热原子的高分辨率光谱及偏振调制四波混频的研究
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摘要
热原子蒸气是激光光谱实验中常用的光学介质。通常情况下,因为热原子间的碰撞、原子运动产生的多普勒效应等原因会使谱线增宽,导致光谱的分辨率下降。提高热原子光谱的分辨率可以有许多方法,如饱和吸收光谱技术、双光子光谱的消多普勒技术等。本文主要讨论热原子的Dicke窄化光谱和偏振调制四波混频(PMFWM)光谱。
     全文共分五章:
     第一章:绪论。主要介绍与本文相关的激光光谱、电磁感应透明(EIT)及四波混频(FWM)的发展。
     第二章:基本理论。主要介绍激光光谱学的吸收和色散等基本概念以及常用的理论如偶极近似、密度矩阵方程等。
     第三章:受限热原子的高分辨率光谱。分析了通过设计一个平行平面电介腔的超薄样室(ETC)实现高分辨光谱的方法。当ETC中充以稀薄热原子蒸气时,原子的平均自由程与其所处容器的几何形状有关,可使原子的平均自由程表现为各向异性。以较大速度沿平行于电介质表面飞行的原子,不会产生多普勒效应。而在两壁间往返飞行的原子和探测光场之间的相互作用是一种瞬态机制。因此在很多情形下会出现亚多普勒结构(Dicke窄化)的单光子吸收光谱及选择反射光谱。如果满足双光子光谱的消多普勒配置,则Dicke窄化光谱可以取得极高的分辨率。因此在实验上可以以一个简单的受限热原子系统替代昂贵的冷原子囚禁系统得到高分辨率的光谱。
     第四章:电磁感应透明(EIT)。由于ETC中热原子的消激发效应和慢原子效应的增强,可以实现消多普勒增宽。在抽运光作用于系统的情况下,探测光使得ETC中处于定态的基态慢原子演化为另一个基态的布居。这种相干的相位布居所产生的偶极矩与仅探测光作用于系统时产生的偶极矩大小相等方向相反,因此可以认为EIT来自于斯塔克分裂以及由于抽运光产生的两个缀饰态之间的量子干涉效应。而ETC中慢原子效应的增强,使多能级原子系统产生了高分辨率的EIT峰和超慢光效应。
     第五章:偏振调制四波混频(PMFWM)。EIT窗中的FWM信号在热原子样品池里可以低损耗地传播,从而形成高效率的FWM信号。探测光及抽运光的偏振对EIT特别是FWM信号有调制作用。当探测光水平偏振或抽运光垂直偏振时,EIT窗中的FWM信号均具有较大的幅度。改变探测光或抽运光的偏振均可改变FWM信号的强度。
     与非缀饰四波混频(NDFWM)信号的强度相比,单缀饰光作用下的四波混频信号(SDFWM)的强度在偏振调制光为线偏振及椭圆偏振的情况下受到了不同程度的抑制。而双缀饰四波混频(DDFWM)信号在受到缀饰光的偏振调制时均可产生抑制。另外,缀饰场除了对输出光强有抑制作用以外,还能改变输出光的偏振状态。因此可以有效地通过偏振调制的方法实现对四波混频信号的抑制或增强的操控。
     与“宏观”样室相似,ETC的使用所形成的双缀饰EIT(DDEIT)及单缀饰FWM(SDFWM)信号分别具有明显的斯塔克分裂及Autler-Townes(AT)分裂以及抽运光产生的缀饰态的相干效应。由于ETC中处于暗态的超慢原子贡献的增强,以及基态原子相干驰豫时间的增大,多个超窄EIT峰可以在受限的多能级原子系统中产生。因为消多普勒的配置和原子与电介面碰撞的消激发效应共同作用的结果使DDEIT及SDFWM信号具有明显的Dicke窄化现象。另外,FWM信号的抑制和增强表明在ETC中也可实现对多波混频(MWM)的操控,并有望将这一简单的实验系统用于非线性光学器件的设计、波长变换及量子信息的处理。
     最后在结论与展望部分介绍了本论文的主要结论及对后续工作的设想,主要包括ETC的双光子光谱、FWM及原子与电介面间的范德瓦尔斯作用(vdW)的理论研究和实验测量等。
Thermal atomic vapor is usually used as optical medium for laser spectroscopy experiments.The line broadening occurs due to the collision and the Doppler effect of moving atoms,thus high resolution spectroscopy is limited.The high resolution spectrum can be obtained through many methods,such as saturated absorption spectroscopy,Doppler free methods of two-photon spectroscopy,and etc.In this thesis,we devote to Dicke-narrowing spectroscopy and polarization modulated four-wave mixing(PMFWM) spectroscopy of thermal atoms.
     There are five chapters in this thesis:
     Chapter 1:Introduction.The development of laser spectroscopy,electromagnetically induced transparency(EIT) and four-wave mixing(FWM) is discussed in this chapter.
     Chapter 2:Basic theory.In this chapter,we mainly give some basic theory of absorption, dispersion,dipole approximation and the equation of density matrix elements.
     Chapter 3:High resolution spectroscopy of confined atoms.By designing an extremely thin cell(ETC) with two parallel dielectric planes,high resolution spectroscopy can be achieved.In such a tnin cell,the dilute atomic vapor is confined,the average free path of atoms corresponding to the shape of the cavity is anisotropic.Atoms flying along the surface of the plane do not contribute to the Doppler broadening,while atoms flying to or away from the surfaces interact with the light in a transient regime.Thus one-photon sub-Doppler absorption and selective reflection spectroscopy(Dicke-narrowing spectroscopy) are achieved for many cases.In a two-photon situation,much higher resolution Dicke-narrowing spectroscopy can be obtained as Doppler free configuration is satisfied.It is feasible to implement an experiment instead of a cold atomic system for high resolution spectroscopy.
     Chapter 4:Electromagnetically induced transparency(EIT).Doppler free can be achieved due to de-excited and slow thermal atoms in ETC.Stationary atoms in a ground state evolve into the population of another ground state induced by the probe laser as the pump fields are applied to the system.When a pump laser is applied to the system,the dipole from the coherent population has the same magnitude and opposite sign for the probe laser applying to only.Thus EIT is interpreted as ac-Stark splitting and quantum interference of two dressing states.The high resolution dips of EIT and the effects of supper slow group velocity occur due to the effect of slow multi-level atoms in ETC.
     Chapter 5:Polarization modulated four-wave mixing(PMFWM).FWM signal propagates with less loss in thermal atomic cell in the windows of EIT,thus high efficient FWM signal is achieved.EIT,especially FWM signal can be modulated by the polarization of the probe and the pump fields.The magnitude of FWM signal is larger than other situation as the horizontally polarized probe beam and the vertical polarized pump beam are applied to the system.The strength of FWM signal can be changed through different polarization of the probe or the pump fields.
     Comparing with FWM without dressing fields(NDFWM),the strength of the singly-dressing FWM(SDFWM) signal is suppressed when the modulation-field is linear or elliptically polarized.In doubly-dressing FWM(DDFWM) situation,the suppression occurs in all polarized situation of the modulated fields.We also find that both the strength and the polarization of FWM signal can be modulated by the polarization of the dressing fields,and thus FWM signal is successfully suppressed or enhanced by the methods of polarized modulation.
     It is similar to the situation of a macroscopic cell,ac-Stark(Autler-Townes(AT)) splitting and the coherent effect from the dressing state created by the pump laser in doubly-dressing EIT(DDEIT) and singly-dressed FWM(SDFWM) are observable in ETC. The enhancement of the contribution from slow atoms in dark state and the increase of coherent relaxation time of ground state atoms result in multi-dip EIT lines in multi-level confined atoms.Dicke-narrowing appears both in DDEIT and SDFWM lines due to Doppler free configuration and the de-excited effects of atoms in ETC.Suppression and enhancement of FWM suggest that multi-wave mixing(MWM) can be controlled.It is possible to apply ETC experiment to the design of nonlinear optical devices,wavelength conversion and quantum information processing.
     Finally,we give a summarization of the whole thesis and some of the possible new research topics that followed,such as the theory and experiment of two-photon spectroscopy, FWM and atom-surface van der Waals interaction(vdW) in ETC.
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