Effective refractive index for determining ray propagation in an absorbing dielectric particle

The ray-tracing technique can be employed to simulate the scattering of light by a dielectric particle whose characteristic dimension is much larger than the incident wavelength. When a scattering particle is absorptive, a localized electromagnetic wave refracted into the scatterer is inhomogeneous, which requires the use of an effective refractive index to determine the propagation direction of the refracted ray. The effective refractive index for the first-order reflection–refraction event (i.e., the case for the ray-transmission from air into a particle) has been previously derived by the authors. In this study, we further develop recurrence formulae for the effective refractive indices associated with higher-order reflection–refraction events when the ray-transmission is from a particle to air. It is shown from the new formulae that effective refractive indices in this case depend upon ray history. Numerical results indicate that the real and imaginary parts of the effective refractive index are larger and smaller, respectively, than the real and imaginary parts of the inherent complex refractive index of an absorbing particle. Furthermore, if the particle faces associated with two sequential internal reflections are parallel to each other, the corresponding effective refractive indices are the same.     

Effect of absorption on light scattering by agglomerated debris particles

We study the influence of material absorption on light scattering by agglomerated debris particles whose sizes are comparable with the wavelength. We find that the angular profile of linear polarization is extremely sensitive to the imaginary part of refractive index, and there are some unique features that may assist in the retrieval of physical properties of particles using remote-sensing techniques. Most notably, the position of the positive polarization maximum α_max changes monotonically with the imaginary part of refractive index, allowing it to be used to characterize this property. In addition, the amplitude of the negative polarization branch (NPB) is significantly greater for dielectric particles than for non-dielectric particles. It disappears in the transition region between dielectric and conducting particles before reappearing as the imaginary part of the refractive index is increased further. Further increasing the imaginary part of the refractive index may see the NPB disappearing and reappearing in quasi-periodic fashion. This recurrent NPB has a much smaller amplitude than that of dielectric particles. This suggests that the cometary circumnuclear haloes, which have significant NPBs, cannot contain significant quantities of absorbing particles. In addition, combined observations suggest that the polarization maximum of circumnuclear haloes are relatively small P_max～12%, and occur at relatively small phase angles α_max～60°, which is also consistent with dielectric particles.     

Accurate prediction of refractive index of inorganic oxides by chemical formula

A method is proposed for estimation of mean refractive index of a complex inorganic oxide whose chemical composition is known. This method is based on using mean refractive indices of simple oxides. Applicability of the method has been tested for a representative set of ∼120 known oxide crystals. It has been shown that mean refractive index can be predicted with possible error of 0-15% depending on the chemical nature of the compound. The majority of oxides show acceptable difference <5% between theoretical and experimental refractive index values and the mean error level for a set of 124 oxide crystals is as low as ∼5%. Refractive indices of a set of simple oxides are estimated by the Lorenz-Lorentz relation. Full table of refractive indices of simple oxides for all elements of Periodic Table is proposed.     

The parameter of the optical indicatrix of guanidinium aluminum-sulfate hexahydrate crystals

The absorption coefficient (220–1200 nm) and refractive index (800–450 nm) spectra of a strongly anisotropic guanidinium aluminum-sulfate hexahydrate (GASH) crystals are measured. It is shown that the optical anisotropy of GASH crystals in the visible spectral region is significant, while the crystal in the polar direction becomes isotropic. The contribution of IR oscillators to the electronic polarizability of GASH is sharply anisotropic. It is found that the constants of the Sellmeier dispersion formula qualitatively agree with the vacuum ultraviolet spectra calculated from first principles. The experimental refractive indices coincide with the calculated values with an accuracy of 0.10 (n _e ) and 0.1% (n _o ). It is proposed to use the GASH crystal for development of optical phase difference compensators.     

介质中含孤立发光中心的纳米粒子的辐射寿命概率分布

含发光中心的纳米粒子在介质中随机分布,造成局域介质相对折射率的随机分布,导致了纳米粒子辐射寿命的涨落.分析纳米粒子数量在线度为发射光波长量级的体积内的概率密度,由此得出局域介质相对折射率的概率密度.通过纳米粒子辐射寿命对折射率的依赖关系,得到纳米粒子辐射寿命概率密度的解析表达.在此基础上,分析了Meltzer等人的实验中,含Eu³⁺离子的Y₂O₃粒子在甲醇溶液中辐射寿命的相对偏移量的涨落,并得出辐射寿命的不确定度(均方差)与纳米粒子体积、体积密度等物理量的关系.     

吸收介质膜/金属空芯光纤的太赫兹波传输特性

介质/金属结构空芯光纤是一种有发展前景的太赫兹波传输媒质.介质膜在有效增加内面反射率从而降低传输损耗的同时,其材料吸收会引起附加损耗.讨论了介质材料吸收对太赫兹空芯光纤结构参数的影响.计算结果表明,相比于无吸收的理想介质,吸收介质的最优膜厚变小.最优折射率变大.综合考虑了光纤内直径、介质膜折射率和传输波长等因素,分析了介质膜的材料吸收容限.分析结果表明,吸收容限随光纤内直径减小或传输波长增大而减小.当光纤内直径很小或传输波长很大时,吸收容限可能不存在.分析结果对介质/金属太赫兹空芯光纤的设计和材料选择具有重要参考价值.     

含负折射率材料的一维光子晶体的光学传输特性

采用光学传输矩阵方法,模拟研究了由正折射率材料和负折射率材料交替组成的一维光子晶体的光学传输特性.计算了这种含负折射率材料的一维光子晶体的透射谱和色散关系.结果表明,在正入射时,含负折射率材料的光子晶体的带隙要比传统的光子晶体要大得多,并具有狭窄的透射带,从光学薄膜理论的色散关系出发解释了形成上述现象的原因.讨论了在不同的偏振模式下,光以中心波长入射时,反射率随着入射角度的变化关系.发现含负折射率材料的一维光子晶体具有更好的角度特性,可以用来实现对中心波长的全方位反射.     

一种基于光子晶体异质结构的新型多通道波分复用器

在二维三角光子晶体环形腔的周围增加六个散射介质柱,构成一个新的环形腔结构,该结构使光波的透射率达到90%,带宽也比较小。通过改变光子晶体介质柱的折射率,使环形腔的选择波长不断改变,能够明显地区分出两个不同波长,且分波波长在通信波长范围之内。将不同折射材料的光子晶体连接在一起,构成一种新的光子晶体波分复用器,相比同种材料,它具有高效率,多波长选择的优点。利用这种异质结构可以构建一个多波长的波分复用结构,它也为制作多通道波分复用器奠定了基础。     

Propagation and the kurtosis parameter of Gaussian flat-topped beams in uniaxial crystals orthogonal to the optical axis

The analytical formulae for the Gaussian flat-topped beams propagating in uniaxial crystals orthogonal to the optical axis are derived. The numerical results show that the Gaussian flat-topped beams spread at different rates in the directions parallel and orthogonal to the optical axis due to anisotropic crystals. An analytical expression for the kurtosis parameter of the Gaussian flat-topped beams propagating in uniaxial crystals is derived and illustrated with numerical examples. It is shown that the evolution of the kurtosis parameters K_x and K_y depend on the ratio of extraordinary to ordinary refractive indices.     

Precise measurement of group refractive indices and temperature dependence of refractive index for Nd-doped yttrium orthovanadate by intracavity spontaneous mode locking

We report on a novel method based on intracavity spontaneous mode locking to precisely measure the group refractive indices and temperature dependence of refractive index of Nd:YVO(4) crystal at the wavelength of 1064 nm. All the experimental results are found to agree very well with the most recent measured values. We also confirm that the developed method is applicable to measuring the group refractive indices and the temperature dependence of the refractive indices of other vanadate crystals, as well as nonlinear crystals.     

Diffractional Nonreciprocity at Electromagnetic Wave Diffraction on a Slit

The diffraction on a slit in a metal screen located between two media with different refractive indices was experimentally investigated. It was shown that being a function of the wavelength the intensity of diffracting wave depends on the refractive index of medium, which the electromagnetic wave penetrates.     

Quasi-Rayleigh light scattering by internal elements of a metamaterial in a light-carrying core of a single-mode optical fiber based on the Veselago effect

At present, single-mode optical fibers composed of metamaterials—so-called “left-handed” optical media—for the far- and mid-IR ranges have already been created. In the near future, left-handed singlemode optical fibers for the visible and near-IR ranges will be created, light-carrying cores of which will be composed by an ordered structure of dielectric elements, the dimensions of which will be much smaller than the light wavelength, while the effective refractive index of the structure will be negative; i.e., the structure will possess the so-called “Veselago effect.” We show that, because the dimensions of these dielectric elements many times exceed the dimensions of molecules of optical media, the elements should strongly scatter light, with this scattering considerably exceeding the Rayleigh (molecular) light scattering that occurs in conventional quartz single-mode optical fibers. We propose to term this phenomenon the quasi-Rayleigh light scattering. Numerical estimates of the quasi-Rayleigh light scattering for left-handed single-mode optical fibers at a light wavelength of λ = 1.55 μm have been made.     

Methods for investigating thin dielectric films in the millimeter range

An original method based on determining the characteristics of open Fabry-Perot resonator modes with different polarizations is proposed for measuring parameters of dielectric plates and films with a thickness smaller than λ/2 in the millimeter and submillimeter wavelength ranges. This method is used for determining the refractive index and tanδ, as well as the thickness of films made of isotropic materials. For anisotropic films of known thickness, the method makes it possible to measure the permittivity tensor components. Popular film materials such as Teflon (polytetrafluoroethylene, PTFE), lavsan (Mylar, polyethyleneterephthalate, PETP), and polyamide with a minimal thickness of ∼5 μm are investigated. Appreciable anisotropy of roll film materials and the dependence of the dielectric properties on the thickness, which is associated with manufacturing technology, are revealed. The dependence of the refractive index and tanδ on the air humidity is investigated     

Theory of chirped photonic crystals

In chirped photonic crystals, the structural parameters describing a unit cell are progressively varied from a unit cell to the nearest ones. Geometric and dielectric response functions can be affected by this modulation, but here we only investigate the effect of a long-range, slowly varying, modulation of the refractive index. The Bloch modes are modified by essentially being modulated by an envelope function which adapts to the long-range dielectric function perturbation. It is shown that this envelope function obeys a simple linear Schrödinger equation of classical (non-quantum) origin. Close to a band extremum, at a gap edge, the envelope functions can be interpreted as wave functions of particles, called “energy carriers”. These carriers have a mass and come as two species, referred to as “effective photons” (for positive band curvatures) or “photonic holes” (for negative band curvatures). The energy transfer through the chirped structure can be viewed as resulting from the migration of these particles under forces implied by the long-range dielectric function modulation.     

Polarisation properties of comet NEAT C/2001 Q4

Comets exhibit high (up to 25 %) amount of optical polarization when they are observed through ground based or space telescopes. These polarizations are caused due to the scattering of cometary dust. The observed linear polarisation of comets is generally a function of the wavelength of incident light (λ), the scattering angle (θ), the geometrical shape and size of the particle and the composition of dust particles in terms of the complex values of the refractive index. The scattering properties of cometary dust will help to know the nature of cometary dust. In the present work, the observed linear polarization data of Comet NEAT are studied through simulations using Ballistic Particle-Cluster Aggregate (BPCA) and Ballistic Cluster-Cluster Aggregate (BCCA). Using Superposition T-matrix code, the best-fitting values of complex refractive indices are calculated which can well fit the observed polarization data of Comet NEAT C/2001 Q4. The best fitting values of complex refractive indices coming out from the present analysis correspond to mixture of both silicates and organics.     

Improvement of absolute band gaps in 2D photonic crystals by anisotropy in dielectricity

Two-dimensional (2D) photonic band gaps (PBG) structure fabricated from anisotropic dielectric is studied by solving Maxwell's equations with use of plane-wave expansion method. Numerical simulations show that absolute photonic band gaps can be substantially improved in two dimensional square and triangular lattices of cylinders by introducing anisotropy in material dielectricity. Owing to different refractive indices for electromagnetic waves with E- and H-polarization, the quasi-independent adjustment of band gaps for the E- and H-polarization modes can be implemented by uniaxial crystals with their extraordinary axis parallel to the cylinders. Large absolute band gaps can be created for uniaxial cylinders in air with a positive anisotropy. In the case of air holes in background uniaxial dielectric with even a weak negative anisotropy, the absolute band gap can be increased 2-3 times. Large absolute band gap can also be obtained in other complex configurations of uniaxial and biaxial materials and this enables a full exploitation of potential utilization for anisotropic materials available in nature. Such a mechanism of band gap adjustment should open up a new scope for designing band gaps in 2D PBG structures. Received 26 January 1999     

Brillouin scattering measurement for refractive indices of anisotropic crystals

An experimental method is proposed for measuring the refractive indices of anisotropic crystals. The results of the first measurements are in good agreement with those of other methods.     

基于光纤M-Z干涉的高灵敏度液体折射率传感器的实验研究

提出并制备了一种基于Mach-Zehnder干涉效应的高灵敏度光纤液体折射率传感器.分别利用NaCl溶液和甘油溶液,研究了传感器的透射光谱和外界介质折射率的关系.实验结果表明,随着周围介质折射率的增大,传感器干涉谱的极小值点对应的波长向长波方向漂移,在1.333~1.356的折射率变化范围内,极小值点对应的波长的漂移量和折射率的变化具有较好的线性关系,对应的灵敏度约为4 086 nm/refractive-index.该传感器制作简单、结构紧凑,在生物和化学测量中具有较好的应用前景.     

Specific features of waves in magnetic crystals

On the basis of the asymmetric velocity tensor introduced we develop an orthogonal representation of wave propagation in anisotropic magnetic dielectrics. Simple expressions are obtained for the direct dependence of polarization vectors, refractive indices, and phase and group velocities of isonormal waves in a magnetic dielectric on the vectors of principal velocities. We show that the presence of magnetic properties is manifested in the optical range in the fact that the velocities of wave propagation in uniaxial crystals are determined by three main refractive indices, and therefore, both waves except for specific directions become extraordinary, whereas in biaxial crystals (also because of a large number of parameters) the cones of internal and external refraction become elliptic. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 5, pp. 597–600, September–October, 1999     

Wave refraction and backward magnon–plasmon polaritons in left-handed antiferromagnet/semiconductor superlattices

Characteristics of the bulk electromagnetic waves in teraHertz frequency region are examined in a left-handed superlattice (SL) which consists of alternating layers of nonmagnetic semiconductor and nonconducting antiferromagnetic materials. General problem on the sign of the refractive index for anisotropic media is considered. It is shown that the phase refraction index is always positive while the group refractive index can be negative when some general conditions are fulfilled. Effective permittivity and permeability tensors of the SL are derived for perpendicular and parallel orientation of the magnetic anisotropy axis with respect to the plane of the layers. Problem of anomalous refraction for transverse electric and transverse magnetic-type polarized waves is examined in such media. Analytical expressions for both the phase and group refractive indices are obtained for various propagated modes. It is shown that, in general, three different types of the refracted waves with different relative orientation of the phase and group velocity vectors are possible in left-handed media. Unusual peculiarities of the backward modes corresponding to the coupled magnon–plasmon polaritons are considered. It is shown, in particular, that the number of the backward modes depends on the free charge carrier's density in semiconductor layers, variation of which allows to create different frequency regions for the wave propagation.