Design and manufacture of metal/dielectric long-wavelength cutoff filters

Thin films of high reflecting metal, such as Ag, have a high reflectance in the long-wavelength region. When they are combined with dielectric layers, it is possible, through thin film interference effects, to induce transmission in certain shorter wavelength regions. Thus, they are useful components for the design of long-wavelength cutoff filters with a broad rejection region. In this paper, metal/dielectric multilayer designs based on this principle are numerically investigated. Three designs with different cutoff wavelengths and with very broad transmission regions in the visible or near-IR spectral ranges are presented. An excellent rejection on the long-wavelength side extends beyond 20 μm. Experimental results for one of the designs produced in our magnetron sputtering system are given.     

Absorption-coefficient-determination method for particulate materials

A method is presented for determining the optical absorption coefficient, or the imaginary refractive index, of particulate material that has been collected from aerosols or hydrosols by means of filtration. The method, based on the Kubelka-Munk theory of diffuse reflectance, is nondestructive and requires no other knowledge of the sample than the amount present, the specific gravity, and an estimate of the real index of refraction. The theoretical development of the method is discussed along with an analysis of photometric and gravimetric errors. We test the method by comparing results obtained for powdered didymium glass with measurements made before the glass was crushed. An example of the method's application to the determination of the absorption coefficient of atmospheric dust at UV, visible, and near-IR wavelengths is also presented.     

基于紫外/可见光谱技术的李子硬度无损检测

目的探究采用紫外/可见光谱技术结合化学计量学预测李子硬度的可行性。方法以“红”李子和“青”李子为研究对象,采用光谱采集系统获取李子样本的平均光谱;采用标准正态变换对原始光谱数据进行预处理,并利用连续投影算法(SPA)和竞争性自适应重加权算法(CARS)从全光谱的1024个波长中分别提取2个(513.04 nm和636.72 nm)和10个(230.01,244.67,274.71,287.66,290.90,300.59,311.78,423.08,515.39,631.31 nm)特征波长;分别建立基于全光谱和提取的特征波长预测李子硬度的误差反向传播(BP)网络模型。结果将采用SPA和CARS特征波长选择方法提取的特征变量作为BP网络输入,明显提升了BP网络模型的运行效率,且SPA-BP网络模型具有相对较好的李子硬度预测能力(rp=0.695,预测样本集均方根误差为1.610 kg/cm2)。结论采用紫外/可见光谱技术结合特征波长提取方法可实现李子硬度的快速无损检测。     

Remote sensing of the ocean contributions from ultraviolet to near-infrared using the shortwave infrared bands: simulations

In the remote sensing of the ocean near-surface properties, it is essential to derive accurate water-leaving radiance spectra through the process of the atmospheric correction. The atmospheric correction algorithm for Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) and Moderate Resolution Imaging Spectroradiometer (MODIS) uses two near-infrared (NIR) bands at 765 and 865 nm (748 and 869 nm for MODIS) for retrieval of aerosol properties with assumption of the black ocean at the NIR wavelengths. Modifications are implemented to account for some of the NIR ocean contributions for the productive but not very turbid waters. For turbid waters in the coastal regions, however, the ocean could have significant contributions in the NIR, leading to significant errors in the satellite-derived ocean water-leaving radiances. For the shortwave infrared (SWIR) wavelengths (approximately > 1000 nm), water has significantly larger absorption than those for the NIR bands. Thus the black ocean assumption at the SWIR bands is generally valid for turbid waters. In addition, for future sensors, it is also useful to include the UV bands to better quantify the ocean organic and inorganic materials, as well as for help in atmospheric correction. Simulations are carried out to evaluate the performance of atmospheric correction for nonabsorbing and weakly absorbing aerosols using the NIR bands and various combinations of the SWIR bands for deriving the water-leaving radiances at the UV (340 nm) and visible wavelengths. Simulations show that atmospheric correction using the SWIR bands can generally produce results comparable to atmospheric correction using the NIR bands. In particular, the water-leaving radiance at the UV band (340 nm) can also be derived accurately. The results from a sensitivity study for the required sensor noise equivalent reflectance, (NE Delta rho), [or the signal-to-noise ratio (SNR)] for the NIR and SWIR bands are provided and discussed.     

Modeling the temperature dependence of the index of refraction of liquid water in the visible and the near-ultraviolet ranges by a genetic algorithm

A simple formula describing the dependence of the index of refraction of water on wavelength in the visible and the near-UV ranges and at temperature from 0 degrees C to 100 degrees C is given. Parameters of the formula were determined by minimization of discrepancies between calculated and experimental data by use of an elite genetic algorithm with adaptive mutations. This algorithm was devised with a particular application in mind, the determination of model parameters. Its superiority over the simple genetic algorithm in locating the global minimum was demonstrated on a family of multiminima test functions for as many as 100 variables.     

Colloidal silicon quantum dots: synthesis and luminescence tuning from the near-UV to the near-IR range

This review describes a series of representative synthesis processes, which have been developed in the last two decades to prepare silicon quantum dots (QDs). The methods include both top-down and bottom-up approaches, and their methodological advantages and disadvantages are presented. Considerable efforts in surface functionalization of QDs have categorized it into (i) a two-step process and (ii) in situ surface derivatization. Photophysical properties of QDs are summarized to highlight the continuous tuning of photoluminescence color from the near-UV through visible to the near-IR range. The emission features strongly depend on the silicon nanostructures including QD surface configurations. Possible mechanisms of photoluminescence have been summarized to ascertain the future challenges toward industrial use of silicon-based light emitters.     

Spectral and global diffuse properties of high-performance translucent polymer sheets for energy efficient lighting and skylights

A visible and near-IR spectral study is presented for a translucent smooth polymer sheet in which dopant particles are clear polymer with a refractive index close to that of the clear polymer host. Diffuse, specular, and total reflectance and transmittance and absorptance as a function of sheet thickness and dopant levels approach ideal behavior for lighting applications. A fourth optical parameter, side loss S(T), is introduced to fully account for the measured data. This covers radiation that is trapped by total internal reflection (TIR) and travels sideways sufficiently far, including to the sheet's edges, to miss detection on exit. S(T) has a strong spectral character, whereas total T and R spectra closely follow the spectrally flat wavelength dependence of the undoped clear sheet. Three distinct regimes are identified for the behavior with wavelength of the specular and diffuse components and are linked to rear surface TIR and side loss.     

Controlling the red boundary of the tunneling photoeffect in nanodimensional carbon structures in a broad (UV-IR) wavelength range

The tunneling photoeffect (PE) has been studied in a microdiode with an electrostatic field localized at an emitter based on a nanodimensional carbon structure. It is established that, when the carbon nanoemitter is exposed to laser and LED radiation photons of low energy (below work function) in the spectral range from near-UV (380 nm) to near-IR (1150 nm) at micro- and milliwatt optical power, a tunneling photocurrent can be initiated by controlling the electric field strength in the emitter-anode gap. The observed phenomenon can be adequately interpreted using a modified Fowler-Nordheim equation for non-equilibrium photoelectrons. Specific features of the construction and operation of photodetectors based on the tunneling PE with a controlled long-wavelength threshold (red boundary) of photoelectron emission are considered. The bandwidth of photoelectron emitters is evaluated, and the possibility of their operation in the wavelength range from UV up to far-IR is predicted.     

Models for the wavelength dependence of the index of refraction of water

Models for the index of refraction of water are reviewed and reduced to wavelength-dependent formulas. A simple three-term nonlinear model derived from Quan and Fry [Appl. Opt. 34, 3477-3480 (1995)], originally developed for the visible region, fits the available data well over an extended range covering the UV to the near-IR (200-1100 nm).     

Spectral multivariate calibration with wavelength selection using variants of Tikhonov regularization

Tikhonov regularization (TR) is a general method that can be used to form a multivariate calibration model and numerous variants of it exist, including ridge regression (RR). This paper reports on the unique flexibility of TR to form a model using full wavelengths (RR), individually selected wavelengths, or multiple bands of selected wavelengths. Of these three TR variants, the one based on selection of wavelength bands is found to produce lower prediction errors. As with most wavelength selection algorithms, the model vector magnitude indicates that this error reduction comes with a potential increase in prediction uncertainty. Results are presented for near-infrared, ultraviolet-visible, and synthetic spectral data sets. While the focus of this paper is wavelength selection, the TR methods are generic and applicable to other variable-selection situations.     

Palladium-based plasmonic perfect absorber in the visible wavelength range and its application to hydrogen sensing

We report on the experimental realization of a palladium-based plasmonic perfect absorber at visible wavelengths and its application to hydrogen sensing. Our design exhibits a reflectance <0.5% and zero transmittance at 650 nm and the operation wavelength of the absorber can be tuned by varying its structural parameters. Exposure to hydrogen gas causes a rapid and reversible increase in reflectance on a time scale of seconds. This pronounced response introduces a novel optical hydrogen detection scheme with very high values of the relative intensity response.     

Optimum sensors for color constancy in scenes illuminated by daylight

The apparent color of an object within a scene depends on the spectrum of the light illuminating the object. However, recording an object's color independent of the illuminant spectrum is important in many machine vision applications. In this paper the performance of a blackbody-model-based color constancy algorithm that requires four sensors with different spectral responses is investigated under daylight illumination. In this investigation sensor noise was modeled as gaussian noise, and the responses were quantized using different numbers of bits. A projection-based algorithm whose output is invariant to illuminant is investigated to improve the results that are obtained. The performance of both of these algorithms is then improved by optimizing the spectral sensitivities of the four sensors using freely available CIE standard daylight spectra and a set of lightness-normalized Munsell reflectance data. With the optimized sensors the performance of both algorithms is shown to be comparable to the human visual system. However, results obtained with measured daylight spectra show that the standard daylights may not be sufficiently representative of measured daylight for optimization with the standard daylight to lead to a reliable set of optimum sensor characteristics.     

Near-infrared radiance measurements as a ground reflection signature

Observations of the spectrum of light from thunderstorms indicate that the radiance amplitude of the near-IR portion of the spectrum is often relatively greater in comparison with the amplitude of the radiance of the visible portion of the spectrum than it is in the spectrum of solar radiation at the surface. Observational data suggest that two mechanisms may cause this effect. The first is the reduction in the shorter wavelengths due to selective scattering, and the second is selective reflection from foliage.     

离子束溅射沉积Co膜光学特性的尺寸效应研究

本文采用离子束溅射沉积了不同厚度的Co膜,利用Lambda-900分光光度计,对不同厚度的Co膜从波长为310nm到1300nm范围测量了薄膜的反射率和透射率.选定波长为310、350、400、430、550、632、800、1200nm时对薄膜的反射率、透射率和吸收率随薄膜厚度变化的关系进行讨论.实验结果显示,Co膜的光学特性都有明显的尺寸效应.对在可见光范围内同一波长时的反射率和透射率随薄膜厚度变化关系的实验结果作于同一图上,发现反射率曲线与透射率曲线都有一个处在网状膜阶段的交点,这个交点对应的厚度作为特征厚度,该厚度可认为是金属薄膜生长从不连续膜进入连续膜的特征判据.     

Statistics of spatial cone-excitation ratios in natural scenes

For some sets of surfaces, the spatial ratios of cone-photoreceptor excitations produced by light reflected from pairs of surfaces are almost invariant under illuminant changes. These sets include large populations of spectral reflectances, some of which represent individual natural surfaces but not their relative abundances in nature. The aim of this study was to determine whether spatial cone-excitation ratios are preserved under illuminant changes within the natural visual environment. A fast hyperspectral imaging system was used to obtain populations of 640,000 reflectance spectra from each of 30 natural scenes. The statistics of spatial cone-excitation ratios for randomly selected pairs of points in these scenes were determined for two extreme daylights. Almost-invariant ratios were common, suggesting that they represent a reliable property of the natural visual environment and a suitable foundation for visual color constancy.     

Realization of an extremely low reflectance surface based on InP porous nanostructures for application to photoelectrochemical solar cells

Extremely low reflectance was obtained from InP porous nanostructures in UV, visible, and near-infrared ranges. Porous samples were electrochemically prepared on which 130-nm-diameter nanopores were formed in a straight, vertical direction and were laterally separated by 50-nm-thick InP nanowalls. The reflectance strongly depended on the surface morphology. The lowest reflectance of 0.1% in the visible light range was obtained after the irregular top layer had been completely removed. Superior photoelectrochemical properties were obtained on the InP porous structures due to two unique features: the large surface area inside pores, and the large photon absorption enhanced on the low reflectance surface.     

Broadband absorption spectroscopy in turbid media by combined frequency-domain and steady-state methods

A technique for measuring broadband near-infrared absorption spectra of turbid media that uses a combination of frequency-domain (FD) and steady-state (SS) reflectance methods is presented. Most of the wavelength coverage is provided by a white-light SS measurement, whereas the FD data are acquired at a few selected wavelengths. Coefficients of absorption (mu(a)) and reduced scattering (mu(s)') derived from the FD data are used to calibrate the intensity of the SS measurements and to estimate mu(s)' at all wavelengths in the spectral window of interest. After these steps are performed, one can determine mu(a) by comparing the SS reflectance values with the predictions of diffusion theory, wavelength by wavelength. Absorption spectra of a turbid phantom and of human breast tissue in vivo, derived with the combined SSFD technique, agree well with expected reference values. All measurements can be performed at a single source-detector separation distance, reducing the variations in sampling volume that exist in multidistance methods. The technique uses relatively inexpensive light sources and detectors and is easily implemented on an existing multiwavelength FD system.     

Suitability of surface acoustic wave oscillators fabricated using low temperature-grown AlN films on GaN/sapphire as UV sensors

Epitaxial AlN films were prepared on GaN/sapphire using a helicon sputtering system at the low temperature of 300 degrees C. Surface acoustic wave (SAW) devices fabricated on AlN/GaN/sapphire exhibited superior characteristics compared with those made on GaN/sapphire. An oscillator using an AlN/GaN/sapphirebased SAW device is presented. The oscillation frequency decreased when the device was illuminated by ultraviolet (UV) radiation, and the downshift of the oscillation frequency increased with the illuminating UV power density. The results showed that the AlN/GaN/sapphire-layered structure SAW oscillators are suitable for visible blind UV detection and opened up the feasibility of developing remote UV sensors for different ranges of wavelengths on the III-nitrides.     

Spatial distributions of the intensity of optical emission lines studied during bismuth ferrite film deposition in high-frequency discharge plasma

We have studied spatial distributions of the intensity of optical emission lines in plasma of the RF discharge in oxygen during the deposition of bismuth ferrite (BiFeO₃) films. The intensities of characteristic emission lines in the visible, near-UV and near-IR spectral ranges have been measured as functions of the distance from the probed plasma layer to the target. The intensity profiles of the emission lines of oxygen ions, oxygen atoms, and iron atoms exhibit different behavior. Differences in the spatial distributions of intensity have been also observed for the two characteristic emission lines (at 613.7 and 688.6 nm) of iron.     

Mechanism of optical absorption enhancement of surface textured black silicon

Micro-pyramids and nano-pores structures on the silicon surface are fabricated. The reflectance of both textured samples is drastically suppressed compared with that of the non-textured sample for wavelengths from 250 to 1,100 nm. The reflectance rises from 1,000 nm for non-textured sample. However, 30 nm red-shift is observed for both textured samples. The absorptance of both textured samples is obviously enhanced in the ultraviolet–visible wavelengths for the reduction of reflectance. Without introducing any adulterant into samples, both textured samples exhibit strong absorptance in the infrared wavelengths where the non-textured sample becomes optically transparent. The absorptance of both textured samples is 70 % at wavelengths of 1.1 μm, and the absorptance of 30 % for micro-pyramids structure sample and 10 % for nano-pores structure sample are observed at wavelengths longer than 1.1 μm. Furthermore, 110 and 60 nm red-shift in bandgap wavelengths exist in micro-pyramids structure sample and nano-pores structure sample compared with non-textured sample.