Storage Of Optical Pulses In Multi-Level Atomic Systems And Their Optimization

The quantum coherence effect of atoms,molecules,quantum dots and other quantum emitters has been an important research topic in atom and molecule physics.Many interesting phenomena can be realized via quantum coherence effect,including coherent population trapping(CPT),lasing without inversion(LWI),electromagnetically induced transparency(EIT),and enhanced Kerr nonlinearity,etc.Despite the development of relevant research for nearly 30 years,there is no decline in people's research enthusiasm and new results are still emerging.In recent years,much attention has been paid to the investigation of EIT phenomenon in atomic gases.EIT is a typical quantum interference effect occurring in resonant multilevel atomic systems.The simplest scheme for EIT has two laser fields(probe field and control field)which interact with ?-type three-level atomic system.The EIT schemes can be used not only to cancel optical absorption in resonant quantum systems,but also to bring many novel optical effects at weak-light level.By utilizing EIT and slow light,people can realize considerable applications,one of which is optical quantum memory,which is very promising for quantum repeaters,and all optical information processing.However most of the researches focus on the atomic gases work in free space,the diffraction effect in those systems can not be neglected,which makes the optical pulses unstable during propagation as well as during storage and retrieval.In addition,a larger power is needed for operating the storage and retrieval in those systems since the coupling between light and atoms is weak in free space.In order to solve these challenging problems,it is necessary to explore the physical properties of both EIT and its light storage schemes.It is also necessary to propose improved theoretical or experimental schemes.In this dissertation,based on the experimental and theoretical investigations on light memory in past two decades,we propose schemes to improve storage efficiency,lifetime and fidelity.Firstly,we show the possibility of the storage and retrieval of weak-light solitons in kagome-structured hollow-core photoniccrystal fiber via EIT.Secondly,based on EIT effect and four-wave mixing,the storage and retrieval of both probe/Stokes pulses is studied systematically and the method of optimizing storage efficiency and fidelity is proposed.Thirdly,we propose scheme based on active Raman gain(ARG)for storing fast light.The main contents are composed of the following aspects:1.Formation and propagation of ultraslow weak-light solitons and their memory in the atomic gas filled kagome-structured hollow-core photonic crystal fiber.We show that,due to the strong light-atom coupling contributed by the transverse confinement of the HC-PCF,the EIT and hence the optical Kerr nonlinearity of the system can be largely enhanced,and hence optical solitons with very short formation distance,ultraslow propagation velocity,and extremely low generation power can be realized.We also show that the optical solitons obtained can not only be robust during propagation,but also be stored and retrieved with high efficiency through the switching off and on of a control laser field.2.Storage and retrieval of optical pulses based on four-wave mixing in four-level double-? system.We first show the coupled propagation of probe and Stokes pulses in this system and then we prove that the high memory efficiencies for both input pulses can be achieved with excitations from only”slow mode”(one dispersion branch of this double-? system).In the next step,we study the formation and propagation of coupled ultraslow optical solitons.Using the standard method of multiple scales,we derive coupled nonlinear Schrodinger equations that govern the nonlinear evolution of the envelopes of the probe/Stokes fields.We demonstrate that for weak input fields and suitable operation conditions,the dispersion effect and self-and cross-phase modulation effects are balanced,a pair of coupled optical solitons moving with remarkably slow propagating velocity can be formed and thus we can further improve the memory efficiencies and fidelities by using these optical solitons.3.Storage and retrieval of light pulses in a fast-light medium via the mechanism of active Raman gain.The system under consideration is a four-level atomic gas interacting with three(pump,signal,and control)laserfields.We show that a stable propagation of signal light pulses with superluminal velocity is possible in such a system through the ARG contributed by the pump field and the quantum interference effect induced by the control field.We further show that a robust storage and retrieval of light pulses in such a fast-light medium can be implemented by switching on and off the pump and the control fields simultaneously.In this thesis,the physical properties of the resonance interaction between light and multi-level quantum systems are revealed and the nonlinear optical theory for weak light is extended.The reports herein may have potential applications for light information processing and transmission using multi-level atomic systems.