Surfaces with high solar reflectance and high thermal emittance on structured silicon for spacecraft thermal control

Presented here is an examination of unstructured and structured (by anisotropic etching), monocrystalline silicon wafers coated with sputter deposited aluminum and chemical vapor deposited silicon dioxide for high solar reflectance and high thermal emittance, respectively. The topography of the samples was characterized with optical and scanning electron microscopy. Optical properties were examined with reflectance and transmittance spectroscopy, partly by usage of an integrating sphere. The measurement results were used to estimate the equilibrium temperature of the surfaces in space. The suitability of the surfaces with high solar reflectance and high thermal emittance to aid in the thermal control of miniaturized, highly integrated components for space applications is discussed. A silicon dioxide layer on a metal layer results in a slightly lower reflectance when compared to surfaces with only a metal layer, but might be beneficial for miniaturized space components and modules that have to dissipate internally generated heat into open space. Additionally, it is an advantage to microstructure the emitting surface for enhanced radiation of excess heat.     

Hydrothermal derived nanostructure rare earth (Er,Yb)-doped ZnO: structural,optical and electrical properties

The structural, optical and electrical properties of undoped and rare-earth (Er, Yb) doped zinc oxide (ZnO) nanopowder samples synthesized by hydrothermal method were investigated. The obtained samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive spectroscopy. The optical properties of undoped and rare-earth (Er, Yb) doped ZnO were carried out with UV–visible diffuse reflectance spectroscopy techniques. XRD results reveal that Yb and Er doped ZnO nanopowders have single phase hexagonal (Wurtzite) structure without any impurities. SEM analysis indicate that dopants with different radii affected the surface morphology of ZnO nanostructures. The optical band gap of all samples were calculated from UV–Vis diffuse reflectance spectroscopy data. We have obtained band gap values of undoped, Er and Yb doped ZnO as 3.24, 3.23, 3.22 eV, respectively. Electrical characterization of the samples were made in the 280–350 K temperature range using Van der Pauw method based on Hall effect measurement. The carrier concentrations decreased for both Er and Yb doping while the Hall mobility and electrical resistivity increased with Yb, Er doping compared to undoped ZnO nanopowder at room temperature. The temperature dependent resistivity measurements of Er doped ZnO showed a metal–semiconductor transition at about 295 K, while Yb doped ZnO showed characteristic semiconductor behavior.     

Defect formation in hafnium dioxide thin films

Hafnium dioxide thin films were deposited by reactive electron-beam evaporation at six different substrate temperatures on fused-silica substrates. During the depositions, the scattering of light caused by the growth of defects in the films was recorded with in situ total internal reflection microscopy. After deposition the films were analyzed by angle-resolved scatterometery, spectrophotometric measurement of film reflectance and transmittance, atomic force microscopy, and x-ray diffraction. We explore the effects of film defect formation on film optical properties and film surface topography using these data.     

Optical characterization of porous silicon and crystalline silicon by the Kramers-Kronig method

The electronic structure of porous silicon (PS) has been characterized by optical reflectance spectra analyses. Using a Cary-500 spectrometer, the reflectance spectra of PS are measured in the photon energy range of 0.4-6 eV. The spectral responses of optical constants are calculated for PS and Si by Kramers-Kronig analysis. The analysis clarified strong evidence for widening and direct bandgaps for PS samples. Also, the optical constants of PS layers as a function of porosity have been studied. Our results indicate that PS retains some of the characteristic optical features of crystalline Si. However, in the visible region, PS shows that the imaginary part of the complex refractive index increases, and the real part decreases as porosity increases. This feature could be related to the surface roughness of PS and its role in surface absorption and scattering.     

Substrate biasing effects on the microstructure of magnetron-sputtered AlN films investigated by in situ reflectance interferometry

In situ reflectance interferometry (RI) at 400 nm wavelength was used to investigate the effect of the substrate negative bias on the microstructure of aluminium nitride (AlN) films deposited at room temperature on Si substrates by magnetron sputtering. Their surface reflectance recorded during film deposition promptly yields real-time information on the microstructures developed under oxygen contamination and bias change. Specifically, the refractive index n and the extinction coefficient k are deduced from reflectance using appropriate multilayer optical models and validated by spectroscopic ellipsometry. These optical constants correlate appreciably with the microstructure that evolves between columnar-crystallized and purely amorphous phases including in-between amorphous states containing dispersed nano-AlN grains. These microstructures were identified using ex situ energy dispersive X-ray spectroscopy, transmission electron microscopy and diffraction, X-ray diffraction and Auger electron spectroscopy. The simple and cost-effective in situ RI thus appears a powerful tool in controlling the microstructures of thin AlN films for desired applications.     

Influence of Al concentration on structural and optical properties of Al-doped ZnO thin films

Undoped ZnO and Al-doped zinc oxide (ZnO:Al) thin films with different Al concentrations were prepared onto Si (100) substrate by pulsed filtered cathodic vacuum arc deposition system at room temperature. The influence of doping on the structural and optical properties of thin films was investigated. The preferential (002) orientation was weakened by high aluminum doping in films. Raman measurement was performed for the doping effects in the ZnO. Atomic force microscopy images revealed that the surface of undoped ZnO film grown at RT was smoother than that of the Al-doped ZnO (ZnO:Al) films. The reflectance of all films was studied as a function of wavelength using UV–Vis–NIR spectrophotometer. Average total reflectance values of about 35 % in the wavelength range of 400–800 nm were obtained. Optical band gap of the films was determined using the reflectance spectra by means of Kubelka–Munk formula. From optical properties, the band gap energy was estimated for all films.     

Combination of surface texturing and nanostructure coating for reduction of light reflection in ZnO/Si heterojunction thin film solar cell

In this study, we report a single heterojunction solar cell based on n-type zinc oxide/p-type silicon. Three different solar cells were fabricated based on ZnO thin film on Si substrate, ZnO nanorods on Si substrate, and ZnO nanorods on micro-pyramidal structure of Si substrate. The comparison between these three kinds of solar cells was studied. Pyramidal structure of silicon was fabricated using chemical etching technique of p-type Si (100). The chemical solution consists of NaOH, isopropyl alcohol and hydrazine hydrate. The results showed that Si micro-pyramids can enhance optical absorption of Si substrates by increasing surface area and entrapping of incident light. For fabrication of uniform ZnO nanorods, a seed layer of ZnO was deposited on Si substrates via radio frequency magnetron sputtering technique. This layer can be used as an active n-type material in heterojunction solar cells as well. ZnO nanostructures can increase light absorption due to their high specific surface area. The combination of ZnO nanorods and Si micro-pyramids can enhance light trapping effect and increase the efficiency of solar cells. The structural and morphology of samples were studied using field-emission scanning electron microscopy, atomic force microscopy and X-ray diffractometry while the optical properties were investigated using photoluminescence and reflectance spectrometry. The efficiency and fill factor of solar cells were obtained from current–voltage characteristics using a solar simulator and a source-meter. The results showed that the efficiency of solar cell based on nanostructures of ZnO/micropyramids of Si is highly increased due to high anti-reflective behavior of this sample.     

Transparent superhydrophobic surfaces for applications of controlled reflectance

This work involves a new optical application for transparent superhydrophobic materials, which enables low-energy optical contact between a liquid and solid surface. The new technique described here uses this surface property to control the reflectance of a surface using frustration of total internal reflection. Surface chemistry and appropriate micro-scale and nano-scale geometries are combined to produce interfaces with low adhesion to water and the degree to which incident light is reflected at this interface is controlled by the movement of water, thereby modifying the optical characteristics at the interface. The low adhesion of water to superhydrophobic surfaces is particularly advantageous in imaging applications where power use must be minimized. This paper describes the general approach, as well as a proof-of-principle experiment in which the reflectance was controlled by moving a water drop into and out of contact with a superhydrophobic surface by variation of applied electrostatic pressure.     

Structural, morphological and optical properties of thermal annealed TiO thin films

Structural, morphological and optical properties of TiO thin films grown by single source thermal evaporation method were studied. The films were annealed from 300 to 520 °C in air after evaporation. Qualitative film analysis was performed with X-ray diffraction, atomic force microscopy and optical transmittance and reflectance spectra. A correlation was established between the optical properties, surface roughness and growth morphology of the evaporated TiO thin films. The X-ray diffraction spectra indicated the presence of the TiO₂ phase for the annealing temperature above 400 °C.     

总气压对直流反应磁控溅射制备TiO2薄膜的光学性质的影响

用DC(直流)反应磁控溅射设备在硅基底上制备TiO2薄膜，在固定电源功率、氩气流量42．6sccm、氧流量15sccm、溅射时间30min的条件下，通过控制总气压改变TiO22薄膜的光学性质。应用n＆k Analyzer 1200测量，当总气压增加时薄膜的平均反射率降低，同时反射低谷向短波方向移动，总气压对消光系数k影响不大；随着总气压的增加薄膜的折射率出现了下降的趋势，但当总气压达到一定量值时折射率的变化趋于稳定。通过XRD和SEM表征发现，随着总气压的增加TiO2的晶体结构由金红石相向锐钛矿相转变，薄膜表面的颗粒度大小由粗大变得微小细密。     

Dynamics of tissue optics during laser heating of turbid media

The dynamics of the optical behavior of tissue during the photothermal interaction of laser radiation with tissue could significantly affect the optimization of light doses for effective and safe applications of lasers in medicine. Characterization of the dynamics of tissue optics during laser heating was performed by means of simultaneous measurements of the total transmittance, diffuse reflectance, and surface temperature of fresh and thermally coagulated human skin and canine aorta during long-pulsed Nd:YAG laser heating with a double integrating-sphere system and an infrared camera. Thermally induced changes in the optical properties of tissue caused a decrease in the total transmittance and an increase in the diffuse reflectance of both fresh and precoagulated skin and aorta samples. For fresh tissue, these changes were primarily reversible until photocoagulation occurred, then both the reversible, as well as the irreversible, changes were observed. However, for precoagulated tissue the reversible changes in the optical properties were dominant, whereas the irreversible changes were insignificant. Results from this study indicate the existence of the nonlinear behavior in the optics of turbid biological media during pulsed laser heating. Possible mechanisms responsible for this nonlinear optical behavior are discussed.     

Dust absorption averaged over the Sahara inferred from moderate resolution imaging spectroradiometer

We estimated the average dust single scattering albedo (omega(0)) over the Sahara using four years of moderate resolution imaging spectroradiometer data. The method employed is based on the theory that the critical surface reflectance (rho(c)) for which the reflectance at the top-of-the-atmosphere is not influenced by the variability of dust optical thickness depends on omega(0). The average dust absorption over the Sahara was estimated to be smaller than that previously reported in the literature, and this may be causing the cooling of the climate system. Our method enables one to estimate omega(0) from data with a variety of aerosol optical thickness values using samples in the vicinity of rho(c). Use of satellite data over large areas and for long periods enables quantification of the average dust absorption over the entire Sahara.     

Experimental and simulated angular profiles of fluorescence and diffuse reflectance emission from turbid media

Given the wavelength dependence of sample optical properties and the selective sampling of surface emission angles by noncontact imaging systems, differences in angular profiles due to excitation angle and optical properties can distort relative emission intensities acquired at different wavelengths. To investigate this potentiality, angular profiles of diffuse reflectance and fluorescence emission from turbid media were evaluated experimentally and by Monte Carlo simulation for a range of incident excitation angles and sample optical properties. For emission collected within the limits of a semi-infinite excitation region, normalized angular emission profiles are symmetric, roughly Lambertian, and only weakly dependent on sample optical properties for fluorescence at all excitation angles and for diffuse reflectance at small excitation angles relative to the surface normal. Fluorescence and diffuse reflectance within the emission plane orthogonal to the oblique component of the excitation also possess this symmetric form. Diffuse reflectance within the incidence plane is biased away from the excitation source for large excitation angles. The degree of bias depends on the scattering anisotropy and albedo of the sample and results from the correlation between photon directions upon entrance and emission. Given the strong dependence of the diffuse reflectance angular emission profile shape on incident excitation angle and sample optical properties, excitation and collection geometry has the potential to induce distortions within diffuse reflectance spectra unrelated to tissue characteristics.     

Far-ultraviolet reflectance measurements and optical constants of unoxidized aluminum films

The far-UV reflectance of thin unoxidized aluminum films prepared and maintained in ultra-highvacuum conditions was measured versus the angle of incidence, and the complex refractive index was obtained from those measurements on several wavelengths from 82.6 to 113.5 nm. Measurements were made on two perpendicular planes of incidence to deal with the unknown of the polarization state of the radiation beam. The surface roughness was characterized by atomic force microscopy. The refractive index is obtained for the first time, to our knowledge, from direct optical measurements in this spectral range. Current results match well the former values in the literature that were calculated through the Kramers-Kronig analysis by using in the above interval reflectances estimated from electron-energy-loss spectra and from optical measurements on surfaces of unstated roughness.     

Influence of Sputtering Gas on Morphological and Optical Properties of Magnesium Films

The influence of sputtering gas(He & Ar) on the structural properties of Mg thin films has been investigated.The optical property(reflectance) that results from the growth of films at varying substrate temperatures(Tsub) was also studied.The deposited films were characterized by using X-ray diffraction(XRD),field emission scaning electron microscopy(FE-SEM),atomic force microscopy(AFM) and UV-Vis-NIR spectrophotometer.The smaller crystallite size and lower deposition rate were observed in the presence of Helium atmosphere compared to Argon.Morphology of the films shows 2D hexagonal geometry of grains in the deposition temperature range(Tsub≈50-150℃) in both the sputtering gases.The surface roughness of the polycrystalline films were found to increase with increase in the deposition temperature of both ambient gases.Optical reflectance of Mg films was measured in near infrared region and larger reflectance was observed from Mg films sputtered in He atmosphere compared to that in argon.     

Synthesis of AgBr/ZnO nanocomposite with visible light-driven photocatalytic activity

AgBr/ZnO nanocomposite was synthesized via chemical precipitation from pure ZnO nanowires, AgNO₃, and NaBr. Inductively coupled plasma optical emission spectroscopy, X-ray diffraction, and high resolution transmission electron microscopy results confirmed the forming of AgBr/ZnO nanocomposite. High resolution transmission electron microscopy results of the as-synthesized AgBr/ZnO nanocomposite revealed that AgBr nanoparticles were attached to the surface of ZnO nanowires. UV-vis diffuse reflectance spectra of both pure ZnO and AgBr/ZnO nanocomposite displayed a band gap edge at about 350-380 nm. However, compared with pure ZnO, an additional broad tail from approximately 400 nm to 700 nm appeared in the UV-vis diffuse reflectance spectrum of AgBr/ZnO nanocomposite. The photocatalytic studies indicated that the as-synthesized AgBr/ZnO nanocomposite was a kind of promising photocatalyst in remediation of water polluted by some chemically stable azo dyes under visible light.     

稀疏植被地区气溶胶光学厚度反演

研究了稀疏植被地区地表反射率的精确确定方法,构建了精确确定稀疏植被地区地表反射率的地面光谱模型,实现了该类地区气溶胶光学厚度的卫星数据反演.分析表明,所构模型的地表反射率确定误差在0.015以内.通过对气溶胶光学厚度反演的不确定性分析,确定稀疏植被地区的气溶胶光学厚度的卫星反演误差在0.15以内,并使用MODIS数据反演了北京市区及周围地区的气溶胶光学厚度.用太阳分光光度计测量的气溶胶光学厚度对MODIS数据的气溶胶光学厚度反演结果进行了验证.     

A new optical coefficient of biological tissue that determines the diffuse reflectance light distribution on the tissue surface

When a light beam is incident normally upon a tissue, the spatial distribution of diffuse reflectance on the tissue surface is affected by the optical properties of the tissue. The purpose of this paper is to find the relationship between the spatial distribution of diffuse reflectance on the tissue surface and optical properties of the tissue. The diffuse reflected light distribution is found to be determined mainly by a new optical coefficient μ_as. A qualitative relationship between μ_as and the diffuse reflected light distribution was developed by Monte Carlo simulations. Measurements of the diffuse reflected light distribution on solutions of Intralipid-10% with added ink confirm the Monte Carlo simulations. The new optical coefficient μ_as was measured from several tissue examples by the qualitative relationship. The value of μ_as was useful for determining the components of the tissue.     

高效光催化降解气相苯纳米TiO2微球的制备EI北大核心CSCD

以四氯化钛为钛源,尿素为沉淀剂前驱物,硫酸钠为分散剂,利用水热法在水-醇体系中制备出纳米TiO2微球。运用X射线衍射、电子显微镜、N2吸附-脱附和紫外-可见光谱等手段表征样品的结构和性质,并考察了水热温度对纳米TiO2微球结构及光催化降解气相苯活性的影响。结果表明,此类微球由纳米颗粒组成,且比表面积大,介孔结构明显,光吸收出现明显的“蓝移”。光催化结果显示,微球具有很高的光催化活性,尤其是180℃下制备的微球仅用20min将苯完全去除,但生成CO2的量仍随时间有所增加,表明微球的强吸附性能促进其光催化降解过程,且矿化率高达5.5,是P25(2.7)的2倍。     

A feature parameter of optical diffuse reflectance on normal and malignant human breast tissue

Light that is delivered to the tissue surface undergoes multiple elastic scattering and absorption; part of it returns to the surface as optical diffuse reflectance, carrying quantitative information on the structure and composition of the measured tissue. In this paper, we utilized a well tested and publicly available Monte Carlo simulation software to simulate optical diffuse reflectance on normal and malignant human breast tissue in the visible wavelength region 450–650 nm. Based on the Monte Carlo simulation results, we discovered a feature parameter of optical diffuse reflectance on the simulated tissue surface. Normal and malignant human breast tissue may be discriminated by the value of the feature parameter. The values of the feature parameter are shown for normal and malignant human breast tissue in the visible wavelength region 450–650 nm. Experiments with tissue phantoms showed that the feature parameter is correlated to the component of the phantom. So the feature parameter is useful for the non-invasive optical diagnosis of biological tissue.