Polarimetric weak-localization effect in scattering of natural light in the region of small phase angles

Results of laboratory measurements and interpretations of the polarimetric effect of weak localization (negative polarization) appearing under scattering of natural light by a dark ultradisperse surface (soot with albedo of 2.5%) are presented. The measurements were carried out in red light in a range of phase angles of 0.2°–4.2°. It was revealed that the soot, despite the low albedo, shows the polarimetric weak-localization effect inherent in bright surfaces such as the surface formed by smoked MgO. The results of measurements are interpreted using numerical simulation of multiple light scattering in a medium consisting of particles whose characteristics are close to those observed for soot. As the result of simulation, it was found that the scattering with the multiplicity exceeding two can give rise to a negative polarization branch, which becomes narrower with increasing scattering order. For the case of soot, four orders of scattering is sufficient to describe the observed polarimetric effect of weak localization with a necessary accuracy.     

Effect of particles size on the optical constants of iron/polystyrene composites via UV-radiation

The present study deals with the optical characteristics of polystyrene (PS) composites containing iron particles of different sizes: 5, 40, 110, and 250 μm. The optical absorption spectra were collected in the wavelength range 300–800 nm using a UV-spectrophotometer. The optical results obtained were analyzed in terms of the absorption formula for non-crystalline materials. The optical energy gap and other basic optical constants such as refractive index, dielectric constants, and optical conductivity were investigated and showed a clear dependence on the iron particles size. It was found that the optical energy gap for the iron-filled composites is less than that for the neat PS, and it decreases as the iron particle size decreases. The refractive index of the prepared composites was determined from the collected transmittance and reflectance spectra. It was found that the calculated dielectric constant and refractive index of the composites increase when the iron particles size decreases. The optical dispersion behavior of the composites was described by the single-oscillator model. Enhancement in the optical conductivity was observed with decreasing the iron particles size.     

Grain-size effect in BaTiO3 ceramics: study by far infrared spectroscopy

The article offers comparative study of two nanocrystalline (50 and 100?nm averaged grain size) and two coarse-grain (1.2 and 10?µm averaged grain size) ceramics by means of Fourier transform infrared spectroscopy in a broad temperature range (10–570?K). Far infrared reflectivity spectra were fitted with the factorized model of the dielectric function and the evaluated dielectric function was compared with the results of low-frequency dielectric measurements. It appears that the stiffening and weakening of the overdamped soft mode is the only reason for the reduced dielectric permittivity of nanocrystalline ceramics in the paraelectric phase, but a strong grain-size dependence of the dielectric constant in the ferroelectric phase is not connected with the changes in lattice dynamics. All single-crystal symmetry changes were detected in all of the samples, but no phase-transition discontinuities was seen in nanocrystalline ceramics. A coexistence of more than one phase is suggested. Strong dependence of the Curie-Weiss temperature on the grain size, earlier revealed by others, is confirmed.     

Light scattering by slightly nonspherical particles on surfaces

We investigate the shape dependence of the scattering by dielectric and metallic particles on surfaces by considering particles whose free-space scattering properties are nearly identical. The scattering by metallic particles is strongly dependent on the shape of the particle in the region near where the particle touches the surface. The scattering by dielectric particles displays a weaker, but nonetheless significant, dependence on particle shape. These results have a significant effect on the use of light scattering to size and identify particles on surfaces.     

Dielectric layer-dependent surface plasmon effect of metallic nanoparticles on silicon substrate

The electromagnetic interaction between Ag nanoparticles on the top of the Si substrate and the incident light has been studied by numerical simulations. It is found that the presence of dielectric layers with different thicknesses leads to the varied resonance wavelength and scattering cross section and consequently the shifted photocurrent response for all wavelengths. These different behaviours are determined by whether the dielectric layer is beyond the domain where the elcetric field of metallic plasmons takes effect, combined with the effect of geometrical optics. It is revealed that for particles of a certain size, an appropriate dielectric thickness is desirable to achieve the best absorption. For a certain thickness of spacer, an appropriate granular size is also desirable. These observations have substantial applications for the optimization of surface plasmon enhanced silicon solar cells.     

Effect of phase transformation on optical and dielectric properties of zirconium oxide nanoparticles

Zirconium oxide nanoparticle (ZrO₂) is synthesized by the hydrothermal method at different calcination temperatures. The structural analysis is carried out by X-ray diffraction and Raman spectra. The sample prepared at 400 °C and 1100 °C showed the cubic and monoclinic phase, respectively, and the sample calcined at 600 °C and 800 °C showed the mixed phase with co-existence of cubic and monoclinic phases. Furthermore, the morphology and particle size of these samples were investigated by scanning electron microscope (SEM) and transmission electron microscope (TEM) analysis. The band gap estimated from UV–Vis spectra of ZrO₂ (zirconia) nanocrystalline materials calcined at different temperatures from 400 °C to 1100 °C was in the range of 2.6–4.2 eV. The frequency dependence of dielectric constant and dielectric loss was investigated at room temperature. The low frequency region of dielectric constant is attributed to space charge effects.     

可见光隐身涂料设计

根据颗粒粒径与波长相比的不同情形，分别用粗粒子理论、细粒子理论和桥理论求得颗粒的 光散射系数和吸收系数，从而得到含颗粒涂料的漫反射率.讨论了涂料参数如黏结剂和颗粒 的光学常数、颗粒粒径和体积比等对漫反射率的影响.并以含钛白粉颗粒的涂料为例，介绍 了可见光隐身涂料的计算机设计方法. 关键词： 非均匀涂料 涂料设计 可见光隐身     

Kramers Kronig analysis of the optical properties of small silver particles

In order to complete a preceding paper the dielectric constant? of the particle material of small silver particles with diameters between 210 and 25 Å is computed in the wavelength region 365≦λ≦455 nm from the measured spectra of thesmall particle plasma reasonance absorption. For this purpose a properKramers Kronig relation is derived, and is checked by applying to particles with Drude free electron gas. The results, concerning the silver particles, are that the real part of? changes slightly, whereas the imaginary part is markedly enhanced (up to the ten-fold of the bulk values) if the particle size decreases. This size dependence of? can quantitatively be described with thefree path effect within the accuracy of the measured values. Conversely, thebulk dielectric constant of silver is obtained by applying the free path effect to the measured dielectric constant of the small particles.     

Evolution of the optical reflectivity of a monolayer of nanoparticles during its growth on a dielectric thin film

The reflectivity variations of a dielectric thin film during the deposition of a disordered monolayer of metallic nanoparticles are studied. We present experimental results and provide theoretical physical insight into the behavior of the reflectivity signal and its dependence on the dielectric thin-film thickness and structural features of the monolayer of nanoparticles. A closed-form expression is used to describe the reflectivity of a disordered monolayer of particles on a flat substrate within the frame of a coherent-scattering model approach. It is shown that the model reproduces qualitatively the behavior of the reflectivity signal during the experiment. Finally we study the optical response in the limit of small particles for low surface coverage fractions of the monolayer to evidence the main parameters that dictate the evolution of the reflectivity signal during the growth of a monolayer of nanoparticles.     

Enhanced photocatalytic performance of TiO2 particles via effect of anatase–rutile ratio

In the present work anatase–rutile transformation temperature and its effect on physical/chemical properties as well as photocatalytic activity of TiO₂ particles were investigated. The characterisation of the synthesised and annealed TiO₂ particles were determined by X-Ray Powder Diffraction (XRD), scanning electron microscope (SEM), dynamic light scattering (DLS) and Brunauer–Emmett–Teller surface area analysis (BET). The refraction in the ultraviolet–visible (UV–vis) range was assessed using a dual-beam spectrophotometer. The photocatalytic performance of the particles was tested on methylene blue solution. The XRD data indicated that the percentage of rutile increased with the annealing temperature and almost 100% of anatase transformed to rutile at 1000 °C. In addition, the phase transformation was a linear function of annealing temperature so phase composition of TiO₂ can be controlled by changing the annealing temperature. The SEM and BET results presented the increase of agglomerate size and the decrease of specific surface area with the increasing annealing temperature. This proved that anatase has smaller particle size and higher surface area than rutile. The photocatalytic activity of the annealed TiO₂ powders reduced with the increase of annealing temperature. The samples annealed at 900 °C and 925 °C with anatase: rutile ratio of 92:8 and 77:23, respectively, showed the best activity. These results suggested that the photocatalytic activity of TiO₂ particles is a function of phase composition. Thus it can be enhanced by changing its phase composition which can be controlled by annealing temperature.     

Effects of saturation on optical bistability with coupled surface plasmons

We present exact numerical results for a symmetric layered medium (prism/Ag film/nonlinear dielectric/Ag film/prism) where the middle dielectric slab is assumed to have a saturation-type nonlinearity. We show bistable behaviour in the power dependence of the reflectivity ofp-polarized light under the condition when coupled surface modes are excited in the structure. Moreover, we study the effect of saturation on the bistable behaviour to show that multivalued character is inhibited by saturation effects. The field distributions corresponding to the minimum reflectivity states of the nonlinear structure are also presented.     

Design and Fabrication of GaInAsP/InP VCSEL with Two a-Si/a-SiN x Bragg Reflectors

We report on the design and fabrication of a 1.55 μm wavelength Vertical Cavity Surface Emitting Lasers (VCSELs) which consists of two dielectric Bragg mirrors and a InGaAsP-based active region. The dielectric materials are amorphous silicon and amorphous silicon nitride. Layers of such materials have been deposited by magnetron sputtering and analyzed in order to determine their optical properties. A large refractive index difference of 1.9 is found between these materials. Distributed Bragg Reflectors (DBRs) based on these dielectric materials quarter wave layers have been studied by optical measurements and confronted to theoretical calculations based on the transfer matrix method. A maximum reflectivity of 99.5% at 1.55 μm and a large spectral bandwidth of 800 nm are reached with only four and a half periods of a-Si/a-SiN_x. The VCSEL was fabricated by metallic bonding process. This method allows to bond an InP-based active region as the gain medium on a Si substrate thanks to the formation of a Au–In alloy. This process is performed at a low temperature of 240°C without damaging the optical properties of the microcavity. This VCSEL has been characterized by an optical pumping experiment with a low and a high-density optical power and a laser emission has been obtained at room-temperature.     

Growth and magnetic behavior of bismuth substituted yttrium iron garnet nanoparticles

Crystal growth and the magnetic properties of bismuth substituted yttrium iron garnet (Bi-YIG) nanoparticles were studied with particular focus on the bismuth composition dependence of the magnetic properties of the particles and the effects of annealing on the garnet phase formation. The Bi-YIG nanoparticles of 47–67 nm in size can be chemically synthesized when they are annealed at 650–850 °C. Both the lattice constant and the magnetization of the garnet nanoparticles linearly increase when the bismuth composition in the Bi-YIG particles increases. We have found that chemically synthesized nanoparticles transform from the amorphous to the garnet phase when annealed at temperatures below 650 °C, while the onset of magnetic moment of iron in the garnet nanoparticles is observed slightly above 650 °C. According to Mössbauer effect measurements, the hyperfine fields of ⁵⁷Fe at the tetrahedral and octahedral sites in the garnet are 39 and 48 T, respectively.     

Laser diagnostics of nanoscale dielectric films on transparent substrate by integrating differential reflectivity and ellipsometry

The effect of nanometer dielectric films on the differential reflection characteristics of linearly polarized light from non-absorbing materials is investigated in the long-wavelength approximation. The second-order formulas for changes in the reflectance of s- and p-polarized light caused by ultrathin layer are obtained. A detailed analysis of the influence of ultrathin film to the reflectivity of p-polarized light in the vicinity of the Brewster angle is carried out. The novel methods are developed for determining the thickness and refractive index of uniform (or the average values of refractive index of nonuniform) nanometer-scale films by differential reflectivity and ellipsometric measurements.     

Evidence for ordered regions in poly(n-butyl)methacrylate from light-scattering studies

From light-scattering studies on polybutylmethacrylate, a polymeric glass, the variation of the velocity and attenuation of thermally excited hypersonic phonons with temperature has been measured. Measurement of the temperature dependence of the ratio of the intensity of the Rayleigh line to the Brillouin lines is interpreted as due to a configurational rearrangement within the glass above the glass transition temperature, T_g . Only light scattered from longitudinal phonons was observed. The distinct change in the temperature dependence of the velocity, attenuation and intensity ratio identified the glass transition.

For samples annealed well above T_g, T_g was found to be about 0°C from the light-scattering studies, 12°C from differential scanning calorimetry (DSC), and 20°C from refractive index measurements. For an unannealed sample the behaviour of the above parameters with temperature was found to be different. T_g for the unannealed sample was 14°C from light-scattering, 18°C from DSC and 20°C from index of refraction measurements.     

Size-dependent denaturing kinetics of bovine serum albumin adsorbed onto gold nanospheres

We have used localized surface plasmon resonance (LSPR) to monitor the kinetics of thermal denaturing of bovine serum albumin (BSA) adsorbed onto gold nanospheres of size 5 nm-100 nm. The effect of the protein on the LSPR was monitored by visible extinction spectroscopy. The wavelength of the peak extinction (resonance) is affected by the conformation of the adsorbed protein layer, and as such can be used as a very sensitive probe of thermal denaturing that is specific to the adsorbed (as opposed to free) protein. The time dependence of the denaturing is measured in the temperature range 60 °C–70 °C, and the lifetimes are used to calculate an activation barrier for thermal denaturing. The results show that thermally activated denaturing of proteins adsorbed onto nanoparticles has a nanoparticle-size-dependent activation barrier, and this barrier increases for decreasing particle size. This may have important implications for other protein-nanoparticle interactions.     

High-reflectivity high-contrast grating focusing reflector on silicon-on-insulator wafer

A high-contrast grating(HCG) focusing reflector providing phase front control of reflected light and high reflectivity is proposed and fabricated.Basic design rules to engineer this category of structures are given in detail.A 1550 nm TM polarized incident light of 11.86 mm in focal length and 0.8320 in reflectivity is obtained in experiment.The wavelength dependence of the fabricated HCGs from 1530 nm to 1580 nm is also tested.The test results show that the focal length is in the range of 11.81-12 mm,which is close to the designed focal length of 15 mm.The reflectivity is almost above 0.56 within a bandwidth of 50 nm.At a distance of 11.86 mm,the light is focused to a round spot with the highest concentration,which is much smaller than the size of the incident beam.The FWHM of the reflected light beam decreases to 120 nm,and the intensity increases to 1.18.     

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.     

Thermally tunable double-peak phase-conjugate reflectivity in InP:Fe at 1.32 μm

We report on double-peak thermally tuned reflectivity of optical phase conjugation in InP:Fe under dc fields. A single-pump four-wave mixing geometry generates a phase-conjugate reflectivity up to 6.7% for a pump intensity of 28 mW/cm² at 1.32 μm wavelength. The phase-conjugate reflectivity exhibits strong temperature dependence that is attributed to the phase-shift variations of the index grating due to optical power density of the intensity distribution. Two strong peaks in phase-conjugate reflectivity are observed near room temperature and are separated by 3 °C. Our theoretical predictions, which are based on the modified single-defect model, are in good agreement with the experimental findings. Received: 6 December 2000 / Revised version: 13 February 2001 / Published online: 27 April 2001     

金属光栅表面增强喇曼散射TE模的计算分析

以纳米量级金属履带矩形光栅为模型,研究了表面增强喇曼散射的特性.针对TE模的入射光,采用耦合波原理对金属表面的衍射场进行了讨论,并用数值计算方法讨论了光栅周期、光栅深度等参量对表面增强的影响以及增强因子随第一级瑞利系数的变化关系.结果表明：在入射光波长为700 nm、光栅周期p=400 nm、 光栅深度d=150 nm、占空比为1/3、入射角度为10°时,获得最大增强,增强因子G可达102.