Spectral invariance and non-invariance of light generated by partially coherent sources

A basic formula is derived for the spectrum of the field generated by a planar source of any state of spatial coherence. The formula shows clearly the contribution due to diffraction and due to source correlations. The distinction between these two contributions has not been appreciated in some previous publications. Examples are presented illustrating the effect of source correlations on the spectrum of the far field. A brief preliminary account is also presented of a recent investigation by H. Chen and E. Wolf which has shown that it is possible to produce a source whose degree of coherence will act on the spectrum of the source as a low-pass filter with a sharp frequency cut-off.Dedicated to H. Walther, with best wishes, on the occasion of his 60th birthday     

Size and Shape Effect of Gold Nanoparticles in “Far‐Field” Surface Plasmon Resonance

The “far‐field” surface plasmon resonance (FSPR) of metal nanoparticles, which have built a facile way to emission enhancement of red, green, blue, and white with nice reproducibility, has big potential application in solution‐processed organic light‐emitting diodes (OLEDs). According to the theory of the “far‐field” effect, the reflectivity of the metal surface and the phase shift at the reflection play an important role in enhancing ratio, which strongly relate to the size and shape of nanoparticles. In this work, gold nanospheres with different sizes and nanorods are synthesized in order to determine the size and shape effect of FSPR. The results demonstrate that the one with higher reflectivity in a certain range induces a better emission enhancement in the luminous efficiency and the maximum brightness. The nanoparticles with bigger sizes and shape of rods have higher reflectivity, which is consistent with the simulation based on FSPR effect. The phase shifts of different nanoparticles are optimized by the distance between gold nanoparticles and emitters. The metal NPs with a high reflectivity and the applicable phase shift will have big potential for the emission enhancement in OLEDs.     

用于制作光纤光栅的Talbot干涉仪的分析与建模

对用于制作光纤光栅的Talbot干涉仪进行了分析.当考虑准分子激光器的空间相干性和时间相干性时,几何光学中的光线法并不能给出Talbot干涉仪干涉场的真实强度分布情况,而通过光的电磁理论,则可以提供对干涉场的详细描述.基于电磁理论,建立了一个Talbot干涉仪干涉场计算模型,并首次将Talbot干涉仪的反射镜转角作为模型计算参数加以考虑,随后进行了一系列的计算.计算结果显示了干涉场的分布,同时表明空间相干性对Talbot干涉仪的近场干涉和远场干涉影响都很大,其中远场干涉对空间相干性的要求更严格,且对于远场干涉,反射镜转角的影响不可忽视.     

基于单发光区芯片的大功率光纤耦合激光器的输出远场特征分析

为研究光纤耦合激光器的输出远场特征,基于ZEMAX光学设计软件,模拟了基于单发光区激光器芯片的多种光纤耦合结构,分析了不同耦合结构的输出远场特征。模拟结果表明:单管耦合输出远场分布通常为中间亮、边缘暗的圆形光斑。当准直后的光束快慢轴光束尺寸基本一致时,远场输出光斑均匀性会得到极大改善;当存在光纤轴心角向误差(大于1°)时,远场输出光斑的均匀性会明显降低。多单管耦合时,单管之间的台阶高度若大于准直后的快轴光斑尺寸,则对应的远场输出为有暗区的同心圆环,单管的数量对应圆环的数量。为了提高输出远场分布的均匀度,应严格控制合束单管之间的台阶高度。     

On the interaction of a sound pulse with the shear layer of an axisymmetric jet,II: Heated jets

The fluctuating field of a heated jet excited by a sound pulse is simulated numerically. The fluctuations in both the flow field and the far field are studied. The flow field results depend crucially on the stability properties of the heated jet. The altered stability properties due to the heating cause a reduction in the overall fluctuations and a shift of the instability waves into lower frequencies. These changes cause a similar downward shift in the far field spectrum and a reduction in the total far field noise. The results provide a partial explanation in terms of stability theory of experimentally observed properties of the noise of heated jets.     

单程拼接光栅压缩池系统中光栅缝隙的衍射效应

利用傅里叶光学方法，建立了一个基于远场的高斯光束数学模型.该模型能够分析单程拼接光栅压缩池系统中拼接光栅的缝隙对远场时空特性的影响.数值模拟得到了不同系统参数时远场时空特性关于缝隙宽度的变化曲线.揭示了光栅缝隙对远场的影响规律：对于双光栅拼接的压缩池系统，若只是压缩池中第二块光栅为拼接光栅，则光栅缝隙导致远场时域略微变窄；若压缩池中的两块光栅都是拼接光栅，则缝隙导致远场时域展宽或者变窄，依赖于入射脉冲的傅里叶变换极限型脉宽大小；光栅缝隙对于远场强度分布的影响是导致中央主峰能量下降，并且出现两个子峰，子峰的峰值随缝宽的增大而增大.     

Vectorial structure of the far field of an elegant Hermite–Gaussian beam

Based on the vector angular spectrum method and the method of stationary phase, an analytical expression for the vectorial structure of the far field of an elegant Hermite–Gaussian beam is derived. The analytical formulae of the energy flux distributions of the TE term, the TM term, and the whole beam are presented in the far field. Analytical expressions for the ratios of the powers of the TE and TM terms to those of the elegant Hermite–Gaussian beam are obtained without any approximation. The physical pictures of the far field of an elegant Hermite–Gaussian beam are demonstrated and compared with those of the far field of the corresponding standard Hermite–Gaussian beam. This research reveals the internal vectorial structure of the far field of an elegant Hermite–Gaussian beam from an alternative viewpoint.     

Analytical vectorial structure of an apertured hollow Gaussian beam in the far field

Based on the vector angular-spectrum and the complex Gaussian expansion of the aperture function, the structure of an apertured hollow Gaussian beam in the far field is given in a integral form. In virtue of the method of stationary phase approximation, the analytical vectorial structures are derived. Starting from the analytical expressions, the propagation properties of apertured hollow Gaussian beams with different order in the far field are illustrated graphically. In addition, the influence of the truncation parameter on far field distribution is studied detailedly. This research can shed light on the further understanding of the vectorial property of an apertured hollow Gaussian beam.     

Analytical structure of an apertured vector Gaussian beam in the far field

Based on the angular spectrum representation of the Maxwell’s equations and the complex Gaussian expansion of the aperture function, the structure of an apertured vector Gaussian beam in the far field is presented in the integral form. By means of the method of stationary phase, the analytical vectorial structures are obtained. According to the analytical expressions, the characteristics of vectorial structure of an apertured Gaussian beam are investigated in the far field. The influence of a linearly polarized angle on the vectorial structure is also studied in the far field. This research provides a novel approach to further comprehend the vectorial property of an apertured Gaussian beam.     

Nonaxial generation in cylindrical specimens of neodymium glass

Nonaxial generation in cylindrical neodymium glass specimens with polished sides was studied. The true ray path within the specimen was determined from the distribution of radiated energy in the near and far zones. The time correlation of emission at various points in the far zone was investigated. It is shown that in the case of a cylindrical specimen, nonaxial generation is associated with closed linear ray paths within the specimen.The authors are grateful to P. A. Bazhulin for his unflagging interest in the present study, and to S. G. Rautian for his useful comments.     

Simulation of circular array ultrasound transducers for intravascular applications

The beam shape of a circular array transducer that is commonly used in intravascular ultrasound catheters was investigated in linear mode of operation. For this use, a simulation program which can simulate the radio frequency (rf)-response of a number of scatterers has been developed. The program is based on the impulse response method, which is implemented in the frequency domain. Due to the unusual geometry of the transducer, the far field gets peculiarly shaped for large apertures. Instead of having a far field with its maximum intensity in a single lobe on the acoustical axis, the far field splits up into a dual-lobe far field with maximum intensity in two lobes off the acoustical axis. A formula is derived that predicts the occurrence of these beam shapes.     

STUDY ON THE RADIATION ACOUSTIC FIELD OF RECTANGULAR RADIATORS IN FLEXURAL VIBRATION

In this paper, the radiation acoustic field of a rectangular radiator in flexural vibration is studied. The radiator is a rectangular thin plate in flexural vibration with simply supported boundary conditions. Based on the theory of Rayleigh integral, the acoustic pressure distribution of the radiator in its far field is obtained analytically and the near acoustic field of the radiator is also computed numerically. In the near field, the three-dimensional acoustic pressure distribution is obtained. In the far field, the two- and three-dimensional acoustic pressure distribution patterns are calculated. The dependence of the radiation acoustic pressure of the radiator on the vibrational order of the plate in flexural vibration is analyzed. It is demonstrated that the radiation acoustic pressure along the central axis of the radiator is always zero in the near and far acoustic fields, and the radiation near acoustic field and the far acoustic field are both directional. This is different from the acoustic field generated by an oscillating piston. On the other hand, when the vibrational order is increased, the acoustic pressure distribution of the radiator becomes complex.     

用单晶硅在Nd：YAG激光器中实现被动锁模

在Ｎｄ：ＹＡＧ激光器中，用单晶硅实现了被动锁模，得到了脉冲定义为２８ｐｓ左右，能量为约３μＪ的锁模脉冲，并对锁模机理作了初步解释。     

Propagation of nonparaxial vector hollow Gaussian beams through a circular aperture

Based on the vectorial Rayleith-Sommerfeld formulae, the nonparaxial propagation properties of the vector hollow Gaussian beams (HGBs) through a circular aperture are studied in detail. We describe the derivation of the integral expressions of the propagation of nonparaxial vector HGBs through a circular aperture. The derived expression is independent the approximation of paraxial and far field, which are valid for either far and near field and for the systems in which aperture radius is comparable to or even smaller than wavelength. And it is also strict integral formula for the light field on the axis. Numerical calculation results indicate that there is no difference between derived formulae and the Collins formulae in the situation of paraxial approximation. Using the formula deduced, we calculate the propagation properties of HGBs. The calculated results indicate that the propagation field of vector hollow Gaussian beams is asymmetric in near field, while the propagation field is symmetric in far field. These research results could well shed light on the further understanding of the vectorial property of HGBs through a circular aperture, and would play a guiding role in the practical application of HGBs.     

光子晶体发光二极管光抽取效率及远场辐射特性的数值研究

运用三维时域有限差分法,研究了完美/缺陷光子晶体特定参量的改变对发光二极管光抽取效率影响,得出优化参量.基于近场远场转换,进一步分析了两种不同类型的缺陷引入及其周边空气孔半径的改变对光子晶体发光二极管远场辐射特性的影响.数值研究的结果表明,通过引入缺陷以及减小缺陷周围空气孔半径能够同时提高光子晶体发光二极管的光抽取效率和远场辐射方向性.     

Computational simulation of wave propagation problems in infinite domains

This paper deals with the computational simulation of both scalar wave and vector wave propagation problems in infinite domains. Due to its advantages in simulating complicated geometry and complex material properties, the finite element method is used to simulate the near field of a wave propagation problem involving an infinite domain. To avoid wave reflection and refraction at the common boundary between the near field and the far field of an infinite domain, we have to use some special treatments to this boundary. For a wave radiation problem, a wave absorbing boundary can be applied to the common boundary between the near field and the far field of an infinite domain, while for a wave scattering problem, the dynamic infinite element can be used to propagate the incident wave from the near field to the far field of the infinite domain. For the sake of illustrating how these two different approaches are used to simulate the effect of the far field, a mathematical expression for a wave absorbing boundary of high-order accuracy is derived from a two-dimensional scalar wave radiation problem in an infinite domain, while the detailed mathematical formulation of the dynamic infinite element is derived from a two-dimensional vector wave scattering problem in an infinite domain. Finally, the coupled method of finite elements and dynamic infinite elements is used to investigate the effects of topographical conditions on the free field motion along the surface of a canyon.     

Contribution of evanescent waves to the far field: the atomic point of view

Evanescent modes of the electromagnetic field are seldom invoked in conventional far-field optics, as their contribution far from the source (a few wavelengths) is negligible. Contradicting this fact, in recent theoretical works, based on a particular decomposition of the free-space Green tensor, it has been asserted that evanescent waves do indeed contribute to the far field, where they appear as an additional ~1/r component of the field. We provide an explicit demonstration that evanescent modes do not contribute to the power radiated to the far field by any dipolar source. First we derive an expression for the free-space field susceptibility in which contributions from evanescent and homogeneous modes are separated, and then we use linear response theory to compute the decay rate for an atomic dipole in vacuum.     

斜向通道雷电电磁场远场近似特性

为了使雷电电磁场的计算更加接近实际情况,并对远场区雷电电磁脉冲场进行模拟,利用偶极子理论对斜向放电通道雷电电磁场解析表达式进行了求解,用解析方法对地表雷电电磁场及其导数在远场区的一阶、二阶近似表达式进行了推导,得到了电磁场及其导数与通道底部电流及其导数的远场近似关系。采用脉冲函数表示通道基电流,对比了精确表达式与近似表达式计算的电磁场及其导数波形。结果表明在远场区,斜向通道雷电电磁场波形与其近似波形的偏差、电磁场导数波形与其近似导数波形之间的偏差均随着距离的增加而减小,且电磁场导数近似公式要比电磁场近似公式适用范围更加广泛。计算结果验证了解析推导近似的可行性。     

斜向通道雷电电磁场远场近似特性

为了使雷电电磁场的计算更加接近实际情况,并对远场区雷电电磁脉冲场进行模拟,利用偶极子理论对斜向放电通道雷电电磁场解析表达式进行了求解,用解析方法对地表雷电电磁场及其导数在远场区的一阶、二阶近似表达式进行了推导,得到了电磁场及其导数与通道底部电流及其导数的远场近似关系。采用脉冲函数表示通道基电流,对比了精确表达式与近似表达式计算的电磁场及其导数波形。结果表明在远场区,斜向通道雷电电磁场波形与其近似波形的偏差、电磁场导数波形与其近似导数波形之间的偏差均随着距离的增加而减小,且电磁场导数近似公式要比电磁场近似公式适用范围更加广泛。计算结果验证了解析推导近似的可行性。