【作者】 寇军；

【导师】 高锦岳；

【作者基本信息】 吉林大学 ， 光学， 2011， 博士

【摘要】 在过去的十几年里,人们发现原子介质的光学特性能够因为光波场作用使原子处于相干叠加态而发生显著的改变,这种原子相干的产生可以使介质对共振探测场的吸收和色散特性发生变化,同时相干介质具有很大的非线性极化率。本文主要从理论和实验两方面系统地研究了铷原子蒸气中的相关非线性光学效应,同时实现了基于相位控制相干粒子数捕获的全光开关。第一部分：倒Y模型原子系统中的双暗共振和增强的交叉Kerr非线性效应自从电磁感应透明现象在各种三能级原子系统中被实验证明以来,引起了人们广泛的兴趣和重视。其特性被应用到许多领域的研究中,例如使光脉冲传播的群速度减慢,进而实现光信息的存储与释放；还可以用于实现高效率的非线性光学过程,如增强的四波混频效应,提高光信号间的交叉相位调制系数,多光子干涉等。对这些相干效应的研究大都集中在原子的V型、A型和三型三能级系统中。随着实验和理论研究的深入,许多更新和更复杂的量子干涉效应是与多光场、多能级系统的相互作用联系在一起的。其中,具有两个透明点和三个吸收峰的双暗态共振和双窗口电磁感应透明因其在双极量子阱激光和量子计算机方面的潜在应用而得到了人们的重视。我们选择倒Y型原子系统作为研究对象。首先在理论上分析了在两个耦合场作用下,原子系统中的吸收和色散特性。然后从实验上观测了在倒Y模型原子系统中双窗口电磁感应透明现象,搭建了一套马赫-曾德(Mach-Zehnder)干涉仪,用来测量原子介质对探测场体现出的色散特性。结果表明,可以通过调节两个耦合场的频率来分别独立地控制两个透明窗口的位置,每一个透明窗口处的色散曲线都有着陡峭的变化,这一特性使在临近的不同频率处分别调控光信号的群速度成为可能,可用于双通道的光信息和光量子位的存储与释放。双窗口电磁感应透明现象的物理本质是由于两个耦合场同时作用于原子系统中,从而在其内部形成了可调控的双暗共振。当双暗共振简并时,两者之间的相互作用达到最大,量子干涉增大了EIT系统的交叉Kerr非线性系数。因此,我们进一步研究在倒Y模型原子系统中基于EIT增强的交叉相位调制现象。在目前的实验条件下,系统的交叉相位调制相移达到12°。我们的实验结果对理解双暗共振相互作用的物理机制有所帮助,同时也为多能级原子系统中非线性效应的相关研究提供了实验参考。第二部分：利用四波混频技术测量原子相干的动态变化由于原子不同能态之间相干性的建立在量子光学的基础研究和实际应用中有着重要意义,人们已经提出了多种实现原子相干的方法,如强磁场感应原子相干、微波场感应原子相干、自发辐射相干等。近些年来,在绝热过程中利用激光场驱动,建立两个能级之间的相干效应备受关注,提出了多种解决问题的方案和技术,例如受激拉曼绝热过程、斯塔克啁啾快绝热过程和分段绝热过程等。同时原子的相干控制与绝热过程的结合,也开创了一个新的研究领域。在这样的背景下,人们开始关注绝热过程中原子相干建立和衰退的演化规律。我们提出采用四波混频(FWM)技术探测随时间变化的原子相干性。在实验方面,分别利用受激拉曼绝热(STIRAP)和部分受激拉曼绝热(fractional STIRAP)技术在铷原子的两个非简并基态能级之间建立原子相干,相干性的大小会受到斯托克斯场、泵浦场拉比频率等参数的控制,而随时间变化。同时一个弱连续场作为探测场激发产生四波混频(也可称之为相干反斯托克斯拉曼散射)信号,理论分析表明四波混频信号的强度与原子基态相干性成正比。因此对实验中得到的四波混频信号进行探测,就可以获得原子相干性随时间的演化过程,实验结果与数值模拟对比,两者相一致。此技术可以用于探索相干控制绝热过程的物理本质,同时在基于原子相干的非线性过程等研究课题中有潜在应用。第三部分相位控制相干粒子数捕获及全光开关的实现在本部分,我们提出一种相位控制的相干粒子数捕获(CPT)效应,以及建立在这种多光子干涉效应基础上的快速响应全光开关。实验结果表明,利用两个耦合场可以将原子制备在含有耦合场振幅和相位信息的叠加态(暗态)上,再通过绝热过程将这一叠加态以原子自旋波的形式存储在原子介质中,这种原子自旋波将参与控制场和探测场之间的量子干涉,与控制场和探测场构成了一套准闭合环系统,三者之间的相对相位差将决定探测场的透过或吸收。由于改变任意一个光波场的相位,同时保持其他三个光波场的相位不变,就可以调节整个系统的相对相位差。我们选择一个弱场作为开关场,可以通过控制探测场的透过或吸收来实现光开关的“开”和“关”状态。我们在实验上演示了这种全光开关,并且进一步考察了原子基态退相干和四波混频效应对全关开关的影响。基于此种机制的全光开光具备弱光水平和高速响应的优点,控制脉冲的能量密度为每原子散射截面(λ~2/2π)10~2个光子,光开关的响应时间在ns量级,可用于高速光通信和量子信息网络等研究领域。同时探测场没有光速减慢效应,这也是与以往基于量子干涉的光开关的不同之处。与现存的光开关技术相比,这种快速响应的光开关在未来的应用中更具备吸引力。更多还原

【Abstract】 In the Past decade, researchers have found that the optical properties of the atomic media can be dramatically altered by the optically inducing the atom into a coherent superposition. An otherwise opaque medium on a probe resonance is made highly transparent, accompanying with large susceptibility. The atomic medium also behaves large nonlinear susceptibility. In this thesis, the related nonlinear optical effects in thermal rubidium vapor are discussed systematically. And a fast optical switching based on the phase controlled quantum interference is implemented.1. Double dark resonance and large cross-Kerr nonlinearity in a four-level inverted-Y atomic systemSince the phenomenon of electromagnetically induced transparency (EIT) has been observed in a variety of three-level atomic systems, it has attracted wide interest and attention. Light manipulation techniques based on EIT allow us to well control the storage, generation, and interaction of classical and quantum light signals in atomic gases and solid samples through coherently enhanced optical nonlinearities with negligible absorption. EIT-based resonant four wave mixing and enhanced cross-phase modulation coefficient are also reported.The studies of these coherent effects are concentrated in theⅤ-type,Λ-type and (?)-type three-level atomic systems. With the development of the experimental and theoretical research, new requirements of multi-level quantum interference effect are presented, because of the potential applications in bipolar quantum well laser and the quantum computers, the phenomenon of double dark resonance has been given even more attention.The electromagnetically induced transparency (EIT) and its dispersion properties in a four-level inverted-Y atomic system are investigated. The absorption spectrum of a weak probe field shows two EIT windows (dark resonances) whose location, width, and depth can be controlled by manipulating the parameters of the coupling fields; the corresponding dispersion properties are also measured by using a Mach-Zehnder interferometer. This kind of system can find important applications in two-channel quantum communication and information storage.Further more, we also studied the EIT enhanced Kerr nonlinearity with reduced linear absorption in a four-level inverted-Y atomic scheme. When detunings of the coupling and control fields are appropriately set, an enhanced EIT window is observed, and the induced phase shift of the probe field due to cross-phase modulation (XPM) is obtained by measuring the dispersive property of the probe transition. The maximal XPM phase shift is about 12°under the current experimental conditions. The experimental measurements agree well with the theoretical calculations. This work could be useful for future experiments on the generation of crossed-Kerr nonlinearities in multilevel systems. The enhanced XPM phase shift in such an atomic system has applications in quantum nonlinear optics and quantum information science.2. Measurement of coherence dynamics based on coherent anti-Stokes Raman scatteringThe atomic coherence has great significance in basic research of quantum optics and the practical applications, several methods has been proposed to achieve atomic coherence, including the strong magnetic field induced atomic coherence, the microwave field induced atomic coherence, Spontaneous Coherent Generation (SGC) and so on.In recent years, the establishment of atomic coherence via adiabatic passage attracted much attention. The adiabatic process techniques, such as stimulated Raman adiabatic passage, Stark-chirped rapid adiabatic passage, and piecewise adiabatic passage, are used to prepare superposition in atomic system. Under such a background, people start to focus on the temporal evolution of the coherence in the system undergoing adiabatic process.In this part, by using STIRAP and fractional STIRAP, a time-dependent coherence is prepared and indirectly monitored by the generated coherent Raman scattering (CARS) signal, the experiment results fit very well with numerical simulations. It is demonstrated that the CARS can be applied to measure the coherence dynamics between two particular levels during adiabatic passage. Such an efficient technique can be used to explore the hypostasis of coherence control adiabatic passage (CCAP), which has attracted much attention recently, and have potential applications in nonlinear process based on dynamical atomic coherence.3. Phase-dependent coherent population trapping and fast all-optical switchingWe demonstrate a new scheme for achieving phase-dependent coherent population trapping and a fast optical switching based on the phase controlled quantum interference is implemented. Two strong coupling lasers establish a coherent superposition (dark state) of two ground states in rubidium atoms.When the coupling lasers are turned off simultaneously, the amplitude and phase information is stored in the atomic coherence. Then the switch and probe pulses are turned on, the optical fields and atomic superposition form a quasi loop configuration, where the quantum interference is dependent on the relative phase of the whole ensemble.The relative phase of the system can be manipulated by adjusting the phase of any optical field coupling the atomic ensemble. So we choose the left circular polarized weak field as the switch field, the other weak field can be absorbed or transmit through the medium. The phenomena are considered as "on" and "off’ state of the all optical switching. We experimentally realized this all-optical switching, and discussed the effects induced by the decoherence between the atomic ground state and four-wave mixing on the optical switching.The all-optical switching based on the phase dependent coherent population trapping has many advantages, for example, the switching speed is not restricted by the relaxation rates, there is no response time delay caused by slow-light propagation effect and the width of probe pulse can be in the nanosecond time domain without shape distortion, and the switch pulse can be achieved at an energy density of 10-2 photon per atomic cross sectionλ2/2π, The all-optical switching will have potential application in high-speed optical communications and quantum information systems.更多还原