Propagation Properties Of Few-Cycle Laser Pulses In An V-Type Three-Level System

The propagation of few-cycle ultraintense and ultrashort laser pulses in nonlinear media exhibits abundant novel optical phenomena which are different from those in nanosecond and picosecond domains, leading the study of the interaction between laser and matter to a completely new field. The dynamical studies of these nonlinear processes enable us to understand the physical mechanism of all kinds of nonlinear effects from the microscopic point of view, provide theoretical basis for the designation and utilization of laser and predict nonlinear properties of the media.In the semi-classical theory, electromagnetic field is described by Maxwell equations, 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 ultrashort laser pulses, the interaction of few-cycle ultrashort laser pulses with matter, static electronic field 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, effect of pulse width on propagation characteristics of the femtosecond ultrashort laser propagating in a V-type three-level atomic medium with different densities are investigated. The study result shows that: When the pulse doesn't split, the pulse amplitude decreases obviously with the pulse width increasing; when the pulse splits, increasing pulse width accelerates pulse splitting, and the number of sub-pulse raises with the pulse width increasing. When the pulse widthτ_p < 5fs, under considering LFC, the sub-pulse appears time delay and the phenomenon of time delay is more remarkable with the pulse width increasing; when the pulse widthτ_p≥5fs, without considering LFC, the sub-pulse presents time delay and the phenomenon of time delay is more obvious with the pulse width increasing. In addition, above phenomenon is closely related to the populations of the medium.In Chapter 4, Effect of pulse width on carrier-envelope phase (CEP) dependence of ultrashort laser pulse and spectrum in a dense V-type three-level atomic medium is investigated by numerical solution. It is shown that: When the pulse width is smaller, the pulse splitting number is less, the spectrum is wider, high-frequency part of the spectrum is significant, CEP has a larger effect on the pulse and spectrum, particularly the effect on the time interval between sub-pulses and strength of high-frequency part is more obvious; with pulse width increasing, the pulse splitting number increases, width of the spectrum marrows and high-frequency part of the spectrum disappears gradually, effect of CEP on the pulse and spectrum weakens gradually.In Chapter 5, this chapter investigated effect of static electronic field on CEP-dependent spectrum of femtosecond laser pulse propagating in a V-type three-level atomic medium. It is shown that: When the static electronic field is absent, variation of CEP has a considerable effect on strength of the lower-frequency components of the pulse spectrum, but only has a weaker effect on the high-frequency components, spectral flatness and continuity are worse; when the static electronic field presents, effect of CEP on the spectral strength, especially on strength of the high-frequency components, increases; by choosing appropriate values of the static electronic field and CEP, a supercontinuum with better flatness and continuity can be obtained. In addition, the static field presenting makes the CEP period corresponding to the same spectral distribution fromπchanging to2π.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), effect of the relative carrier-envelope phase (RCEP) on spatiotemporal evolution of two-color sech-type femtosecond pulses propagating in a dense V-type three-level atomic medium is investigated by using numerical solution of the full Maxwell-Bloch equations without the slowly varying envelope and the rotating-wave approximations. It is shown that: when area of the two-color pulses is smaller, modulation role of RCEP on pulse shape and special property of the two-color pulses in the resonance case is more evidently than that in the detuning case, but in the both cases the spectrum has only a light broadening in the pulses propagation process; different from the case with smaller area, when area of the two-color pulses increases to a certain value, modulation role of RCEP on pulse shape and special property of the two-color pulses in the detuning case is more evidently than that in the resonance case, and in the detuning case we can get spectrum broadening much greater than that in the resonance case, supercontinuum with highest frequency which equals to 18 times of the incident frequency of the enter pulse can appear.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.