Λ型三能级系统中EIT的瞬态响应时间研究

详细研究了Λ型三能级原子系统中电磁感应透明的瞬态过程。我们从理论上分析了系统中各参数(例如失谐、原子能级衰减率、探测场和耦合场强度)对瞬态响应时间的影响,并且通过对密度矩阵演化方程的数值求解,分析了相关密度矩阵元尤其是与吸收和色散相关的矩阵元ρab的瞬态行为。结果发现:在满足双光子共振条件Δ=Δμ的情况下,瞬态响应时间与能级衰减速率以及探测场和原子跃迁的失谐有直接关系;随着失谐的增大,响应时间也相应延长,同时,瞬态响应时间也随着衰减率γ的增大而缩短。而探测场和耦合场的强度变化并未对响应时间造成影响。此外,很多文献都指出,常用的微扰近似稳态解只适用于探测场强度远小于耦合场强度的情况,而本文则通过对比数值解的稳定值和微扰近似稳态解的差异,给出了其适用的确切范围:即应满足PE/h≤0.1Ωμ。     

基于偶极阻塞效应的单光子水平电磁感应透明

对量子化的探测场在一维超冷里德伯原子样品中的传播特性进行研究。基于偶极阻塞效应,四能级原子的稳态电磁感应透明(EIT)光谱表现出典型的单光子水平的非线性现象:未饱和之前,探测场透射率和光子关联依赖于探测场强度。调整两个经典控制场的单光子失谐,能够实现对非线性EIT行为的灵活操控。通过改变拉比频率比例,可观察到四能级原子系统到三能级梯形原子系统的非线性EIT转变。     

多窗口可调谐电磁诱导透明研究

当两束激光以Λ-构型作用于三能级原子系统并满足双光子共振条件时，探测激光场吸收谱呈现电磁诱导透明(EIT)特征.若再加一个微波控制场作用于该三能级系统的两个低能级跃迁之间，会导致探测吸收特性明显变化，EIT窗口将发生劈裂.通过求解相应的密度矩阵方程，揭示了外加微波场作用下EIT窗口的变化规律，并给出了相应的缀饰态解释.研究结果表明，在适当的条件下, 电磁诱导透明呈现三重结构，而EIT窗口的频率位置取决于微波控制场的拉比频率及频率失谐量.因此通过改变微波控制场的参数可以实现多EIT窗口的频率调谐. 关键词： 电磁诱导透明 量子相干 频率调谐 多窗口EIT     

准A型四能级原子系统在弱场中的增益及电磁感应透明

利用数值模拟的方法讨论了稳态下准型四能级原子与两光场相互作用系统对探测场的增益以及与粒子数布居规律之间的关系。结果表明即使在场较弱的情况下，随着探测场和它所对应能级间跃迁频率失谐量的变化，系统在所有拉比频率相位为零时对探测光总会同时呈现三个增益峰和两个固定的产生电磁感应透明（EIT）现象的位置，并且当系统呈现电磁感应透明的同时，探测场所对应的高能级粒子数为零；而拉比频率相位的变化则会导致系统对探测光既有吸收也有增益，并且也呈现电磁感应透明现象，但仅仅相位的变化并不影响高能级的粒子数分布。值得注意的是该结果均在精确求解下所得。     

准Λ型四能级原子系统在弱场中的增益及电磁感应透明

利用数值模拟的方法讨论了稳态下准型四能级原子与两光场相互作用系统对探测场的增益以及与粒子数布居规律之间的关系。结果表明即使在场较弱的情况下,随着探测场和它所对应能级间跃迁频率失谐量的变化,系统在所有拉比频率相位为零时对探测光总会同时呈现三个增益峰和两个固定的产生电磁感应透明(EIT)现象的位置,并且当系统呈现电磁感应透明的同时,探测场所对应的高能级粒子数为零;而拉比频率相位的变化则会导致系统对探测光既有吸收也有增益,并且也呈现电磁感应透明现象,但仅仅相位的变化并不影响高能级的粒子数分布。值得注意的是该结果均在精确求解下所得。     

自发产生相干对探测场的色散和吸收影响

在Y形四能级原子系统中,分析了自发产生相干对探测场的色散和吸收影响. 结果发现,随着自发产生相干的增强,系统呈现的电磁感应透明窗口逐渐变窄,并在共振处出现反常色散,吸收从负值变为正值. 同时,当抽运场和耦合场的相对强度不同时,共振处附近吸收为零或出现增益,而两场强相等或接近时,系统对探测场表现为吸收. 关键词： 非线性光学 电磁感应透明 自发产生相干     

双耦合场作用下的Mollow七峰谱

对双耦合A型三能级系统中Mollow谱线上叠加的新的透明和吸收特性进行了理论研究.若基态能级包括两条精细结构简并能级,强耦合场和探测场作用于激发态能级和第一条基态精细结构能级之间,另一个弱耦合场作用于激发态能级和第二条基态精细结构能级之间,则构成了双耦合A型三能级系统.通过调谐强耦合场的频率失谐量,探测吸收谱线中不仅得到了典型的Mollow三峰失谐谱,还出现了电磁诱导透明(EIT)和电磁诱导吸收(EIA)等新现象,形成了Mollow七峰谱.该文进一步分析了EIT和EIA的位置与耦合场参数之间的关系,并用缀饰态理论做出了解释.     

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。     

利用相干制备的三能级原子介质实现低噪声弱光相位操控

我们发现利用相干制备的三能级原子介质可实现低噪声弱光相位操控.基于两波混频效应,相干制备的原子系统在弱光相位操控方面有许多新特性,例如宽频范围内无损常数相位的实现以及无附加相位的吸收与放大等.通过同时锁定抽运光的光强和双光子失谐,我们发现探测光的吸收为零,且探测光的相位对于频率的变化十分缓慢,因而可以通过选择适当的系统参数在很宽的频率范围内实现无损π相位操控.同时,利用该系统我们可以在单光子水平下实现低噪声、无损的量子相位门,这个系统在光通信和信息处理领域有着重要的应用价值.     

激光诱导的四能级系统电子布居转移H

受激拉曼绝热通道(STIRAP)为我们提供了一种有效转移原子与分子中电子布居的方法.采用数值方法系统地分析了不同的四能级系统在受激拉曼迁移过程中电子布居的转移情况,计算了各种系统处于双光子共振和共振失谐时的转移率,并且讨论了在不同情况下第四能级对转移率的影响.计算结果表明,对于许多原子或分子系统,获得高的转移率所必须满足的条件是非常严格的,而有些四能级系统根本就不能获得高的转移率.(     

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.     

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.     

双光子失谐对慢光和光存储影响的实验研究

利用电磁感应透明(EIT)效应在87Rb热原子气室中进行了慢光和光存储的实验研究,在单光子红失谐650 MHz处测量了双光子失谐对光脉冲延迟和光存储的影响.结果表明:在双光子失谐0—0.5 MHz范围内存在显著的光脉冲延迟和光存储恢复信号,其慢光波形与理论计算结果基本相符;而恢复光脉冲信号随着双光子失谐的变化出现形变,这是由于多个EIT子系统之间的干涉引起的.这一研究结果为连续变量光场在热原子系综中的存储提供了实验参考.     

The Enhancement Absorption of Light Field Induced By Optical Radiation on the off Resonance in Cesium Vapor Cell

A large increase of atomic absorption via atomic coherence is observed in Cesium atoms in the condition of off resonance in degenerate two-level atomic systems. We show that when on resonance, the reduction of atomic absorption via EIT accured. But when we tune the coupling field on the off resonance, the enhancement of absorption via EIA is obtained for the probe field. The influences of the two-photon detuning and intensity of coupling field on the absorption are also studied experimentally.     

双通道原子相干性转移效率与拉曼单光子失谐的关系研究

研究了原子相干性在双通道间的转移效率与拉曼单光子失谐的关系。在冷原子四能级Tripod型系统中,首先利用EIT动力学过程将光信号存储在原子的一个记忆通道上。然后通过拉曼双光子过程,我们进行了原子记忆在两个通道间相干转移的研究,结果表明,在一定的拉曼光强下,原子相干性在双通道间的转移效率随着拉曼单光子失谐发生变化,最大的转移效率可达到94%。     

Comparison of steady and transient optical responses between a four-level Tripod system and a three-level Lambda system

We study and compare the steady and transient optical responses of a four-level system and a three-level system, which are dressed by two and three laser fields, respectively. In the three-level system, a transmissivity window (a reflectivity platform) may be induced on the probe resonance by a traveling-wave (standing-wave) coupling field. In the four-level system, both transmissivity window and reflectivity platform are remarkably modified when a traveling-wave driving field is applied on two-photon resonance with the probe, but change little when the traveling-wave driving field is largely two-photon detuned. This implies that the four-level system can be safely treated as a three-level one in certain frequency regions of interest. Transient behaviors of atomic coherence on the probe resonance are also examined in both three-level and four-level systems to further confirm this conclusion. Our new findings are beneficial to achieve a novel all-optical routing scheme by simultaneously creating a transmissivity window and a reflectivity platform on a single resonance, and may also be used to simplify theoretical treatments in the situations where numerical calculations are complicated and intractable in the presence of at least one standing-wave field.     

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.     

Experimental implementation of a four-level N-type scheme for the observation of electromagnetically induced transparency

A nondegenerate four-level N-type scheme was experimentally implemented to observe electromagnetically induced transparency (EIT) at the ⁸⁷Rb D ₂ line. Radiations of two independent external-cavity semiconductor lasers were used in the experiment, the current of one of them being modulated at a frequency equal to the hyperfine-splitting frequency of the excited 5P _3/2 level. In this case, apart from the main EIT dip corresponding to the two-photon Raman resonance in a three-level L-scheme, additional dips detuned from the main dip by a frequency equal to the frequency of the HF generator were observed in the absorption spectrum. These dips were due to an increase in the medium transparency at frequencies corresponding to the three-photon Raman resonances in four-level N-type schemes. The resonance shapes are analyzed as functions of generator frequency and magnetic field.     

Investigation of atomic coherence in multilevel systems with embedded tunable dark state superposition

The coherent superposition of two-atomic levels induced by coherent population trapping is employed in the two-level system, the standard three-level Λ type scheme and the four-level N-type systems and a weak probe pulse scans across the system. A theoretical analysis about the response of medium to the probe field is given. It is shown that under different initial conditions, the coherent superposition of the dark state exhibits abundant optical phenomena response to the probe field. It can change the absorption or gain and the dispersion relationship in the medium experienced by the probe. In the embedded three-level scheme, the probe experiences a crossover from absorption to transparent and then to amplification. Consequently the group velocity of the probe pulse can be controlled to propagate either as a subluminal, a standard, a superluminal or even a negative speed. In the embedded four-level N-type system, it can give an enhancement to the Giant Kerr effect and overcome the limitation of two-photon absorption, then make the nonlinear properties of the medium richer than the traditional N-type scheme.     

Absorption Changes Induced by Off-Resonance Driving a Degenerate Two-Level System

The alteration of atomic absorption via quantum coherence is observed in the degenerate two-level atomic system. It is shown that when the detuning of coupling field equals to that of probe light, i.e. two-photon resonance, the reduction of atomic absorption via electromagnetically induced transparency occurs. However, when we tune the coupling field to two-photon off-resonance, the enhancement of absorption is obtained for the probe field. The influences of one-photon detuning and intensity of coupling field on absorption are also experimentally demonstrated.