在双Lamder模型中实现可调控光子带隙

研究驻波场相干驱动下四能级双Lamder模型的电磁感应光子带隙。当耦合场是远共振或共振时,来自于两邻近能级间的自发辐射相干效应(SGC)有助于实现一个或两个光子带隙。当自发辐射相干效应不存在时,探测场被原子系统强烈吸收,因此导致光子带隙严重形变甚至无法形成。数值结果表明,光子带隙结构是由SGC效应导致探测场和耦合场之间的相干增强Kerr非线性调制而产生,并改变驻波场的耦合方式,使系统实现单光子带隙转变为双光子带隙的动态调控。     

原子晶格中基于自发辐射相干效应的光子带隙

利用电磁感应透明技术,在一维光晶格中相干驱动四能级Lambda模型冷原子系统,从而实现动力学可调谐电磁感应光子带隙结构。基于两邻近能级间的自发辐射相干(SGC)效应,通过控制耦合场从远共振到共振,使该原子系统实现从两个光子带隙转变为三个光子带隙的动态过程。当自发辐射相干效应不存在时,在探测场共振区域处探测光子被原子系统强烈吸收,因此感应光子带隙严重形变甚至无法形成。通过数值计算证明光子带隙结构的形成源自于自发辐射相干效应下探测场和耦合场之间的三阶交叉克尔(Kerr)非线性调制,并且通过控制耦合场的耦合方式,可以实现系统从两个光子带隙到三个光子带隙的动力学调控。     

一维原子晶格中基于电磁感应透明的可调控光子带隙

利用电磁感应透明技术,在一维光晶格中相干驱动四能级N模型冷原子,实现动力学可调谐电磁感应光子带隙结构.在该原子系统中,光晶格中的原子密度呈高斯分布,使整个介质的密度在空间上呈周期性分布,导致探测场的折射率被空间调制,从而产生带隙结构.理论计算表明:基于两强耦合场的双暗态共振作用,探测场的吸收谱出现两个透明窗口;当满足Bragg条件时,探测场的三个不同频率区域内光子态密度减小,形成光子带隙结构.另外,改变两个耦合场的强度和失谐可以有效地调控光子带隙的宽度和位置.     

二维电磁感应光子带隙的动态生成与调控

研究了由两个垂直传播的强驻波激光场共同耦合的一个四能级Tripod型冷⁸⁷Rb原子介质的稳态光学响应特性. 结果发现, 当两驻波场满足双光失谐条件时, 可在这两驻波场的传播方向上同时获得反射率高达95%以上的电磁感应光子带隙结构; 通过适当调节强激光场, 还可实现一个方向为光子带隙而另一个方向为透明窗口或者两个方向均为透明窗口的结构. 并且光子带隙和透明窗口的频宽和位置是可调谐的. 这种全光控制的二维的信号光禁闭和导通机制可用于实现全光开关和全光路由, 有利于复杂的全光通讯网络的开发. 关键词： 相干诱导光子带隙 电磁感应透明 周期调制原子相干     

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。     

倒Y型四能级系统中自发辐射诱导相干对弱探测场的色散和吸收特性的影响

本文研究了倒Y型四能级系统中自发辐射诱导相干对探测场的色散和吸收特性的影响。在稳态条件下利用密度矩阵微扰理论推导出了密度矩阵的迭代解。数值分析了自发辐射诱导相干对线性和非线性折射率和吸收系数的影响,分析了泵浦场和耦合场偏离双光子共振对弱探测场色散和吸收特性的影响。研究发现自发辐射诱导相干使弱探测光的线性和非线性折射率增强,同时形成一个较宽的透明窗。在没有自发辐射诱导相干的情况下,泵浦场和耦合场偏离双光子共振对探测场的线性和非线性折射率影响明显,在有自发辐射诱导相干的情况下,泵浦场和耦合场偏离双光子共振使得探测场的线性透明窗变窄,非线性吸收增加。     

光子晶体特性在EIT介质中的实验实现

基于原子相干效应,研究了在驻波场作用下原子介质的吸收和折射率的周期性调制所产生的周期性微结构特性及其操控。实验结果表明,在驻波场作用下原子介质存在与光子晶体同样的光子带隙特性,而且可通过调节形成驻波的两束光的频率差实现对其光子带隙位置的操控,实现了光子带隙20MHz的频率移动。     

基于自发辐射相干效应的可调光子带隙反射率的提高方法

光子带隙是指某一频率范围的波不能在周期变化的空间介质中传播,即这种结构本身存在“禁带”,并已成功地应用于滤波器、放大器和混频器等器件的设计中.此前,许多专家都致力于提高带隙的反射率,但其只能逐渐接近1.本文在囚禁于一维光晶格中的冷原子介质中实现两个可调光子带隙,并通过选择两基态为精细结构的三能级∧型原子系统,考虑自发辐射相干效应来探究这两个带隙的反射率.适当调节参数,探测场出现增益,从而获得较高反射率的带隙结构,甚至可以超过1.此外,两个带隙反射率还可以通过调节偶极矩之间的夹角以及非相干驱动场强度等参数来操控.     

四能级双V型原子系统中考虑自发辐射相干的无粒子数反转激光

提出了一个由两个弱探测场和两个强耦合场驱动的四能级双V型原子系统, 研究发现在四波混频共振条件下, 两探测场均可被放大而无须粒子数反转. 值得注意的是, 由于所选择的激发态为超精细结构的两个近能级, 这里必须考虑自发辐射相干效应的影响. 与不考虑自发辐射相干相比, 同样参数条件下探测场的增益得到大幅度提高.而且, 探测场增益对相位非常敏感, 即增益-吸收线型受相位周期性调制, 同时也受两个偶极矩之间夹角θ制约. 此外还分析了相干抽运场 (强耦合场)的失谐对增益谱线产生的影响. 关键词： 无粒子数反转激光 四波混频 自发辐射相干     

五能级87Rb原子系统中相干诱导的可调谐三光子带隙

研究了一个由两个同向驻波场和一个微波场相干驱动的五能级87 Rb原子系统,并在探测场共振频率处实现了可调谐三光子带隙。利用激光场与多能级原子系综相互作用的密度矩阵方程,结合光脉冲在具有空间周期性介质中的相干传输矩阵理论,推导出了描述相干原子系统稳态条件下的反射谱和透射谱。由数值模拟发现,通过改变驱动场的失谐和拉比频率可以动力学地调控光子带隙的位置和宽度,而且这种相干诱导三光子带隙能够同时操控三个不同中心频率光脉冲的传播。     

Photonic bandgap via nonlinear modulation assisted by spontaneously generated coherence

A four-level double-ladder atomic system with two upper states coupled to the excited state by a standing-wave trigger field is explored to generate photonic bandgap (PBG) structure. With the assistance of spontaneously generated coherence (SGC) from the two decay pathways, we can obtain single or double fully developed PBG when the trigger field is far away from resonance or resonant. While in the absence of SGC, the atomic medium becomes strong absorptive to the probe field, and therefore the resulting PBGs are severely malformed or even cannot be opened up. Numerical results show that the PBG structure is originated from the third-order cross Kerr nonlinear modulation between the probe and trigger fields. This mechanism differs from the recent schemes based on linear modulation.     

Dynamically controlled optical nonreciprocity of a double-ladder system with spontaneously generated coherence in moving atomic optical lattice

A four-level double-ladder cold atoms system with spontaneously generated coherence trapped in a moving optical lattice is explored to achieve optical nonreciprocity. When spontaneously generated coherence(SGC) is present, the remarkable contrast optical nonreciprocity of light transmission and reflection can be generated at each induced photonic bandgap in the optical lattice with a velocity of a few m/s. However, when the SGC effect is absent, the optical nonreciprocity becomes weak or even vanishing due to the strong absorption. It is found that the optical nonreciprocity is related to the asymmetric Doppler effect in transmission and reflection, meanwhile the degree and position of optical nonreciprocity can be tuned by the SGC effect and the Rabi frequency of the trigger field.     

Phase control of light amplification in steady and transient processes in an inverted-Y atomic system with spontaneously generated coherence

We investigate the effects of spontaneously generated coherence （SGC） on both the steady and transient gain properties in a four-level inverted-Y-type atomic system in the presence of a weak probe, two strong coherent fields, and an incoherent pump. For the steady process, we find that the inversionless gain mainly origins from SGC. In particular, we can modulate the inversionless gain by changing the relative phase between the two fields. Moreover, the amplitude of the gain peak can be enhanced and the additional gain peak can appear by changing the detuning of the coupling field. As for the transient process, the transient gain properties can also be dramatically affected by the SGC. Compared to the case without SGC, the transient gain can be greatly enhanced with completely eliminated transient absorption by choosing the proper relative phase between the two fields. And the inverted-Y-type system with SGC can be simulated in both atomic and semiconductor quantum well systems avoiding the conditions of SGC.     

Effect of spontaneously generated coherence on left-handedness in a degeneracy atomic system

A theoretical investigation is carried out into the effect of spontaneously generated coherence (SGC) on the left-handedness in a four-level Y-type atomic system with two highest nearly degenerate lying levels. It is found, with the spontaneously generated coherence intensity enhancing, the atomic system gradually displays left-handedness with simultaneous negative permittivity and permeability. And the refractive index enhances with the increasing intensity of SGC. However, the absorption is suppressed by the SGC effect when the SGC has a large intensity. When the probe field is near-resonant coupled to the atomic system, the appearance of SGC doesn’t always change the permeability from positive to negative and allow for lefthanded behavior, unless the SGC reaches a large intensity.     

Doppler展宽的封闭原子系统中无反转增益的相位控制

研究了在具有自发辐射诱导相干性的Doppler展宽的封闭Λ型三能级系统中探测场和驱动场之间的相对位相对探测场无反转增益的控制作用. 研究结果表明: 1) 不管探测场和驱动场是同向传播还是反向传播, 驱动场是失谐还是共振,无反转增益总是随相对位相的改变而作周期性变化,周期为2π. 2) 驱动场共振时,无反转增益极大值随Doppler展宽值的增大而单调减小,且反向传播时比同向传播时减小的速度更快;驱动场失谐时,无反转增益的极大值随Doppler展宽值的增大不再单调地减小或增大. 在以上两种情况下,均可以通过调 关键词： 自发辐射诱导相干 相位控制 Doppler展宽 无反转增益     

Probe gain via four-wave mixing based on spontaneously generated coherence

We have studied the probe gain via a double-Λ atomic system with a pair of closely lying lower levels in the presence of two probe and two coherent pump fields. The inversionless gain can be realized by using nondegenerate four-wave mixing under the condition of spontaneously generated coherence(SGC) owing to near-degenerate lower levels. Note that by using SGC, two probe fields can be amplified with more remarkable amplitudes, and the gain spectra of an extremely narrow linewidth can be obtained. Last but not least, our results show that the probe gain is quite sensitive to relative phases due to the SGC presence which allows one to modulate the gain spectra periodically by phase modulation, and can also be influenced by all laser field intensities and frequencies, and the angles between dipole elements.     

Effect of spontaneously generated coherence on inversionless lasing gain in an atomic system with Doppler broadening

We have studied the effect of the spontaneously generated coherence (SGC) on gain of lasing without inversion (LWI) in a closed three-level $\Lambda $-type atomic system with Doppler broadening. It is shown that, regardless of the driving and probe fields being co- or counter-propagating, at a suitable value of the Doppler width, we can obtain a much larger LWI gain with SGC than that without SGC; and the region of the LWI gain spectrum with SGC is obviously larger than that without SGC. When the Doppler width takes a constant value, the gain does not monotonically decrease or increase with increasing strength of SGC, the largest LWI gain can be obtained by adjusting strength of SGC. Generally speaking, the co-propagating probe and driving fields is favourable to obtain a larger LWI gain.     

Absorption-amplification response with or without spontaneously generated coherence effect in a four-level atomic system

We discuss and analyze the absorption properties of a weak probe field in a typical four-level atomic system in the presence of a spontaneously generated coherence (SGC) term. The influences of the SGC and a coherent pump field on the probe absorption-amplification are investigated. The results show that the absorption of such a weak probe field can be dramatically enhanced due to the SGC effect. At the same time, the probe-absorption profile exhibits a two-peak structure and the probe-absorption peak gradually decreases as the pump intensity increases. On the contrary, the amplification of such a weak probe field near the line center of the probe transition can be achieved by adjusting the coherent pump field intensity in the absence of the SGC effect.