Population Transfer And Frequency Conversion Based On Atomic Coherence In Ladder System

Light-atom interaction is one of the most important subjects in modern physics. By using the quantum interference effects which results from coherent light field and atomic system interaction, such as electromagnetically induced transparency (EIT), lasing without inversion (LWI), slow light, optical information storage and quantum entanglement, the optical properties of atomic media can be changed significantly. These interesting phenomena have many potential applications, and can be used to solve problems in light physics and techniques.The physics behind atomic coherence is coupling different atomic levels using certain methods to generate quantum interference between different transition channels. This technique has many applications such as improving nonlinear energy conversion efficiency, light signal processing, x-ray laser, high power laser and optical communication, et al. Many methods have been proposed to realize atomic coherence, like atomic coherence using weak magnetic field, strong magnetic field, vacuum field and micro-wave field. In recent years, applying Raman driven field in adiabatic passage to generate coherence between two quantum states has attracted many research interests. Many methods and techniques such as Stimulated Raman Adiabatic Passage (STIRAP), Stark-chirped rapid adiabatic passage (SCRAP) has been proposed and widely investigated. However, most theoretical and experimental research on quantum interference were carried out in atoms with Zeeman degenerated levels, it is rather difficult to realize energy conversion between levels that have a large energy spacing. In order to get UV or even shorter wavelength laser, scientists investigated some effective methods, such as using atomic coherence results from strong coherent driven field to realize gain without population inversion, frequency conversion effect in nonlinear media. Therefore how to generate maximum coherence between two separate atomic levels has been a new subject in the area of quantum optics. B. D. DePaola investigated in a three-level ladder atomic system how to use STIRAP technique to generate maximum coherence, K. Bergmann et al brought the idea of using Stark conversion to realize maximum coherence in a three-level ladder atomic system and gave detailed theoretical analysis.In this thesis, we study three-level and four-level system, by realizing population transfer from ground state to high excited state, to generate large coherence between ground state and high excited state. Thereafter realize enhanced four-wave-mixing in a four-level system, and find a effective method to realize short wavelength laser.There are five parts in this thesis as following:I:Introduce the investigation status of atomic coherence around the world, investigating methods and physics mechanism of related phenomena; introduce STIRAP which is often used to generate atomic coherence and realize population transfer; basic theory and researching status ofπ-pulse; basic theory and researching status of four-wave-mixing.II:Theoretical basis of this thesis, introduce the quantum mechanic equations to describe light-matter interaction, three basic pictures to describe quantum system, transmission equations of classic field and dress-state theory to investigate light-matter interaction. And adiabatic condition of atomic system and representation of Bloch vector.Ⅲ:In this part, we explore the effect of spontaneous emission on coherence and population transfer in a three-level ladder atomic system which is driven by two pulses in counterintuitive order. With adiabatic evolution and the weak-dephasing approximation the detailed analytic solution is carried out. We find that the large coherence and population transfer can be achieved even when the spontaneous decay rate in the exited state exists. The maximum coherence and population transfer decrease with the increasing of the spontaneous decay rate from the highest state to intermediate state. This effect can be improved by shortening the pulse width and enlarging the delay time under the limit of approximation. On the other hand, the coherence generation and the population transfer almost independent with the spontaneous decay rate from intermediate state to the ground state. The validity of the analytic solution is examined by numerical calculations.Ⅳ:In this part, we explore a new technique which combines a STIRAP or F-STIRAP and aπ-pulse in a four level atomic system. The maximum population transfer and coherence generation can be realized in this system by controlling the delay time ofπ-pulse from the Stokes pulse and the pulse width ofπ-pulse. This technique is presented by numerical simulations based on self-consistent sets of density matrix equations and Maxwell equations. The results support the possibility of preparing the maximum atomic coherence generation and population transfer between ground state and higher Rydberg state.V:In this part, a four-level ladder atomic system, driven by two continue fields, is considered. We show that higher coherence and population transfer can be created between the ground state and the middle state. The short wavelength signal can be obtained by stimulated Raman when the probe pulse is applied between level |3> and|4>. We get the condition for maximum frequency conversion efficiency by numerical simulation and theoretical analysis. The influences of the intensity of the two coherent fields, the pulse width, the detuning and the various atomic parameters on the frequency conversion efficiency are studied.