Electromagnetically Induced Transparency in a Four-Level Atomic System

In this paper, we analyze and discuss the absorption properties of the probe beam in a four-level atomic system with a nearly hyper-fine doublet structure of two higher-lying excited levels based on electromagnetieally induced transparency (EIT) for the two cases of transient process and steady-state process. The main gain of this work is to investigate theoretically the influence of the nearly hyper-fine levels on the probe absorption. For the transient process, using the numerical calculations by a simple Mathematica code we find that the magnitude of the probe absorption at the line center is small compared to the typical three-level atomic system in the context of electromagnetieally induced transparency. For the case of the steady state, our results show that the probe absorption can be completely eliminated at the line center of the probe transition just as the usual three-level EIT scheme.     

Electromagnetically Induced Transparency in a Four-Level Atomic System

In this paper, we analyze and discuss the absorption properties of the probe beam in a four-level atomic system with a nearly hyper-fine doublet structure of two higher-lying excited levels based on electromagnetically induced transparency (EIT) for the two cases of transient process and steady-state process. The main gain of this work is to investigate theoretically the influence of the nearly hyper-fine levels on the probe absorption. For the transient process,using the numerical calculations by a simple Mathematica code we find that the magnitude of the probe absorption at the line center is small compared to the typical three-level atomic system in the context of electromagnetically induced transparency. For the case of the steady state, our results show that the probe absorption can be completely eliminated at the line center of the probe transition just as the usual three-level EIT scheme.     

在一个具有超精细结构的四能级原子系统中的拉曼散射

在一个具有超精细结构的四能级原子系统中，为了压抑暂态过程和稳态过程两种情况下的弱探测光束的吸收，我们提出和分析了一种有效的拉曼散射计划。对于暂态过程，借助一个漂亮的Mathematic程序进行数值计算，我们发现在探测跃迁线中心探测吸收的幅度与一般的基于电磁感应透明的三能级原子系统相比要小。对于稳态情况，结果表明探测吸收在线中心能够被彻底地消除，正像通常的三能级电磁感应透明方案。特别地，我们的结果表明在拉曼共振的条件下探测吸收能够被彻底地消除，也就是说，对于稳态过程我们仅仅要求双光子失谐量在超精细二能级频率间隙内是零。与标准的三能级电磁感应透明方案相比，我们的四能级拉曼计划的优点之一是在拉曼共振条件下我们能观察到透明窗口，不需要严格地单光子和双光子失谐量为零。因此，原子超精细结构对于获取电磁感应透明不是一个障碍。     

利用微波场抑制电磁感应透明的功率展宽

电磁感应透明的重要参量之一是透明窗口的光谱线宽。在Λ-型三能级原子系统中,电磁感应透明的光谱线宽由两低能级间的相干失相速率决定,若两低能级同属于原子基态的精细结构,则电磁感应透明窗口的极限线宽很窄。但较强的耦合场作用往往会导致电磁感应透明窗口的功率展宽,而减弱耦合场又会影响电磁感应透明的对比度和深度。为此,通过引入微波控制场共振作用于基态精细能级间构成三场作用下的准Λ-型四能级系统,利用微波控制场来抑制耦合场所引起的电磁感应透明窗口的功率展宽。结果表明,引入微波控制场不仅得到了双窗口电磁感应透明,而且在保持较好对比度的条件下,使得电磁感应透明的光谱线宽明显小于不加微波场的情况。     

Effects of spontaneously induced coherence on absorption of a ladder-type atom

This paper investigates the effects of spontaneously induced coherence on absorption properties in a nearly equispaced three-level ladder-type system driven by two coherent fields. It find that the absorption properties of this system with the probe field applied on the lower transition can be significantly modified if this coherence is optimized. In the case of small spontaneous decay rate in the upper excited state, it finds that such coherence does not destroy the electromagnetically induced transparency (EIT). Nevertheless, the absorption peak on both sides of zero detuning and the linewidth of absorption line become larger and narrower than those in the case corresponding to the effects of spontaneously induced coherence; while in the case of large decay rate, it finds that, instead of EIT with low resonant absorption, a sharp absorption peak at resonance appears. That is, electromagnetically induced absorption in the nearly equispaced ladder-type system can occur due to such coherent effects.     

射频驱动精细结构能级跃迁引起的非线性效应

研究了射频驱动精细结构能级跃迁引起的电磁诱导透明（EIT）和电磁诱导吸收（EIA）的转化。通过采用射频场同时驱动激发态和基态精细结构能级跃迁,使系统中出现EIT和EIA。研究结果表明,调谐射频场频率从与基态精细结构能级共振到与激发态精细结构能级共振的过程中,EIT和EIA相互转化,而且射频场的拉比频率取值不同,EIT和EIA的变化规律也不同。     

Coulomb broadening of the peak of electromagnetically induced transparency in plasma

We have measured the shape of the Autler-Townes doublet and the peak of electromagnetically induced transparency (EIT) under plasma conditions. We compare the experimental results with the calculated spectrum of the probe field of a three-level ArII Λ-scheme by taking into account Coulomb collisions. We show that the Coulomb broadening of the EIT peak is small (less than 40%), while the saturation resonance is broadened under the experimental conditions by a factor of 3. In contrast to the saturation resonance attributable to the Bennett dip in the velocity distribution of the population, the EIT peak is a coherent effect and is broadened mainly through Coulomb dephasing.     

射频驱动下电磁诱导透明窗口的分裂和增益的出现

在Λ型三能级系统的基础上引入两个共振射频场, 通过详细讨论系统的探测吸收特性随两个射频场Rabi频率取不同值时的变化规律, 得出电磁诱导透明(EIT)的分裂规律以及EIT上出现增益现象的产生条件.研究结果表明: 两个射频场对系统所起的控制作用不同, 控制基态精细结构能级之间跃迁的射频场对EIT的分裂起作用, 而控制激发态精细结构能级之间跃迁的射频场不会导致EIT的分裂; 而且, 只有当控制基态精细结构能级之间跃迁的射频场的Rabi频率大于控制激发态精细结构能级之间跃迁的射频场的Rabi频率时, 才能产生EIT与增益相叠加的新特性. 关键词： 射频场 电磁诱导透明 增益 精细结构能级     

Ultrafast GaN/AlN modulator based on quantum dot for terabit all-optical communication

An ultrafast all-optical modulator based on spherical quantum dot using electromagnetically induced transparency (EIT) technique in GaN/AlN structure, associated with inter-sublevel transitions, is proposed and analyzed. The aim of this paper is to modify and enhance the main parameters of an all-optical modulator such as, power consumption, modulation depth, maximum bit-rate and band-width. To realize these points, we have proposed a suitable quantum dot structure based on EIT in a strained GaN/AlN, with an internal barrier. Simulations show this barrier enhances the dot optical properties, such as dipole matrix element, linear susceptibility, and hence absorption coefficient, reducing the power consumption and increasing the modulation depth. Furthermore, using control signal in EIT process, carriers are driven from an upper state to a lower state from where they rapidly decay to the ground state increasing the modulation bit-rate and band-width.     

N-型四能级系统的EIT和MOLLOW谱分析

构建双耦合场作用下的N-型四能级系统,详细讨论系统的探测吸收特性随两个耦合场Rabi频率取不同值时的变化规律,得到EIT、Mollow和Autler-Townes双峰效应及其它们之间的相互转化规律。通过找出N-型四能级系统中的多个跃迁通道,将系统按跃迁通道拆分成多个子系统,不同的拆分方法实现了不同跃迁通道之间的量子干涉,从而使系统呈现出三种产生条件和物理本质不同的非线性效应。     

Manipulating stored images with phase imprinting at low light levels

Coherent manipulation of stored images is performed at low light levels based on enhanced cross-Kerr nonlinearity in a four-level N-type electromagnetically induced transparency (EIT) system. Using intensity masks in the signal pulse, quadratic phase shifts with low nonlinear absorption can be efficiently imprinted on the Fraunhofer diffraction patterns already stored in the EIT system. Fast-Fourier-transform-based numerical simulations clearly demonstrate that the far-field images of the retrieved probe light can be flexibly modulated by applying different signal fields. Our studies could help advance the goals of nonlinear all-optical processing for spatial information coherently stored in EIT systems.     

闭合Λ型4能级系统中的电磁诱导透明和电磁诱导吸收

准Λ型4能级系统具有电磁诱导吸收(EIA)和电磁诱导透明(EIT)两种特性.准Λ型4能级系统包括两个基态精细结构能级和两个激发态精细结构能级,除光学耦合场和探测场外,附加了一个射频场作用于两个激发态精细结构能级之间.若对此系统进行拓展,在两个基态精细结构能级之间引入一个驱动场,则构成4场作用下的闭合Λ型4能级系统.本文对新引进的驱动场的作用规律进行了重点研究.研究结果表明,当驱动场和射频场的Rabi频率满足不同关系时,系统呈现EIA或EIT两种不同特性,探测吸收曲线的整体轮廓也随之改变.     

基于自发辐射相干实现光学前驱动场

研究一个具有两个靠得足够近激发能级的双Lambda模型, 在矩形脉冲中分离出光学前驱动场. 当耦合场是大失谐时, 该原子系统在自发辐射相干效应的影响下产生一个透明窗口并伴随着陡峭的色散曲线. 因此, 由于透明窗口中的慢光效应可以使光学前驱动场从主脉冲场中分离出来. 另外, 我们通过调控脉冲波形序列研究探测场相位和幅值的累积光学前驱动波. 数值结果表明: 由于累积光前驱动场与脉冲主场的相长干涉大大地提高瞬态脉冲的能量.     

Absorption-Dispersion Properties in a Four-Level Atomic System with Vacuum-Induced Coherence

We discuss and analyze absorption-dispersion response for the probe field in a typical four-level atomic system with vacuum-induced coherence (VIC) arising from the cross coupling pathways associated with a pair of upper excited hyperfine levels. We find that VIC effect can preserve electromagnetically induced transparency (EIT) by using the detailed numerical simulations based on the density-matrix equations and analytical calculations in the dressed-state picture. We also show that the atomic hyperfine structure cannot be a hindrance to obtaining EIT.     

梯型四能级系统中超窄电磁感应透明与无反转增益

研究了两个光场作用下梯型四能级系统中Rabi频率及多通道辐射对电磁感应透明的影 响-在梯型四能级系统中上驱动场的Rabi频率对于超窄电磁感应透明及无反转增益的获得起 重要作用-不同的激发方式可改变电磁感应透明的位置-研究了偶极矩关联度在量子干 涉效应中所起的作用-利用缀饰态理论对数值计算结果给出了解释- 关键词： 梯型四能级系统 量子干涉效应 电磁感应透明 无反转增益     

Absorption-Dispersion Properties in a Four-Level Atomic System with Vacuum-Induced Coherence

We discuss and analyze absorption-dispersion response for the probe field in a typical four-level atomic system with vacuum-induced coherence (VIC) arising from the cross coupling pathways associated with a pair of upper excited hyperfine levels. We find that VIC effect can preserve electromagnetically induced transparency (FIT) by using the detailed numerical simulations based on the density-matrix equations and analytical calculations in the dressed-state picture. We also show that the atomic hyperfine structure cannot be a hindrance to obtaining EIT.     

Controllable shape-matched propagation of two signal beams in a double-Λ atomic ensemble

We study a four-level double-Λ atomic ensemble interacting with two time-dependent signal fields and two stationary control fields. Though, in each Λ channel, a pair of signal and control fields couple resonantly with the two lower levels of atoms, the occurrences of electromagnetically induced transparency (EIT) is affected by the coherence of the four fields. In the discussion of atomic susceptibilities, we show that the quantum coherence between the two lower levels can be either formed or released according to the phase matching of the four fields. We analyze the propagation equation of the two signal fields, and find two characteristic solutions: the stationary transmission wave and the transient decay wave. The former corresponds to a correlated EIT effect in which two signal pulses are shape-matched. The latter is an opposite effect to the correlated EIT in which two pulses quench simultaneously, thus named as the correlated two-signal absorption (CTSA). We propose the CTSA condition in correspondence with the EIT condition. The numerical simulation shows that the double-Λ configuration is capable of manipulating synchronous optical signals and thus provides multiplicity and versatility in quantum information process.     

Gain-assisted giant Kerr nonlinearity in a ${\sf \Lambda}$-type system with two-folded lower levels

We propose a four-level Λ scheme with a one-mode active Raman gain core and two-folded lower levels to obtain new linear and nonlinear optical responses. We show that this scheme is fundamentally different from that based on electromagnetically induced transparency (EIT). Firstly, it is gain-assisted and thus capable of eliminating all attenuation of probe field. Furthermore, due to the quantum interference effect introduced by a coupling field a gain doublet appears in gain spectrum, and hence the distortion of the probe field during propagation can be effectively avoided. In addition, in such system a large and rapidly responding Kerr nonlinearity can be produced, which is much (more than 10 times) larger than that obtained in the EIT-based scheme with the same energy-level configuration.     

Studying parity-nonconservation effect in the conversion transition between the levels of the (5/2)± doublet in the 229Pa nucleus

Effects of weak nucleon interaction are calculated for the ground-state doublet of 5/2^± levels of the strongly deformed nucleus ²²⁹Pa₉₁. A parity-nonconservation effect in the doublet states can be observed in the conversion spectrum for the isomeric transition between the doublet levels. By using a generalized model of the nucleus, the matrix element of the effective one-nucleon weak-interaction potential, which determines the weight of the opposite parity admixture in the doublet components is estimated in the single-particle approximation. The reduced probabilities of the E1 and M1 nuclear transitions between the doublet states are calculated within various models of the deformed nuclear potential. The effect of Coriolis forces on the dipole electric transition in question is considered. The lifetime of the upper doublet state is estimated.