Absorption-Amplification Response with or Without Spontaneously Generated Coherence in a Coherent Four-Level Atomic Medium

We discuss and analyze the absorption-amplification properties of a weak probe field in a typical four-level atomic system in the presence of an additional coherence term, the spontaneously generated coherence term. Theinfluences of the spontaneously generated coherence and a coherent pump field on the probe absorption (amplification)are investigated in detail. We show that the absorption of such a weak probe field can be dramatically enhanced dueto the presence of the spontaneously generated coherence. At the same time, the probe-absorption profile exhibitsthe double-peak structure and the probe-absorption peak gradually decreases as the pump intensity increases. On thecontrary, the amplification of such a weak probe field near the line center of the probe transition can be achieved byadjusting the coherent pump field intensity in the absence of the spontaneously generated coherence.     

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.     

Generation of coherent X-rays from a relativistic electron beam backscattered by a CO2 laser

On the basis of Palmer's criterion for the self-bunching effect of a relativistic electron beam in a helical magnetic field, it is shown that coherent X-rays can be generated from relativistic electron beams backscattered by laser beams without the need of any resonant cavity.     

无序介质后向散射光的偏振增强效应

无序介质后向散射光的强度增强效应可以看作为一种与偏振性质有关的相干效应。我们给出后向散射光的偏振度 P 与两个强度增强的因子 E 和 E 的关系,从而得到偏振度增强结果。偏振度的实验曲线证实了后向散射光的偏振增强效应。我们还讨论了后向散射光的部分偏振性质以及偏振增强效应与相干效应之间的关系。     

Steady-state analysis of three-photon absorption spectra via density-matrix method in a three-coupled-quantum-well nanostructure

We numerically simulate three-photon absorption spectra in a three-coupled-quantum-well nanostructure interacting with a pump field,a coherent coupling field,and a probe field.We find that the three-photon absorption spectra can be dramatically influenced due to the intensities of the coupling field and pump field changing under the three-photon resonance condition.The effect of the frequency detuning of the pump field on the three-photon absorption spectra is also discussed.The study in our case is much more practical than the study in the case of its atomic counterpart in the sense of flexible design and the wide adjustable parameters.Thus it may open up some new possibilities for technological applications in optoelectronics and solid-state quantum information science.     

From slow to fast light in amplified negative refraction using atomic coherence assisted by an incoherent pump

We propose and demonstrate a scheme to realize large negative refraction dispersion when a probe field is amplified by cooperation between a coherent field and an incoherent pump. The probe pulse can propagate either subluminally or superluminally in the left-handed amplification medium by modulating parameters of the coherent and incoherent fields.     

Dynamical switching and memory via incoherent pump assisted optical bistability

We analyze the dynamical behavior of all-optical switching and memory based on tunable optical bistability in a three-level Λ-type atomic system driven by a probe field circulating in an optical ring cavity, a coherent coupling field and an incoherent pump field outside cavity. Owing to the incoherent pump process, the absorption and Kerr nonlinearity near the atomic resonance change dramatically as a result of varied population distribution and atomic coherence, and then the switch-up and switch-down thresholds, as well as the width (or area) of bistable curve, can be manipulated effectively. By tuning the intensity of either coherent coupling field or incoherent pump field, we can make the system output flip between the lower and upper branches of different bistable hysteresis loops while the cavity input keeps to a constant value. Accordingly, all-optical switching and memory can be implemented via dynamical control of the bistable behavior under the assistance of incoherent pump. The proposed scheme can find potential applications in all-optical communication and computation.     

光子晶体光纤反常色散区抽运产生超连续谱的相干特性分析

使用复互相干度的定义对超连续谱的相干性进行了数值计算,得到了不同功率抽运情况下的脉冲谱展宽以及超连续谱相干性的变化.结果表明孤子自频移以及色散波辐射是抽运波长位于光纤反常色散区情况下超连续谱展宽的主要物理机理,而超连续谱的相干性则主要受到调制不稳定性的影响.调制不稳定性放大抽运脉冲自身携带的随机噪声,使得非线性效应产生的光谱成分具有随机的相位与幅度,引起超连续谱相干性的下降. 抽运功率越高, 调制不稳定性增益越高,噪声对超连续谱产生的作用越强, 超连续谱的相干性越差.要获得高相干的超连续谱, 需采用峰值功率较小的脉冲进行抽运.要获得大谱宽高相干的超连续谱, 则需要合理选择抽运脉冲功率.     

Convective amplification of parametrically generated acoustic wave in a piezoelectric semiconductor plasma

Using hydrodynamic model of semiconductor plasmas and coupled-mode theory of interacting waves, we have analytically investigated parametric interaction in a magnetised piezoelectric semiconductor plasma in non-relativistic domain. The temperature dependence of momentum transfer collision frequency of electrons due to their heating by the pump is assumed to induce nonlinearity in the medium. We have derived a dispersion relation which finally gives four unstable acoustic modes; two forward amplifying modes and two backscattered attenuating modes. We have also obtained an expression for the critical pump amplitude ( ) at and around which gains and phase velocities of amplifying acoustic modes become least dependent on the pump amplitude and static magnetic field . The required can be readily obtained from a pulsed 10.6 μm CO₂ laser. The magnetic field is found to shift the critical point towards lower pump amplitudes. Received 5 September 2000 and Received in final form 5 March 2001     

Noise sensing via stochastic quantum Zeno

The dynamics of any quantum system is unavoidably influenced by the external environment. Thus, the observation of a quantum system (probe) can allow the measure of the environmental features. Here, to spectrally resolve a noise field coupled to the quantum probe, we employ dissipative manipulations of the probe, leading to so-called Stochastic Quantum Zeno (SQZ) phenomena. A quantum system coupled to a stochastic noise field and subject to a sequence of protective Zeno measurements slowly decays from its initial state with a survival probability that depends both on the measurement frequency and the noise. We present a robust sensing method to reconstruct the unknown noise power spectral density by evaluating the survival probability that we obtain when we additionally apply a set of coherent control pulses to the probe. The joint effect of coherent control, protective measurements and noise field on the decay provides us the desired information on the noise field.     

相干瞬态的量子干涉效应和Berry相位

利用解析方法描述了相干瞬态量子体系中不同类型的量子干涉效应,分别讨论了光学干涉与量子干涉所起的作用,分析了在时域对称光场作用下,几何相位在量子干涉效应中所扮演的角色,从理论上证明了通过适当改变抽运场脉冲面积,可实现对几何相位的测量. 同时研究也发现利用啁啾抽运场可以实现对量子干涉效应的有效控制. 关键词： 相干瞬态 量子干涉 几何相位 啁啾脉冲     

Intracavity weak nonlinear phase shifts with single photon driving

We investigate a doubly resonant optical cavity containing a Kerr nonlinear medium that couples two modes by a cross phase modulation. One of these modes is driven by a single photon pulsed field, and the other mode is driven by a coherent state. We find an intrinsic phase noise mechanism for the cross phase shift on the coherent beam which can be attributed to the random emission times of single photons from the cavity. An application to a weak nonlinearity phase gate is discussed.     

Lasers with saturable absorber driven by a coherent field

We derive the semiclassical equations governing the response of a solid-state monochromatic laser with saturable absorber driven by an external coherent field. The behavior of the emitted field intensity versus either the driving field intensity or the pump parameter of the amplifying atoms displays the same qualitative behavior. Three possible situations can appear, depending on the values taken by the relevant parameters: for a given external field intensity or pump parameter there can be one, two or three stable values of the emitted field intensity. In this way we have an example of a system which displays mono-, bi- or tristability.Chercheur Qualifié F.N.R.S.     

The distribution of headways in traffic noise simulation

In recent years researchers in jet turbulence and jet noise have become increasingly interested in what is termed “large scale coherent jet structures”. There is now considerable evidence that azimuthally coherent structures can be generated by acoustically forcing a jet from upstream. However, the evidence for such structures in unforced jets, except close to the nozzle at low Reynolds numbers, is, at best, circumstantial. The role of such structures in subsonic jet noise production is also completely unproven. In an attempt to establish a link between azimuthally coherent structures and the jet noise field a number of experimenters have recently made azimuthal cross-correlation measurements of either the near field pressure or far field noise, and used the observed coherence to infer the existence of an azimuthally coherent source field. The term azimuthally coherent is used here to infer that the source region is dominated by low order azimuthal components, with relatively little contribution coming from the higher azimuthal components. The purpose of this paper is to question the interpretation of that data. Specifically the sound field generated by a simple ring source with various types of azimuthal coherence is considered theoretically. It is shown that the azimuthal coherence of both the near and far field pressures is principally a function of the Helmholtz number and in many cases of practical interest is relatively insensitive to any coherent structure of the source.     

Acoustic scattering by a three-dimensional elastic object near a rough surface

The ensemble-averaged field scattered by a smooth, bounded, elastic object near a penetrable surface with small-scale random roughness is formulated. The formulation consists of combining a perturbative solution for modeling propagation through the rough surface with a transition (T-) matrix solution for scattering by the object near a planar surface. All media bounding the rough surface are assumed to be fluids. By applying the results to a spherical steel shell buried within a rough sediment bottom, it is demonstrated that the ensemble-averaged "incoherent" intensity backscattered by buried objects illuminated with shallow-grazing-angle acoustic sources can be well enhanced at high frequencies over field predictions based on scattering models where all environmental surfaces are planar. However, this intensity must compete with the incoherent intensity scattered back from the interface itself, which can defeat detection attempts. The averaged "coherent" component of the field maintains the strong evanescent spectral decay exhibited by flat interface predictions of shallow-angle measurements but with small deviations. Nevertheless, bistatic calculations of the coherent field suggest useful strategies for improving long-range detection and identification of buried objects.     

超衍射极限相干反斯托克斯拉曼散射显微成像技术及其探测极限分析

理论上提出一种突破衍射极限限制的相干反斯托克斯拉曼散射显微成像方法, 并对其探测极限进行分析.通过引入环形附加探测光与艾里斑周边的声子作用, 实现点扩展函数的改造, 提高相干反斯托克斯拉曼散射显微成像系统的横向空间分辨率. 随着分辨率的提高, 信号强度也随之降低, 尤其当应用于生物学、医学研究时, 样品分子数密度通常很低, 这将导致信号探测更加困难. 因此分析系统的探测极限, 确定超分辨体积元内的最小可探测分子数是展开超衍射极限相干反斯 托克斯拉曼散射显微成像实验研究的重要前提. 当泵浦光、斯托克斯光、探测光光强均达到极大值, 分辨率约40 nm三维空间内, 超衍射极限相干反斯托克斯拉曼散射显微成像系统的散粒噪声信噪比由曝 光时间与样品分子数密度决定. 曝光时间若取20 ms, 探测极限约为10³, 样品分子数目只有大于探测极限, 才能保证信号可以从噪声背景中提取出来. 关键词： 突破衍射极限 相干反斯托克斯拉曼散射 非线性光学 探测极限     

Quantum phase properties of the field in a micromaser: Effect of cooperative atomic interactions, cavity losses and pump fluctuations

In this paper, we study the effect of cooperative atomic interactions, cavity losses, and pump fluctuations on quantum phase properties of the field in a one-photon micromaser. We consider, initial coherent state of the radiation field and atoms initially in the excited and coherent superposition of their atomic states, respectively. We find that quantum phase properties of the field in a micromaser are highly sensitive to two-atom events and cavity losses. Both contribute to the randomization of the well-defined phase structure associated with the initial coherent state. However, the approach towards the randomization is quite different in the two cases. We also find that the fluctuations, associated with the random injection of the atoms, affect the phase structure of the coherent state.     

Proposal of a Microwave-Driven Semiconductor Superlattice Oscillator for Generation of THz Radiation

We present the proposal of a microwave-driven semiconductor superlattice oscillator. We show that the interplay of a microwave pump field with a synchronous harmonic field can make a semiconductor superlattice to a gain medium for the harmonic field. Placing the superlattice in a resonator for the harmonic field allows the operation of an oscillator. The gain mechanism is based on Bloch oscillations of miniband electrons. The gain is mediated either by the interaction of the high-frequency field with the single electrons or with space charge domains or with both. The microwave-driven superlattice oscillator should be suitable for generation of coherent radiation up to several THz.     

Higher-order squeezing and sub-poissonian photon statistics in CARS and CAHRS

The Squeezing and sub-poissonian photon statistics of an optical field are a purely quantum mechanical phenomenon and has been accepted as means of achieving noise below the quantum shot-noise limit. The effect of higher-order squeezing and sub-poissonian nature of an optical field in coherent anti-Stokes Raman scattering (CARS) and coherent anti-Stokes hyper Raman scattering (CAHRS) are investigated under short-time approximation. The coupled Heisenberg equations of motion involving real and imaginary parts of the quadrature operators are established and solved under short-time scale. The dependence of squeezing on the number of photons is also investigated. It is also shown that higher-order squeezing allows a much larger fractional noise reduction than lower-order squeezing. The occurrence of amplitude squeezing effects of the radiation field in the fundamental mode is investigated in both the processes. The present work shows that squeezing is greater in CAHRS than the corresponding squeezing in CARS. It is also shown that squeezing is greater in stimulated process than corresponding squeezing in spontaneous interaction. The conditions for obtaining maximum and minimum squeezing are obtained. The photon statistics of the pump mode in the processes has also been investigated and found to be sub-poissonian in nature.     

Non-monotonic driven vortex noise in HTSC

We present noise measurements on YBCO thin films in different conditions of magnetic field and driving current. Noise spectra for non-driven and driven cases (in the flux-creep region) evidence deep differences in vortex dynamics between these two regimes. For the driven case, the effect of applying magnetic field is a reduction in noise, which can be explained by the increase in the fraction of vortices that undergo flux-flow. For the non-driven case, magnetic field has no significative influence on noise, probably due to the absence of Lorentz force that causes coherent movement of vortices. For all magnetic fields studied in this work (0-154 mT) the effect of increasing current is an increase of noise, which is in contrast to the results from other authors. This behavior can be explained by an increase of current induced vortex-antivortex annihilation events. We propose that driven noise has a non-monotonic behavior due to the competition between annihilation events and driving force which causes opposite effects on noise.