Beaming effect in multimode photonic crystal by using coupled waveguides

Beaming effect of a multimode photonic crystal (PC) covered by a waveguide array on the exit plane is investigated theoretically. The simulation results show that the multimode PC can make the incident light split into two beams, which can be regarded as secondary sources radiating light into the waveguide array. As a result, many light beams can be generated in the array by the coupling among the waveguides, and the interference of these light beams after passing the system leads to directional emission. Additionally, the directional emission is greatly affected by the beam distribution on the exit plane of the system. Once the main light beams are formed on the exit plane of the system by modulating the system size, steady beaming effect can be obtained in the horizontal direction.     

脉冲式可控偏振态变换器及其应用

本文提出一种基于晶体双折射效应的脉冲式可控偏振态变换器,可以实现偏振光六个偏振分量的脉冲变换.将它应用于偏振型光传感信号的单光路分时检测,可以提高信号检测灵敏度和传感信号的抗干扰能力.文中给出了利用铌酸锂晶体和脉冲电场对光传感信号的X、Y线性偏振分量进行脉冲变换的实验结果.     

Reduction of light signal pulse distortion in cascaded sum-frequency-generation and difference-frequency-generation wavelength conversion

Cascaded sum-frequency-generation (SFG) and difference-frequency-generation (DFG) can implement a wavelength conversion between arbitrary combinations of input and output signal wavelengths. By using a tunable wavelength pump light, the output wavelength can be tuned to a desired wavelength. As in many wavelength conversion devices using the nonlinear optical effect, the group velocity difference between light pulses with different wavelength causes a walk-off effect deforming the output pulse shape. Thus the device length should be kept short to avoid the walk-off effect resulting in limited conversion efficiency. In this report, we propose a method for a quasi-phase matched (QPM) device to maintain the pulse shape of the SFG light pulse along the propagation distance. The output DFG light pulse deformation is suppressed and the conversion efficiency can be increased by extending the device length.     

Longitudinal conductivity in ABC-stacked trilayer graphene under irradiating of linearly polarized light

We study the effect of linearly polarized light on the band structure and longitudinal conductivity in ABC-stacked trilayer graphene. The linearly polarized light can induce a pair of additional points in ABC-stacked trilayer graphene, where conduct and valence bands touch. The locations of these points are determined by the amplitude of the light. Furthermore, the layer pseudospin polarization can be controlled by the light. When the Fermi energy locates at Dirac points, i.e., E_f=0, the longitudinal conductivity shows resonance phenomena when the light is present. Away from the Dirac points, the longitudinal conductivity is unchanged as varying E_f for weak light field at larger Fermi energy, and the amplitude of longitudinal conductivity can be controlled by tuning the light field amplitude. Moreover, the effect of linearly polarized light on resonance phenomena in k-cubic Rashba-Dresselhaus system under the irradiating of linearly polarized light is discussed.     

光自旋霍尔效应中的交叉偏振特性研究

从光束角谱理论出发建立了描述光自旋霍尔效应的傍轴传输模型, 利用这一模型分析了光自旋霍尔效应中的交叉偏振特性. 通过分析交叉偏振效应强度和入射角变化的规律, 发现交叉偏振效应越强, 光自旋霍尔效应中的自旋分裂越大. 为便于实验观察, 将入射角选在光自旋霍尔效应较强的布儒斯特角附近, 观测到了强的交叉偏振效应. 增大交叉偏振分量的同时减小初始偏振分量, 可显著增强光自旋霍尔效应. 这一调控方法为研制基于光自旋霍尔效应的新型光子器件提供了理论基础.     

增益对一维周期结构慢光传输特性影响

本文以一维周期结构为例,对增益与慢光之间的影响做了更进一步的深入研究. 研究发现,增益对有限长和无限长一维周期结构中的慢光效应具有不同的影响. 对无限长结构,增益的加入会减弱结构的慢光效应. 而对于实际应用中存在的有限长周期结构,增益补偿在一定情况下会增强其慢光效应. 文章的研究对促进周期结构作为慢光器件尤其是作为可调谐慢光器件在高密度光子集成中的广泛应用具有积极意义. 关键词： 增益 慢光效应 一维周期结构 有限长     

Slow light and slow current

It is shown that the effect of hole-burning under conditions of coherent population oscillations as well as the light pulse delay in a saturable absorber (a modification of the so-called slow light effect) can be interpreted, in a comprehensive way, in terms of the intensity spectrum of the light and intensity-related susceptibility of the medium. The physical content of these effects is illustrated by a simple electric circuit with a non-linear resistor which realizes a full analog of the saturable absorber. In this case the effect of hole-burning in the absorption spectrum of the medium is converted into the effect of hole-burning in the frequency dependence of resistance of the resistor and the effect of slow light into the effect of slow current.     

灵敏电流计实验中的温差电效应

由于光点式灵敏电流计内部存在照明光源,系统很鸡避免温差,产生温差电流,从而引起光标附加偏转,本文以测量灵敏电流计常量和内阻为例,表明了温差电动势的存在,介绍了消除其影响的方法。     

A new approach for noninvasive skin blood imaging in microcirculation

A new noninvasive method is proposed for optically imaging blood under skin. A mini-mirror is used to guide the light incident onto the skin surface, while blocking specularly reflected light from the tissue. Moreover, by using two linear polarizers at right angles, the specular reflection can be reduced further, while also blocking the rough surface scattered light. A circular polarizer may be used to obtain the same effect. As a result, the blood flow beneath the surface of skin can be visualized clearly. The effect of the mini-mirror is also analyzed theoretically and experimentally.     

Space-variant geometrical phases in focused cylindrical light beams

We show that the depolarization caused when light is focused with a high-numerical-perture lens is accompanied by a space-variant geometrical phase. This phase results in the formation of modes with helicities and phase singularities that differ from those of the original beam. We show that this effect can be explained as a transverse shift of the rays, which is reminiscent of the recently discovered optical Hall-Magnus effect. Our results show that the asymmetric focal spot associated with the focus of linearly polarized light can be explained through geometrical effects.     

A scheme for Sagnac-effect quantum enhancement with Fock state light input

Sagnac effect enhancement can improve optical gyro precision. For a certain input intensity, we suggest that the other input port of beam splitter(BS) should be fed with some quantum light to break through shot noise limit(SNL) to improve Sagnac effect without increasing radiation-pressure noise(NRP). We design a Sagnac effect quantum enhancement criterion(SQEC) to judge whether some quantum light can enhance Sagnac effect and present a Sagnac effect enhancement scheme that utilizing Fock state light and parity measurement technique to extract the output phase. The results of the theoretical analysis show that the maximum sensitivity can be reached at θ = 0, and the phase precision can break through SNL and even achieve Heisenberg limit(HL). When the Fock state average photon number n is far less than coherent state, the minimum measurable angular rate is improved with (2n+1)~(1/2) times, which can deduce shot noise and increase NRP little.     

Phase-coupling-induced ultraslow light propagation in solids at room temperature

We show that the group velocities of light pulses can be decelerated dramatically by the use of a dispersive phase-coupling effect through a wave mixing process. We have observed experimentally such a phase-coupling-induced ultraslow light propagation with a group velocity as low as 0.05 m/s in a photorefractive Bi12SiO20 crystal at room temperature. Moreover, the ultraslow light is amplified in the Bi12SiO20 crystal because of the unidirectional energy transfer from a coupling beam to the ultraslow light. This technique to produce ultraslow light propagation is valid for all nonlinear wave mixing processes with a dispersive phase-coupling effect.     

超构材料中的光学量子自旋霍尔效应

电子的量子自旋霍尔效应的发现推进了当今凝聚态物理学的发展,它是一种电子自旋依赖的具有量子行为的输运效应.近年来,大量的理论和实验研究表明,描述电磁波场运动规律的麦克斯韦方程组内禀了光的量子自旋霍尔效应,存在于界面的倏逝波表现出强烈的自旋与动量关联性.得益于新兴的光学材料:超构材料(metamaterials)的发展,不仅能够任意设定光学参数,同时也能引入很多复杂的自旋-轨道耦合机理,让我们能够更加清晰地了解和验证其中的物理机理.本文对超构材料中量子自旋霍尔效应做了简要的介绍,内容主要包括真空中光的量子自旋霍尔效应的物理本质、电单负和磁单负超构材料能带反转导致的不同拓扑相的界面态、拓扑电路系统中光量子自旋霍尔效应等.     

利用保角变换实现环形光栅的Talbot效应

Talbot效应是一种近场自成像效应,通常只有周期光栅可以产生Talbot效应,而环形光栅无法产生.本文通过引入保角变换,发现可以在环形光栅外部设计适当的折射率渐变层介质,使得其中也能够产生严格的Talbot效应,并计算了对应的自成像半径表达式.本文利用FDTD软件分别将一个环形光栅放置在真空中以及人工设计的折射率渐变层中进行了模拟,并对二者的结果进行了比较分析,发现这种折射率渐变层介质确实对点光源入射的环形光栅的自成像情况有着很好的改善.希望这一工作能够推广Talbot效应的应用范围.     

Slow light in degenerate fermi gases

We investigate the effect of slow light propagating in a degenerate atomic Fermi gas. In particular we use slow light with an orbital angular momentum. We present a microscopic theory for the interplay between light and matter and show how the slow light can provide an effective magnetic field acting on the electrically neutral fermions, a direct analogy of the free electron gas in an uniform magnetic field. As an example we illustrate how the corresponding de Haas-van Alphen effect can be seen in a gas of neutral atomic fermions.     

Dust eruptions by photophoresis and solid state greenhouse effects

We carried out experiments that show a gas pressure dependent ability of light to eject particles from a dust bed. Dust eruptions also occur upon removal of the light source. This can be attributed to a solid state greenhouse effect and photophoretic forces. This ejection mechanism works at light intensities larger than 6 kW/m2 but in extreme cases might work as low as 1 kW/m2. It can be applied to sunlit dust on Mars where it aids or triggers dust lift-off from the surface into the atmosphere. It is of importance for dusty bodies at the inner edge of protoplanetary disks where it leads to light induced erosion. The effect also offers a base for technical applications of dust removal in low pressure environments.     

On the analogy between a single atom and an optical resonator

A single atom in free space can have a strong influence on a light beam and a single photon can have a strong effect on a single atom in free space. Regarding this interaction, two conceptually different questions can be asked: can a single atom fully absorb a single photon and can a single atom fully reflect a light beam. The conditions for achieving the full effect in either case are different. Here we discuss related questions in the context of an optical resonator. When shaping a laser pulse properly it will be fully absorbed by an optical resonator, i.e., no light will be reflected and all the pulse energy will accumulate inside the resonator before it starts leaking out. We show in detail that in this case the temporal pulse shape has to match the time-reversed pulse obtained by the cavity’s free decay. On the other hand a resonator, made of highly reflecting mirrors which normally reflect a large portion of any incident light, may fully transmit the light, as long as the light is narrow band and resonant with the cavity. The analogy is the single atom—normally letting most of the light pass—which under special conditions may fully reflect the incident light beam. Using this analogy we are able to study the effects of practical experimental limitations in the atom-photon coupling, such as finite pulses, bandwidths, and solid angle coverage, and to use the optical resonator as a test bed for the implementation of the quantum experiment.     

Modeling of light coupling effect using tunneling theory based on particle properties of light

Based on the wave-particle duality of light, the Schrödinger equation for a photon as a particle is established to treat the light coupling effect by introducing concepts of the virtual mass and potential for a photon, which is different from the previous method that uses the analogy with quantum mechanics. The virtual mass is physically correlated to the kinetic energy according to Einstein’s energy–momentum relation. As a consequence, this new model has merits of physical simplicity and analytic nature. This new model can well explain the exponential dependence of the light coupling effect on the physical parameters in coupled waveguides on, which can be observed in the experimental and simulation data reported in the literature. Moreover, an explicit expression for the coupling length (coefficient) on the effects of physical parameters can be obtained by virtue of this new model, whereas the previous modal approach and the coupled-wave model resulted in implicit expressions. This new model does not only give a better physical understanding but also offers a possibility to design and fabricate optic devices based on the light coupling effect by optimizing the physical parameters.     

基于光场重构的空间三维显示技术

真实空间三维显示可以通过精确再现三维物体的空间光场分布来实现.依据三维景物空间光场分布特性,提出并介绍基于光场重构的真实空间三维场景显示的基本原理与方法.实验表明,利用光场重构原理,可以运用现有的光学空间光调制器构造出比全息再现更为优越的真实三维显示.     

Optical effects in a rotating frame of reference

The laws governing the propagation of light waves in a rotating frame of reference are investigated. It is shown that a curvature of light beams is possible in a noninertial frame of reference. This effect can be observed experimentally with the use of modern laser techniques.