【作者】 杨旭东；

【导师】 王海；

【作者基本信息】 山西大学 ， 光学， 2009， 博士

【摘要】 光与原子相互作用是量子光学乃至光学中的一个重要研究领域。相干光场与多能级原子之间相互作用可以产生许多新奇而重要的量子现象和量子干涉效应。而利用量子相干制各的相干原子介质更是以其小吸收、强色散、高非线性折射率的理想特性被广泛地应用在众多光学领域中。作为其中的典型代表,电磁感应透明（EIT）效应以及EIT介质受到了人们的广泛关注,与EIT相关的理论和实验成为一个非常活跃的研究领域。本文围绕相干光场与多能级原子相互作用,介绍了电磁感应透明（EIT）效应的基本概念及物理特性,回顾了人们对EIT及其相关效应的研究历史,并在此基础上重点介绍了我们基于EIT效应开展的相关实验和理论研究工作,主要包括光群速度减慢实验、Tripod结构中的EIT增强效应和基于Double EIT的弱光交叉Kerr非线性效应以及共振布拉格散射光谱的研究等。具体如下:1)简要介绍了电磁感应透明效应的相关理论及其在实验上的实现,特别对我们开展的光减速实验进行了简单介绍。我们在基态简并的三能级原子介质中观察到了由EIT效应导致的光减速现象,实验测得的光脉冲群速度被减慢到10000m/s。2)在多能级EIT系统中,观察到了由非对称EIT系统导致的探针光偏振面旋转效应即光致旋光效应。我们在⁸⁷Rb原子D1线中,选取左旋圆偏振耦合光与原子作用,产生了对左旋和右旋圆偏振探针光分量数目非对称的EIT系统,从而破坏了原子介质对探针光两个圆偏振分量折射率的对称性,由此导致了探针光偏振面的旋转。我们设计了一种偏振测量装置,消除了吸收对转角测量的影响,从而较高精度地测量了探针光的偏振面旋转角。实验中,我们用15mW的耦合光实现了探针光45°的偏振面转角。3)在⁸⁷Rb原子D1线所形成的多塞曼子能级原子系统中,研究了一束线性偏振光（探针光）的偏振光谱,并演示了一个基于光偏振变化的光开关。该探针光的偏振光谱信号是由于另一束左旋圆偏振的泵浦光束引起的探针光偏振特性发生变化所导致的。我们同时提供了一个包含多重塞曼子能级模型的理论分析,其计算结果与实验部分达到了定性的吻合。基于此现象,我们演示了一个弱光下的光开关,其开关能量密度为每个原子散射截面（λ²/2π）约68个光子,相应的开关效率达到约3%。4)在Tripod系统中观察到了Two EIT窗口以及两个窗口相互作用引起的EIT增强效应。通过采用左旋圆偏振探针光、右旋圆偏振触发光和左旋圆偏振耦合光与⁸⁷Rb原子D1线作用,构建了四能级Tripod结构,在该结构中观察到了当触发光与耦合光的频率失谐不同时,两束光分别对探针光形成的EIT窗口,并研究了这两个EIT窗口之间的相互作用。结果发现,当触发光与耦合光的频率失谐接近或相等时,耦合光形成的EIT信号会被明显增强。我们对上述实验现象进行了数值计算,结果与实验达到了较好的吻合。5)在⁸⁷Rb原子D1线形成的四能级Tripod系统中,观察到了Double EIT窗口,并研究了基于该Double EIT窗口的弱光交叉Kerr非线性效应。在实验上观察了耦合光在探针光和触发光的吸收谱上同时形成的EIT窗口,并利用Mach-Zehnder干涉仪测量了探针光与触发光之间相互作用导致的交叉Kerr非线性相移及非线性系数。结果显示,当探针光和触发光的透射都超过60%时,触发光的交叉Kerr非线性系数可达2×10^-5 cm²/W。同时,我们还考察了交叉Kerr非线性系数对调制光强度的依赖关系。结果发现,随着调制光强度的增加,交叉Kerr非线性系数逐渐减小,也就是说,我们在越弱光下得到的交叉位相调制系数会越大。6)在铷原子气室中,实验演示了一个基于原子布居数差光栅的高分辨率后向共振布拉格散射（Bragg-scattering）光谱。一对反向传播的泵浦光形成稳定的驻波场,并对二能级原子布居数差的空间分布进行周期调制,从而形成一个布居数差光栅。当一束正交偏振的探针光穿过该布居数差光栅时,就会形成后向共振布拉格散射光谱。实验中,共振布拉格散射光谱信号线宽约为3.5MHz,信噪比可达2000。我们利用此布居数差光栅上的每个体积元偶极振荡发出的弹性瑞利散射光的相干叠加原理,从理论上解释了该后向共振布拉格散射光谱。这其中,属于创新性的工作包括:Ⅰ.在多塞曼子能级原子系统中,得到了由于左旋圆偏振泵浦光引起线性偏振探针光偏振特性发生变化而导致的探针光偏振光谱信号,并基于此演示了一个弱光下的光开关,其开关效率达到约3%。Ⅱ.通过选择合适偏振的光与原子的Zeeman子能级作用,构建了四能级Tripod原子系统。在该系统中观察到了耦合光对探针光和触发光同时形成的EIT窗口,即Double EIT窗口,并第一次实验研究了两束弱光之间基于Double EIT窗口的交叉Kerr非线性效应。我们在弱光下观察到了显著的交叉Kerr非线性效应,并用由两个偏振位移器构成的新型Mach-Zehnder干涉仪精确测量了探针光和触发光之间的交叉位相调制相移。Ⅲ.首次在热原子气室中,实验演示了一个基于原子布居数差光栅的高分辨率后向共振布拉格散射光谱信号,并利用弹性瑞利散射光的相干叠加原理对该共振布拉格散射光谱进行了理论解释。实验得到的后向共振布拉格散射光谱信号线宽约为3.5MHz,信噪比可达2000。更多还原

【Abstract】 The interaction of atoms with light is an important research field in quantum optics, even in optics. Many novel and important quantum phenomena and quantum interference effects are produced in the interaction of multi-level atoms with coherent optical field. The coherently-prepared atomic medium can be applied extensively in many optics areas because of its ideal optical properties of low absorption, steep dispersion and enhanced nonlinearity. As the most typical example, electromagnetically induced transparency （EIT） effect and EIT medium have attracted a great deal of attention. And the EIT-related theory and experiments studies become very active.The dissertation introduces the basic concepts and physical properties of EIT effect around the interaction of multi-level atoms with coherent optical field and reviews the development of the research works on EIT and related effects. On this basis, the experimental and theoretical works we completed based on EIT effect were mainly introduced in this dissertation, including light speed reduction, the enhanced EIT effect and cross-Kerr nonlinear effect between two weak beams based on Double EIT in a tripod system, study on the resonance Bragg-scattering spectroscopy and so on. The main works are as following:1) The related theory and realization in experiment on EIT effect are briefly introduced, in particular the light speed reduction experiment. We observed the light group speed reduction due to EIT effect in the three-level atomic medium with degenerate ground states. The measured group velocity of optical pulse is slowed down to 10000m/s.2) The optical induced polarization rotation resulting from the asymmetry EIT system is observed in a multi-level EIT system in rubidium atoms. By choosing left-circularly-polarized coupling beam to interact with atoms, the asymmetry in the number of EIT subsystems seen by the left- and right-circularly-polarized components of the weak probe beam is produced, which makes the refractive indices of left- and right-circularly-polarized components of the probe beam different and leads to the polarization rotation of the probe beam. We designed a polarization measurement setup to eliminate the influence of absorption on the measurement of polarization rotation angle and measured the rotation angle with the higher measurement precision. In the experiment, a rotation angle of 45 degrees has been achieved with a coupling beam power of only 15mW.3) The polarization spectroscopy of a linearly-polarized optical （probe） field is studied and a light switch based on optical polarization changes is demonstrated in a multi-Zeeman-sublevel atomic system of ⁸⁷Rb D1 line. The polarization spectroscopy signals of the probe light result from the changes in its polarization, which are caused by a left-circularly polarized pumping beam. A theoretical analysis involving multiple Zeeman sublevels is presented and the results are in qualitatively agreement with the experimental observations. Based on this phenomenon, we demonstrated a light switch at low light level （～68 photons perλ/2π） with a switching efficiency of～3%.4) The two EIT windows and enhanced EIT signals resulting from the interaction between the two EIT windows are observed in a tripod system. By choosing left circularly-polarized probe, left circularly-polarized coupling beam and right circularly-polarized trigger beam to interact with the energy levels of D1 line in ⁸⁷Rb atoms, a tripod system is formed. In the four-level tripod system, we observed the two EIT dips produced by a strong coupling beam and a weak trigger beam are in the absorption spectrum of the probe field when the frequency detuning of trigger beam is different from that of the coupling beam. And we studied the interaction between the two EIT windows. The results show that the EIT signal created by coupling beam was significantly enhanced when the frequency detuning of trigger beam is near or equal to that of the coupling beam. We made numerical calculation for the above phenomena and the results are agreement with experimental results.5) The Double EIT windows and the cross-Kerr nonlinear effect between two weak beams based on Double EIT are investigated in a four-level tripod system formed in D1 line of ⁸⁷Rb atoms. We observed the simultaneous EIT windows for probe and trigger fields （double EIT） produced by a strong coupling beam and measured the cross-phase modulation （XPM） phase shifts and nonlinear coefficient between the two fields using Mach-Zehnder interferometer. The experimental results show that the XPM coefficient of trigger beam is larger than 2×10^-5cm²/W when the accompanying transmissions of probe and trigger beams are higher than 60%. Meantime, we studied the dependence of XPM coefficients on the power of modulation beam and found that the XPM coefficients decrease with the increase of modulation power.6) A high-resolution backward Resonance Bragg-scattering （RBS） spectroscopy from a population difference grating （PDG） is demonstrated experimentally in a Rb atomic vapor cell. A stable standing-wave field, which is formed by a pair of counter-propagating pump fields, periodically modulates the space distribution of the population of the thermal two-level rubidium atoms and constructs a PDG. When a probe beam with the polarization orthogonal to that of the pump fields, propagates through the PDG, the backward RBS spectra were observed. The linewidth of ³.5MHz is obtained in the backward RBS spectra and high S/N up to～2000 is achieved. Using the scheme of the coherent superposition of the individual elastic Rayleigh scattering light emitted from the atomic dipole oscillators on each grating volume element of the PDG, the RBS spectroscopy is theoretically explained.The characterized works among the above are as follows:Ⅰ. We observed the polarization spectroscopy signals of the probe light resulting from the changes in its polarization, which are caused by a left-circularly polarized pumping beam. Based on this phenomenon, we demonstrated a light switch at low light level with a switching efficiency of～3%.Ⅱ. By choosing proper polarized light beams to interact with Zeeman-sublevel of atoms, we formed a four-level tripod atomic system. We observed the simultaneous EIT windows （Double EIT） for probe and trigger fields induced by coupling beams in the four-level tripod system. And for the first time, the cross-Kerr nonlinearity between the two weak fields based on the Double EIT windows were experimentally investigated. We observed the significant cross-Kerr nonlinearity （cross-phase modulation （XPM）） at low intensities and accurately measured the XPM phase shifts between the probe and trigger fields by using the new-style Mach-Zehnder interferometer formed by two beam displacing polarizers.Ⅲ. For the first time, we demonstrated experimentally a high-resolution backward Resonance Bragg-scattering （RBS） spectroscopy from a population difference grating in an atomic vapor cell and explained theoretically the RBS spectroscopy by using the scheme of the coherent superposition of the elastic Rayleigh scattering light. The linewidth of the obtained backward RBS spectra is～3.5MHz and the S/N up to～2000.更多还原