second chapter is the theoretical study of an isolated attosecond XUV pulse generation using mixed two color femtosecond pulse, and last chapter is the experimental study of a property of high harmonic generation from He atom. In chapter 1, a 140~350 nm broadband DUV-VUV pulse from Ne gas target using a few cycle laser pulse is demonstrated. Sub 4 fs UV pulse duration had been measured by SD ?FROG method, which use Xe gas as nonlinear medium. An ultrashort UV pulse has various applications. The author observed quantum beating of Xe atom using an ultrashort UV pulse. A broadband UV coherently excite Xe atom from ground state to excited state. A periodic intensity oscillation at 147 nm emission is observed. The oscillation frequencies are well matched with the energy difference of excited states In chapter 2, the effect of the mixing of pulsed two color fields on the generation of an isolated attosecond pulse has been systematically investigated. One main color is 800 nm and the other color (or secondary color) is varied from 0.2 to 2.4 . When secondary field wavelength is longer than 420nm, the continuum length behaves in a similar way to the behavior of the difference in the square of the amplitude of the strongest and next strongest cycle. As the mixing ratio is increased, the optimal wavelength for the extended continuum shifts toward shorter wavelength side. There is a certain mixing ratio of intensities at which the continuum length bifurcates, i.e., the existence of two optimal wavelengths. As the mixing ratio is further increased, each branch bifurcates again into two sub-branches. This 2D map analysis of the mixing ratio and the wavelength of the secondary field easily allows one to select a proper wavelength and the mixing ratio for a given pulse duration of the primary field. The study shows that an isolated sub-100 attosecond pulse can be generated mixing an 11 fs full-width-half-maximum (FWHM), 800 laser pulse with an 1840 nm FWHM pulse. Furthermore the result reveals that a 33 fs FWHM, 800 nm pulse can produce an isolated pulse below 200 as, when properly mixed In chapter 3, the change of high-order harmonics spectra from He due to a quantum path interference between short and long trajectory electron in a strong laser field is investigated. The relative contributions of each trajectory are controlled by the change of focal positions. In case of an intensity lower than barrier suppression ionization, the partial control of quantum path is obtained. The complete control of quantum path, which means the complete suppression of each trajectory, is demonstrated with an intensity higher than barrier suppression ionization.
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