Propagation Properties Of Few-cycle Laser Pulses In An Ladder-type Three-level System

Due to ultrashort laser pulse possesses ultrashort duration and superhigh peak power, it is widely applied to physics, chemistry, biology, and other fields. At present, the interaction of few-cycle laser pulse with atoms and molecules has become one of the most active study subjects and the propagation of few-cycle laser pulse in atomic or molecular media is an important part of the study. Research on the extremely nonlinear optical phenomena induced by few-cycle laser pulses in atomic media etc. can enrich the content nonlinear optics, and promote development of optical physics, quantum information, attosecond science etc. emerging or cross disciplines.In the semi-classical theory, electromagnetic field is described by Maxwellequations, while the atomic or molecular medium is described by the density matrix. Based on previous research work of the other people, we studied propagation properties of few-cycle laser pulses in a ladder-type three-level system in this paper. This paper consists of seven chapters.In Chapter 1, we give a brief introduction of the development and applications of few-cycle ultrashort laser pulses, the interaction of few-cycle ultrashort laser pulses with matter and coherent control, and introduce simply the current research state of this important subject.In Chapter 2, we derive Maxwell-Bloch equations beyond slowly varying envelope approximation (SVEA) and rotation-wave approximation (RWA) and give a detailed exposition for the finite-difference time-domain method and the predictor-corrector methodIn Chapter 3, few cycle ultrashort laser propagating in dilute and dense ladder-type three-level atomic mediums are investigated. The study result shows that, time evolutions of pulse shape and populations in the dense medium are different from those in the dilute medium, and the differences will become more evident with increasing of the initial area of the pulse.When the initial pulse area is 8Ï€, the pulse shape and populations just present smaller changing in propagation process for the dilute medium case, but the pulse splits into sub-pulses with different numbers and shapes and populations vary obviously at different propagation distances for the dense medium case. The cause producing above differences are from two effects: the effect of the Lorentz local-field correction from the near dipole-dipole interaction in the dense medium and effect of stronger polarization field than that in the dilute medium. And the effect of stronger polarization field than that in the dilute medium plays a main role.In Chapter 4, the effect of detuning on few-cycle laser pulse propagation behavior in a ladder-type three-level atomic medium is investigated. The study result shows that, in the resonance case, obvious variation of the pulse form (including carry-envelope phase, pulse duration, oscillation amplitude and frequency) even pulse splitting occurs in the propagation process, the output pulse is much different from the input pulse; in the off-resonance case, generally speaking, varying detuning also can lead to considerable variation of the pulse form in the propagation process; however, when an appropriate detuning presents, self-induced transparency can be realized in the propagation process, the output pulse exact same as the input pulse can be obtained.In Chapter 5, the space distribution properties of few cycle ultrashort laser pulse shape and populations propagating in ladder-type three-level atomic mediums at different moments are investigated. The study result shows that, the space distribution law of pulse shape and populations for different area few-cycle pulses are different evidently. The oscillation times decrease with increasing of the initial area of the pulse, but amplitude, group velocity and the medium populations oscillation times of every level increase. Moreover the space distribution of the population correlates closely to the space distribution of pulses.In Chapter 6, in the cases of single-photon resonance (the central frequencies of two pulses are equal to two atomic system transition frequencies respectively) and complete non-resonance (none of the central frequencies of two pulses and two atomic system transition frequencies are equal), relative carrier-envelope phase dependence of pulse form and spectra characteristic of two-color ultrashort laser pulses in a ladder-type three-level atomic medium is investigated by using the numerical solution of the Maxwell-Bloch equations without the slowly varying envelope (SVEA) and rotating-wave approximations (RWA). The study result shows that: in the single-photon resonance case, relative carrier-envelope phase has evident control of pulse propagating form and spectra characteristic of two-color ultrashort laser pulses with small area. The number of relative carrier-envelope phase decides the amplitude, propagating velocity, pulses splitting and the number of sub-pulses. By changing the number of relative carrier-envelope phase, broader spectral broadening and higher spectral intensity, especially the spectral intensity of high frequency components, can be obtained. When the area of input two-color pulse increases, relative carrier-envelope phase dependence of pulse form and spectra characteristic is weaker. In the non-resonance case, broader spectral broadening can be obtained, but different relative carrier-envelope phases don't have obvious influence on pulse form and spectra characteristic.In Chapter 7, we summarized the main research results of the paper in detail, and we put forward some ideas to the further research work.