Spectral interferometry and reflectometry used to measure thin films

A new method for a precise measurement of the oscillatory part of phase change on reflection (interferometric phase) from a thin-film structure is presented. The method, which is based on phase retrieval from the spectral interferograms recorded at the output of a slightly dispersive Michelson interferometer, is combined with reflectometry. The interferometric phase of the thin-film structure is measured precisely using a reference sample of known phase change on reflection. The spectral reflectance of the thin-film structure is also measured in the interferometer. The feasibility of the method is confirmed in processing the experimental data for SiO₂ thin film on a silicon wafer of known optical constants. Four samples of the thin film are used and their thicknesses are determined. We confirm very good agreement between the thicknesses obtained from the interferometric phase and reflectance measurements. PACS  07.60.Ly; 68.55.Jk; 78.20.Bh     

Temporal phase retrieval from a complex field in digital holographic interferometry

We describe a novel method of processing complex phasors in digital holographic interferometry (DHI). Unlike the commonly used digital phase subtraction method that operates on the phase itself, the proposed method operates on the complex phasor instead. Two temporal phase retrieval algorithms are developed in which the complex phasor of each pixel is measured and analyzed as a function of time. The developed algorithms are demonstrated in profile measurement of step heights. Experimental results show that the proposed phase retrieval algorithms for DHI perform well compared with conventional methods.     

大气CO2反演的地表反射率影响分析与比值反演方法

近红外波段空基大气CO₂反演是一个病态问题,地表特征的不确定性是其重要的影响因素之一。针对大气CO₂高精度反演的需求,研究了地表反射率两大不确定性因素对大气CO₂反演的影响程度,提出了比值光谱法对这种影响进行校正。比值光谱法中CO₂无吸收通道(off-line)的选择至关重要,通过比较和分析多种off-line通道的选择方法,发现多通道多点平均法因多数据平均可降低随机误差的优势,获得的off-line通道对应的反演精度最高,而且离散程度也小。比值光谱法结合多通道多点平均法选取出的off-line通道对地表反射率的不确定性影响有明显的校正效果,有利于大气CO₂反演精度的提高。     

Resonant Lattice Kerker Effect in Metasurfaces With Electric and Magnetic Optical Responses

To achieve efficient light control at subwavelength dimensions, plasmonic and all‐dielectric nanoparticles have been utilized both as a single element as well as in the arrays. Here we study 2D periodic nanoparticle arrays (metasurfaces) that support lattice resonances near the Rayleigh anomaly due to the electric dipole (ED) and magnetic dipole (MD) resonant coupling between the nanoparticles. Silicon and core‐shell particles are considered. Our investigations are carried out using two independent numerical techniques, namely, the finite‐element method and the method of coupled‐dipole equations based on the Green function approach. We numerically demonstrate that choosing of lattice periods independently in each mutual‐perpendicular direction, it is possible to achieve a full overlap between the ED‐lattice resonance and MD resonances of nanoparticles in certain spectral range and to realize the resonant lattice Kerker effect (resonant suppression of the backward scattering or reflection). At the effect conditions, the strong suppression of light reflectance in the structure is appeared due to destructive interference between electromagnetic waves scattered by ED and MD moments of every nanoparticle in the backward direction with respect to the incident light wave. Influence of the array size on the revealed reflectance and transmittance behavior is discussed. The resonant lattice Kerker effect based on the overlap of both ED and MD lattice resonances is also demonstrated.     

Propagation of p-polarized electromagnetic waves obliquely incident on stratified random media: Random phase approximation

We consider the propagation of p-polarized electromagnetic waves obliquely incident on stratified random dielectric media. Using the invariant imbedding method generalized to random media and applying the random phase approximation, we derive a simple analytical expression of the localization length and calculate the disorder-averaged reflectance and transmittance and the fluctuations of the localization length and the reflectance as functions of the incident angle. We also calculate the disorder-averaged intensity profile of the magnetic field inside the random medium. We find that within the random phase approximation, the p wave can be delocalized and transmitted completely at a certain critical incident angle, which is bigger than the Brewster angle in the uniform case.     

Polarization mapping of nanoparticle plasmonic coupling

We propose the use of polarization mapping as a tool to better separate the effects of plasmonic coupling from the local refractive index for molecular imaging and biosensing using gold nanoparticles. Polarization mapping allows identification of the orthogonal excitation mode when the particle dimer orientation is unknown, as may be the case when using plasmonic nanoparticles for cell labeling. This information can be used to sense relative changes in the dielectric environment, or for absolute dielectric sensing with additional a priori interparticle distance information. First, the theoretical scattering by nanoparticle pairs is modeled under parallel and orthogonal polarization orientations and increasing interparticle separation. Second, polarization mapping of substrate bound nanoparticles using dark-field microspectroscopy is investigated as a method to isolate the individual plasmonic coupling modes associated with a pair of nanoparticles without reorientation of the sample. The results of this study provide useful insight toward potential avenues for monitoring distances using plasmonic nanoparticles and sensing the local refractive index using nanoparticle pairs when the pair orientation is not known, as may be the case when using nanoparticles for cell receptor labeling.     

Analysis of reflectance data using the Kramers-Kronig Relations

Kramers-Kronig Relations (KKR) are a well-known tool to interpret reflectance spectra by reconstructing the reflected wave's phase from its modulus with the help of a dispersion relation. However, a unscrupulous application is only possible in the case of semiinfinite media at perpendicular incidence.Here the method is generalized to oblique incidence and layered structures. We discuss the KKR's theoretical foundations and their significance for the phase retrieval problem. Firstly, convergence problems occur, in particular in the case of Fabry-Perot-interferences of layered samples. They are removed by a special normalization procedure which is discussed in detail for IR spectra. Since the phase-retrieval procedure is based on a transformation of the un-physical quantity ln R, KKR cannot be applied straightforward. Additional terms have to be considered due to zeroes of the sample's reflection coefficient in the physical half plane. These terms are explained. A method is introduced to evaluate them with the help of few additional data. Finally we discuss an alternative procedure allowing the phase retrieval without further data using physical confinements. As examples the methods are applied to reflectance spectra of BaF₂ and NaCl measured to investigate the materials' multiphonon processes.     

利用MODIS卫星资料反演北京地区气溶胶光学厚度

大气气溶胶是影响城市环境空气质量的重要因素,同时对人类健康具有重要影响。传统的气溶胶遥感反演方法多适用于海洋及植被等地表反射率较低的区域,对于城市等高亮地表区域,地表反射率较高且难以确定,气溶胶反演面临巨大挑战。针对该问题,提出一种新的地表反射率的确定方法,将下垫面划分为暗地表和亮地表两种类型,分别使用可见光与短波红外的线性关系和利用长时间序列MODIS表现反射率数据使用最小值合成技术构建先验数据集的方法,确定其地表反射率,然后基于辐射传输方程理论利用查找表方法,进行气溶胶光学厚度反演。选择下垫面复杂、空气污染问题严重的北京市作为研究区,应用MODIS数据进行气溶胶反演实验,最后使用北京站、香河站、北京CAMS站和北京RADI站4个AERONET气溶胶地基观测数据和MODIS气溶胶产品对反演结果进行对比验证。结果表明该算法气溶胶反演结果与地基观测数据具有较高的一致性（R2=0.902）,能以较高精度实现城市等高反射率地区的气溶胶反演,反演精度与空间连续性上较MOD04有显著提高。     

High-frequency properties of oil-phase-synthesized ZnO nanoparticles

Monodispersive ZnO nanoparticles each with a hexagonal wurtzite structure are facilely prepared by the hightemperature organic phase method.The UV-visible absorption peak of ZnO nanoparticles presents an obvious blue-shift from 385 nm of bulk ZnO to 369 nm.Both the real part and the image part of the complex permittivity of ZnO nanoparticles from 0.1 GHz to 10 GHz linearly decrease without obvious resonance peak appearing.The real parts of intrinsic permittivity of ZnO nanoparticles are about 5.7 and 5.0 at 0.1 GHz and 10 GHz respectively,and show an obvious size-dependent behavior.The dielectric loss angle tangent(tan 5) of ZnO nanoparticles with a different weight ratio shows a different decreasing law with the increase of frequency.     

基于GOSAT卫星数据的大气甲烷反演误差分析及校正

大气甲烷(CH₄)高精度反演受到多种因素的影响,其中地表特征和大气状态的不确定性是重要的影响因素,如地表反射率、温度、湿度和压力廓线。地表反射率受到诸多因素的影响,难以获得精确的数据,会给反演结果带来较大误差。温度、湿度和压力廓线的不确定性亦是反演误差的重要来源,由此产生的系统误差难以避免,单独利用CH₄吸收带进行反演难以消除此种误差。针对各种参数不确定性的影响,本文提出比值光谱法和CO₂吸收带校正法进行校正。比值光谱法通过将绝对辐亮度谱转化为比值光谱,抑制地表反射率在反演过程中的作用。CO₂吸收带校正法利用CO₂ 1.61 μm吸收带,将CH₄柱含量转化为CH₄体积混合比,校正温度、湿度和压力廓线不确定性引起的系统误差。通过将两种校正方法结合,可同时抑制地表反射率和温度、湿度、压力廓线不确定性产生的影响,减小反演误差。利用温室气体观测卫星(GOSAT)的观测数据进行大气CH₄反演,采用比值光谱法和CO₂吸收带校正上述误差,结果显示校正后的CH₄体积混合比与GOSAT-Level2产品相当接近,反演精度可达-0.24%,反演结果较为稳健可靠。研究表明,比值光谱法和CO₂吸收带校正法可有效校正地表特征和大气状态参数不确定性引起的误差,提高大气CH₄反演精度。     

Phase retrieval using an aperture-array filter under partially coherent illumination

Recently we have proposed a deterministic phase retrieval method using an aperture-array filter to reconstruct a complex-valued object from a single diffraction intensity pattern. We describe here the effect of quasi-monochromatic partially coherent illumination on the object reconstruction by the phase retrieval method, and then present the method of eliminating the effect of the partially coherent illumination from the diffraction intensity pattern via a simple Fourier deconvolution operation provided that the complex degree of spatial coherence of the illuminating beam is known. The usefulness of this method is shown in computer-simulated examples of the object reconstructions under Gaussian Schell-model partially coherent illumination.     

一种改进的光谱图像反演地表目标遥感反射率的方法

提出一种改进的反演地表目标遥感反射率的方法.利用光谱图像中的太阳耀光估计当时气象条件下的大气透过率,分析了天空辐射和色散现象对反演的影响,据此构建遥感反演模型,计算出地表目标的遥感反射率.由于消除了天空漫射辐射带来的额外影响使得可见光波段的反演准确度大为提高,此外将菲涅尔反射率表示为波长的函数进行计算使反演光谱在整个波段范围内能更好地匹配实验室光谱.实验结果表明,改进算法计算的反射率能更好地匹配实验室光谱,具有更高的反演准确度.     

Polarization imaging in atmospheric environment based on polarized reflectance retrieval

A method for calculating the atmospheric parameters measurement accuracy requirement based on polarized reflectance retrieval is proposed. The at-sensor polarization states with different atmospheric parameters content are simulated based on the atmospheric radiative transfer model in order to select the key parameter affecting the polarization observation. The accuracy requirement of atmospheric parameters is derived through the polarized reflectance retrieval method. Experiment results show that retrieval accuracy of polarized reflectance of typical ground objects can be up to 90%. The atmospheric parameters measurement accuracy requirement when the retrieval accuracy is more than 75% is derived.     

Effect of Cu doping on the static dielectric constant of nanocrystalline ZnO

Cu-doped ZnO nanoparticles were synthesized by a simple rheological phase reaction–precursors method using zinc acetate, cupric acetate and oxalic acid for different atomic percentages of Cu doping. X-ray diffraction studies confirmed the correct phase formation and compositions were obtained by energy dispersive X-ray analysis. Particle sizes were recorded from small angle X-ray scattering studies. The static dielectric constant, calculated from the shift in band-gap energy, showed a gradual decrease with Cu doping. A simple theoretical calculation, based on the Penn model, was performed to explain the observed change in dielectric constant with doping. The model could successfully describe the dependence of the static dielectric constant of nanocrystalline ZnO on doping concentration.     

Iterative retrieval of one-dimensional x ray wave field using a single intensity measurement

The problem of retrieving a complex function from the modulus of its Fourier transform has non-unique solutions in one dimension. Therefore iterative phase retrieval methods cannot in general be confidently applied to one-dimensional problems, due to the presence of ambiguities. We present a method for a posteriori reduction of the ambiguities based on the correlation analysis of the solution of a large number of runs of an iterative phase retrieval algorithm with different random starting phases. The method is applied to experimentally measured diffraction patterns from an x ray waveguide illuminated by hard x rays. We demonstrate the possibility of retrieving the complex wave field at the exit face of the waveguide and compare the result with theoretical prediction.     

Athermal metal-free planar waveguide concave grating demultiplexer

An athermal metal-free planar waveguide concave grating demultiplexer is proposed. We designed the dielectric mirror at the grating facet instead of coating with the metal on the back wall of the grating facet. The transfer-matrix method is introduced to design the dielectric mirror and the reflectance spectral responses of the 2D waveguide structure are simulated using the FDTD software (RSoft, Inc.). To reduce the temperature sensitivity of the device, the three-focal-point method is introduced. We use the design example to show the high-reflectance bandwidth of the dielectric mirror. The fabrication errors are also taken into consideration. By using the numerical model of the scalar diffraction theory, the flat-top spectral responses of the channels are simulated.     

Securing complex and multi-plane data in a lens-less digital holographic information system that uses position-phase-shifting geometry

A digital holographic information system can process complex three-dimensional (3-D) object information. We demonstrate a scheme for securing complex and 3-D information in the context of in-line digital holography. Double random phase encoding in the free-space propagation domain of light is used to secure the complex information. Encrypted in-line digital holograms are recorded using the position-phase-shifting method. The encrypted complex image at the CCD recording plane is retrieved from the real-valued digital holograms, and is used for decryption. The robustness of the method has also been studied for various securing keys used in the method against blind decryption. A layer-by-layer information retrieval from the encrypted digital hologram is also discussed. The method can also be used to secure digital complex information in a virtual optics modality using holographic principles.     

Electromagnetic spin–orbit interaction and giant spin-Hall effect in dielectric particle clusters

We report a phenomenon that electromagnetic spin–orbit interactions can be tailored by dielectric nanoparticles, and self-similar giant spin-Hall effect has been observed in the dielectric particle cluster. The near-field phase singularities and phase vorticity in the longitudinal component of scattered field can also be controlled by such a dielectric structure. The origin of phenomena is believed to be due to the collective resonance excitation in the dielectric particle cluster. It is expected to find applications in optics information processing and designing new nanophotonic devices.     

岩石化学成分及复介电常数与光谱特征的关系探究

实验对辽宁省兴城地区的97块岩石样本测量了350～2 500 nm间光谱反射率,岩石化学成分及部分样本的复介电常数值,并用包络线去除法计算了光谱吸收深度。研究基于岩石化学成分光谱特征理论、介电常数相关理论,利用相关性分析的方法求得岩石各化学成分与光谱吸收深度间的相关系数曲线、岩石复介电常数与光谱反射率间的相关系数曲线。通过归纳总结曲线形态特征,获得以下信息：(1)岩石中SiO₂,Al₂O₃,CaO,K₂O,MgO,灼失含量与光谱吸收深度在波段(1 900～2 500 nm)间相关性明显,在1 900,2 200和2 300 nm等化学元素识别特征波段处,出现局部极大/极小值;火成岩中Fe₂O₃含量与光谱吸收深度在铁元素的特征波段(400～550 nm)有着良好的相关性,相关系数达-0.406。探讨岩石光谱吸收特征与其化学成分含量间的关系在通过遥感影像进行成矿预测、识别岩性等方面都有着积极的作用。(2)岩石复介电常数总体与岩石反射率呈负相关,实部相关性优于虚部,实部相关系数在1 900 nm附近达到极小值-0.753。观查曲线可知,介电常数实部与光谱反射率相关系数在1 900和2 200 nm附近都有着良好的相关性,因此实验运用不同的模型拟合了在该处的响应关系函数曲线。介电常数是介质的基本物理性质之一,现有研究中多在微波波段中分析电磁波特征与介电常数间的关系,该研究在可见光和近红外波段范围内探究了二者间的关系,为进一步探讨岩石介电特性和光谱特征奠定了基础。     

Phase behavior of organic-inorganic crystal

We have investigated the structure and phase behavior of nonmolecularly layered silver stearate by means of temperature-dependent diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. Upon heating the sample, remarkable spectral changes took place. The first phase transition took place that might be associated with a premelting event characterized by the formation of gauche conformers at 390-420 K. A second phase transition took place in which silver nanoparticles with a size of ∼4 nm were formed by thermal decomposition of silver stearate at 520-550 K. These silver nanoparticles, derivatized by stearate, were readily spread as a monolayer at air/water interface, and could be packed in 3-D assemblies by the Langmuir-Blodgett method. Received 29 November 2000