无反转放大效应(AWI)对光速减慢与负群速传播的影响

由于光子作为信息的载体有着天然的优越性,例如,传播速度快,信息量大等.所以各国科学家都在不断地寻找各种方法去操控光子.其中电磁感应透明(Electromagnetically Induced Transparency, EIT),电磁感应吸收(Electromagnetically Induced absorption, EIA)和无反转放大效应(Amplification Without Inversion, AWI)作为控制光子的手段一直受到人们的关注.电磁感应透明现象(EIT)可以实现共振光无吸收传播的同时伴随有大的群速度色散效应.我们实现了无缓冲气体的铯泡(Cs)中的光脉冲群速度的减慢,通过在EIT的原子系统上加入非相干泵浦来产生AWI效应,我们发现光脉冲在其中的传播速度随着泵浦光强的增加而明显减小,延迟时间增加一倍,光脉冲的群速度相当于c/40000.此外,我们还利用电磁感应吸收现象(EIA)产生的大的负群速度色散效应,我们观察到了在无缓冲气体的铯泡(Cs)中,光脉冲的负群速度传播的现象.通过EIA原子系统中非相干泵浦光的加入,AWI 效应使得超光速现象变得更加明显,光脉冲的负群速度变得更大.最大的光脉冲超前时间可以达到0.9 μs,相当于光脉冲的群速度为-c/40000.     

四能级Tripod-型原子系统中量子相干导致的交叉Kerr非线性效应

计算了四能级Tripod-型原子系统中探针光极化率随其频率失谐量的变化曲线.结果表明,当触发光作用于该系统的一个共振跃迁能级时,可使探针光的吸收和色散在其电磁感应透明(Electromagnetically Induced Transparency,EIT)窗口(由耦合光产生)处发生显著变化.随着触发光Rabi频率的增加,探针光在EIT窗口的吸收显著降低,色散显著增加.这种由触发光引起的探针光极化率的变化对应着三阶Kerr非线性光学效应,这一效应在偏振量子相位门中有着潜在的应用价值.     

光子带隙材料及亚波长微结构材料中等离子体共振导致的量子相干效应

我们对两类光学材料-光子晶体及左手材料中的量子相干效应进行了理论研究。光子带隙材料通常是指人工制作的具有光子通带和禁带的光学材料，它可以用来控制光场的传输及某些微观过程。光子晶体是典型的光子带隙材料，光子晶体的周期性结构导致其中的原子的量子光学性质与自由空间中明显不同，例如出现光局域化与原子自发辐射的抑制、光子-原子束缚态、二能级原子布居数囚禁等现象。最近的研究还表明心，特殊的态密度分布会导致感应透明现象，使得原子对与其共振的探测光场的吸收趋于零。这与电磁感应透明（EIT）类似，但不需要外加耦合场来建立相干。我们系统地研究了光子晶体特殊态密度产生的量子相干效应，包括三能级系统的感应透明、无反转增益、光速减慢等，及四能级系统的自发辐射和光开关效应，发现强的量子相干效应导致原子辐射与吸收性质产生多方面的改变。     

受限原子极化相干缀饰四波混频光谱

研究了两电介质面间级联四能级系统原子的缀饰四波混频(FWM)光谱.在缀饰场的作用下FWM信号可产生Autler-Townes (AT)分裂,分裂所产生的峰及凹陷的线型及缀饰场对FWM信号的抑制与增强效应均受原子极化相干及受限原子与光场相互作用瞬态机制的调制.     

受限原子极化相干缀饰四波混频光谱

研究了两电介质面间级联四能级系统原子的缀饰四波混频(FWM)光谱.在缀饰场的作用下FWM信号可产生Autler-Townes (AT)分裂,分裂所产生的峰及凹陷的线型及缀饰场对FWM信号的抑制与增强效应均受原子极化相干及受限原子与光场相互作用瞬态机制的调制.     

外加磁场对简并二能级原子系统相干特性的影响

在铯原子简并二能级系统中6S_1/2F=4—6P_3/2 F'=5 的循环跃迁上获得了电磁感应吸收(EIA)峰，并研究了轴向均匀磁场对二能级系统相干效应的影响，即存在轴向均匀磁场时，观测到在铯原子6S_1/2F=4—6P_3/2F'=5 跃迁频率处EIA和电磁诱导透明(EIT)的混合结构，在合适的实验条件下，其结果与Ye等(2002)的理论分析相一致. 关键词： 电磁感应吸收(EIA) 铯原子 简并二能级系统     

激光场线宽对四能级原子系统相干效应的影响

采用密度矩阵方程,在引入激光场线宽的条件下,计算了N型四能级原子系统中介质对探测场的吸收。结果表明:耦合场线宽的引入,消弱了四能级原子相干,对抑制吸收有增强作用,对增强吸收有抑制作用,即耦合场线宽的引入导致了退相干。控制场线宽的引入,在电磁感应透明中削弱了四能级原子相干性,降低了共振吸收,增强了光透明;在电磁感应吸收中,控制场线宽的增加使谱线中心两侧的吸收增强,削弱了谱线中心的吸收增强,从而使四能级系统的电磁感应吸收转换为电磁感应透明。     

真空诱导相干对电磁感应左手效应的影响

研究了具有超精细结构的四能级原子系统在电磁感应下的左手效应.讨论了由交叉耦合自发辐射路径引起的真空诱导相干(VIC)对左手效应的影响.研究表明,在出现左手效应的频率区间VIC效应强弱对介质相对介电常数和相对磁导率实部取值有显著影响,介质的左手效应随VIC效应的增强而增强. 关键词： 量子干涉 真空诱导相干 电磁感应 左手效应     

固体材料中原子相干效应的研究进展

原子相干效应是光与物质相互作用的结果,它导致了一系列重要的物理现象。目前原子相干的实验研究工作主要在原子气体中开展,而与之相比固体材料中的相关实验研究具有更实际的应用前景。本文系统介绍了近年来固体材料中原子相干效应的研究进展,主要涉及电磁感应光透明、光速减慢与相干存储、存储光信息的可控制擦除、基于光存储的全光路由、双光脉冲的速度减慢和可逆存储和基于原子相干的增强四波混频等基本内容,最后还讨论了其在相关领域的应用。     

固体材料中原子相干效应的研究进展

原子相干效应是光与物质相互作用的结果,它导致了一系列重要的物理现象。目前原子相干的实验研究工作主要在原子气体中开展,而与之相比固体材料中的相关实验研究具有更实际的应用前景。本文系统介绍了近年来固体材料中原子相干效应的研究进展,主要涉及电磁感应光透明、光速减慢与相干存储、存储光信息的可控制擦除、基于光存储的全光路由、双光脉冲的速度减慢和可逆存储和基于原子相干的增强四波混频等基本内容,最后还讨论了其在相关领域的应用。     

Optical pumping and coherence effects in fluorescence from a four level system

An effective four-level system around the D2 line of ⁸⁵Rb at room temperature, is experimentally investigated by fluorescent studies under the action of two driving fields L1 and L2. This system exhibits unique features in fluorescence as a function of frequency separation between L1 and L2. In particular, at two-photon resonance, when the Rabi frequency of L1 exceeds that of L2, signatures of Electromagnetically Induced Transperancy effect (EIT) arising from the three-level Λ sub-system is present as a sub-natural dip in fluorescence from the fourth level. At comparable strengths of L1 and L2 the fluorescence features indicate a regime, where the effects arising from optical pumping and EIT effect due to ground hyperfine level coherence coexist. We see in the coexistence regime, saturation effects arising from difference frequency crossing (DFC) resonances and optical pumping around the EIT window. At low strengths of L1, all signs of coherence vanishes from the system and the fluorescent features result from incoherent optical pumping through the Autler-Townes split states of the excited state hyperfine levels, which are split due to the stronger L2 laser. The dominant role of the L1 laser in creating a robust transparency signal even in the presence of an off-resonant excitation is brought out. The results are supported by density matrix calculations.     

Shaping quantum pulses of light via coherent atomic memory

We describe proof-of-principle experiments demonstrating a novel approach for generating pulses of light with controllable photon numbers, propagation direction, timing, and pulse shapes. The approach is based on preparation of an atomic ensemble in a state with a desired number of atomic spin excitations, which is later converted into a photon pulse. Spatiotemporal control over the pulses is obtained by exploiting long-lived coherent memory for photon states and Electromagnetically Induced Transparency in an optically dense atomic medium. Using photon counting experiments, we observe Electromagnetically Induced Transparency based generation and shaping of few-photon sub-Poissonian light pulses.     

Slow-light six-wave mixing at low light intensities

We report an experimental study of resonant six-wave mixing in coherently prepared Rb atoms. Electromagnetically induced transparency in a four-level atomic system suppresses the linear susceptibility and enhances the nonlinear susceptibilities, which leads to the resonantly enhanced, slow-photon six-wave mixing at low light intensities. The light emission in the six-wave mixing process can be viewed as resulting from diffraction of slow light off a resonant nonlinear grating induced in the four-level system by a standing-wave pump field.     

Opening four-wave mixing and six-wave mixing channels via dual electromagnetically induced transparency windows

Highly efficient four-wave mixing (FWM) and six-wave mixing (SWM) processes can coexist in a four-level Y-type atomic system due to atomic coherence. The simultaneously opened dual electromagnetically induced transparency windows in this four-level atomic system allow observation of these two nonlinear optical processes at the same time, which enables detailed studies of the interplay between the FWM and SWM processes. Three-photon and five-photon destructive interferences are also observed.     

四能级原子系统中的电磁诱导吸收

以三个电偶极跃迁(中间跃迁作为探测跃迁、两边分别是耦合跃迁和控制跃迁)构成N型四能级系统为例，研究介质对探测光的吸收.揭示了在N型四能级系统中，既可产生电磁诱导吸收又可产生电磁诱导透明.这不仅取决于非相干转移率的大小，而且还取决于耦合场、控制场的强度和第四个简并能级的衰减速率大小. 关键词： 电磁诱导吸收 电磁诱导透明 原子相干的自发转移 能级的衰减速率     

Controlled shift of optical bistability hysteresis curve and storage of optical signals in a four-level atomic system

The dependence of the shift of an optical bistability hysteresis curve on the nonlinear phase shift induced by a controlling light is observed in a four-level atomic system of 87 Rb inside an optical ring cavity. In the process the intensity of the coupling beam keeps constant and the atomic system is operated at near conditions of coherent population trapping due to atomic coherence. The refractive and absorptive chi3 nonlinearities enhanced by atomic coherence provide the physical mechanism of the phenomena. Based on the effects, all-optical flip-flop and storage of optical pulse signals with a low peak power of several tens of microwatts are implemented.     

Atom Microscopy via Dual Resonant Superposition

An M-type Rb~(87) atomic system is proposed for one-dimensional atom microscopy under the condition of Electromagnetically Induced Transparency.Super-localization of the atom in the absorption spectrum while its delocalization in the dispersion spectrum is observed due to the dual superposition effect of the resonant Reids.The observed minimum uncertainty peaks will find important applications in Laser cooling,creating focused atom beams,atom nanolithography,and in measurement of the center-of-mass wave function of moving atoms.