Polarization-based index of refraction and reflection angle estimation for remote sensing applications

A passive-polarization-based imaging system records the polarization state of light reflected by objects that are illuminated with an unpolarized and generally uncontrolled source. Such systems can be useful in many remote sensing applications including target detection, object segmentation, and material classification. We present a method to jointly estimate the complex index of refraction and the reflection angle (reflected zenith angle) of a target from multiple measurements collected by a passive polarimeter. An expression for the degree of polarization is derived from the microfacet polarimetric bidirectional reflectance model for the case of scattering in the plane of incidence. Using this expression, we develop a nonlinear least-squares estimation algorithm for extracting an apparent index of refraction and the reflection angle from a set of polarization measurements collected from multiple source positions. Computer simulation results show that the estimation accuracy generally improves with an increasing number of source position measurements. Laboratory results indicate that the proposed method is effective for recovering the reflection angle and that the estimated index of refraction provides a feature vector that is robust to the reflection angle.     

3D Printing and Easy Removal of Transparent Thin Polymeric Sheets and their Application in Solar Cells Encapsulation

3D printing of thin transparent sheets is challenging because transparency is compromised due to surface defects. Herein, the 3D printing of thin, transparent, and smooth polymeric surfaces is demonstrated with easy removal and potential application in solar cell encapsulation. Thin disk-shaped objects are printed using a vat-photopolymerization-based 3D-printing technique, and their transparency is measured. The disks are printed directly onto the build plate in three different orientations, whereas a modified build plate is used for each sample. The sample printed on the modified build plate exhibits the highest transparency (≈95%) and is the easiest to remove from the plate. Thin and transparent sheets are prepared with embedded TiO₂ nanoparticles using this approach. TiO₂ addition blocks the ultraviolet-light wavelengths of 200–400 nm, which can reduce phonon generation in silicon solar cells and, thus, lower the panel surface temperature. The results demonstrate an 8 °C reduction in temperature, which can enhance the efficiency of silicon photovoltaics. The proposed method demonstrates the capability of 3D printing of transparent, smooth surfaces with easy removal of thin objects for various applications.     

Infrared holography using a microbolometer array

We describe a series of experiments to demonstrate holography at far-infrared wavelengths using an uncooled microbolometer array. Simple interference patterns and Fresnel zone holograms are recorded with a 10 W cw CO(2) laser illumination in a Mach-Zehnder interferometer setup. A sparse-sampling method is used to sample the hologram at a rate dependent on the bandwidth of the object wavefront rather than the carrier frequency. The samples are then used to reconstruct the complex object wavefront in the hologram plane, which is Fresnel backpropagated for image reconstruction. Uncooled microbolometer arrays are most commonly used in passive mode to image the thermal-blackbody radiation. Their technology has matured to include the wavelength range of far-infrared to submillimeter radiation. The use of microbolometers with active illumination for holography, as described in this paper, suggests their interesting future applications.     

A multimodal approach for individual tracking of people and their belongings

Abstract

In this study, a fully automatic surveillance system for indoor environments which is capable of tracking multiple objects using both visible and thermal band images is proposed. These two modalities are fused to track people and the objects they carry separately using their heat signatures and the owners of the belongings are determined. Fusion of complementary information from different modalities (for example, thermal images are not affected by shadows and there is no thermal reflection or halo effect in visible images) is shown to result in better object detection performance. We use adaptive background modeling and local intensity operation for object detection and the mean-shift tracking algorithm for fully automatic tracking. Trackers are refreshed to resolve potential problems which may occur due to the changes in object’s size, shape and to handle occlusion-split and to detect newly emerging objects as well as objects that leave the scene. The proposed scheme is applied to the abandoned object detection problem and the results are compared with the state of art methods. The results show that the proposed method facilitate individual tracking of objects for various applications, and provide lower false alarm rates compared to the state of art methods when applied to the abandoned object detection problem.     

The preparation of textured Te thin films with pronounced acicular morphology and concomitant high absorptivity

The dependence of surface morphology, and resultant absorptivity, on deposition angle for vacuum deposited Te films has been studied. A deposition angle of 80° was found to yield surface morphologies comprised of acicular crystallites of appropriate dimension and geometry for the trapping of solar incident radiation through multiple reflection. Solar absorptivities across the visible spectrum as high as 95% are manifested by such surfaces.     

Interior imaging using spatial frequency sampling

A new acoustic synthetic aperture geometry is demonstrated in which the image field is sampled in the spatial frequency domain. This means the sampled field is recorded in a particularly convenient form for presentation to the back propagation algorithms used to reconstruct the field at the object plane. The method has applications in interior visualization. It is difficult to image the interior of solid objects using lens based imaging systems because a different lens geometry must be used for each distinct object material. The system presented here overcomes such problems since both the angular variation of the transmission coefficient at the object-water interface and the aberrations introduced by the velocity mismatch at the object surface may be readily compensated for in the back propagation routine. Experimental results are presented illustrating the detection of four half wavelength diameter defects, spaced by three wavelengths, at a depth of eight wavelengths below the surface of an aluminum block.     

Rainbow radiating single-crystal Ag nanowire nanoantenna

Optical antennas interface an object with optical radiation and boost the absorption and emission of light by the objects through the antenna modes. It has been much desired to enhance both excitation and emission processes of the quantum emitters as well as to interface multiwavelength channels for many nano-optical applications. Here we report the experimental implementation of an optical antenna operating in the full visible range via surface plasmon currents induced in a defect-free single-crystalline Ag nanowire (NW). With its atomically flat surface, the long Ag NW reliably establishes multiple plasmonic resonances and produces a unique rainbow antenna radiation in the Fresnel region. Detailed antenna radiation properties, such as radiating near-field patterns and polarization states, were experimentally examined and precisely analyzed by numerical simulations and antenna theory. The multiresonant Ag NW nanoantenna will find superb applications in nano-optical spectroscopy, high-resolution nanoimaging, photovoltaics, and nonlinear signal conversion.     

Characterization of materials for a vacuum-ultraviolet polarization analyzer

Astronomical measurements in the vacuum-UV spectral region (30-190 nm) require space instruments and techniques that are at the boundary between visible and x-ray techniques. In vacuum-UV polarimetry, transmission polarizers must be replaced by reflection polarizers because of the lack of transparent materials in the 30-105-nm range. The general features of a single reflection polarization analyzer for the vacuum UV are introduced and described, with particular emphasis on astrophysical applications. In particular, we discuss the trade-off conditions for optimum polarization and throughput of a single-reflection surface in the vacuum UV, introducing a quality factor parameter. The polarization performances of various reflecting materials are obtained with a reflecting vacuum-UV polarization analyzer laboratory model designed and built to measure the state of linear polarization. On the basis of a comparison of the quality factors, calcium fluoride is determined to be the best-performing material. Finally, we discuss the laboratory polarimetric characterization of the material properties for astronomical application of the polarization analyzer.     

Combined enhanced fluorescence and label-free biomolecular detection with a photonic crystal surface

A 2D photonic crystal surface with a different period in each lateral direction is demonstrated to detect biomolecules using two distinct sensing modalities. The sensing mechanisms both rely on the generation of a resonant reflection peak at one of two specific wavelengths, depending on the polarization of light that is incident on the photonic crystal. One polarization results in a resonant reflection peak in the visible spectrum to coincide with the excitation wavelength of a fluorophore, while the orthogonal polarization results in a resonant reflection peak at an infrared wavelength which is used for label-free detection of adsorbed biomolecules. The photonic crystal resonance for fluorescence excitation causes enhanced near fields at the structure surface, resulting in increased signal from fluorophores within 100 nm of the device surface. Label-free detection is performed by illuminating the photonic crystal with white light and monitoring shifts in the peak reflected wavelength of the infrared resonance with a high-resolution imaging detection instrument. Rigorous coupled-wave analysis was used to determine optimal dimensions for the photonic crystal structure, and devices were fabricated using a polymer-based nanoreplica molding approach. Fluorescence-based and label-free detection were demonstrated using arrays of spots of dye-conjugated streptavidin. Quantification of the fluorescent signal showed that the fluorescence output from protein spots on the photonic crystal was increased by up to a factor of 35, and deposited spots were also imaged in the label-free detection mode.     

Degree of linear polarization in spectral radiances from water-viewing infrared radiometers

Infrared radiances from water become partially polarized at oblique viewing angles through both emission and reflection. I describe computer simulations that show how the state of polarization for water varies with environmental conditions over a wavelength range of 3-15 mum with 0.05-mum resolution. Polarization at wavelengths longer than approximately 4 mum generally is negative (p, or vertical) and increases with incidence angle up to approximately 75 degrees , beyond which the horizontally polarized reflected atmospheric radiance begins to dominate the surface emission. The highest p polarization (~4-10%) is found in the atmospheric window regions of approximately 4-5 and 8-14 mum. In the 3-5-mum spectral band, especially between 3 and 4 mum, reflected atmospheric radiance usually is greater than surface emission, resulting in a net s polarization (horizontal). The results of these simulations agree well with broadband measurements of the degree of polarization for a water surface viewed at nadir angles of 0-75 degrees .     

Automated measurement of the refractive index of fluids

A method for automating refractive-index measurements of fluids has been developed. An encoded rotation stage and position-sensitive detector enable automated reading of angles typically acquired by visual means. Two tunable lasers are used to obtain index measurements at ten discrete wavelengths across the visible spectrum. This method has been implemented on a Hilger-Chance refractometer from which the bulk refractive-index values for various transparent fluids have been measured. An index measurement accuracy of better than one part in the fourth decimal place for distilled water and a few parts in the fourth decimal place for higher index fluids is obtained.     

Real-time objects recognition by photoanisotropic copies

The method of object recognition is described by the example of objects of amplitude transparent type. The method is to obtain a photoanisotropic copy of object images on polarization-sensitive material. At consequent illumination of the photoanisotropic copy with a parallel circularly polarized beam of nonactinic light, the transmitted light becomes elliptically polarized. It is shown that the characteristics of the summary polarization ellipse in the Fraunhofer diffraction region uniquely identify the given object. The real-time determination of the characteristics of the summary polarization ellipse is made by means of diffraction gratings of anisotropic profile and by comparison of these characteristics with etalon from the database.     

Improved broadband and quasi-omnidirectional anti-reflection properties with biomimetic silicon nanostructures

Nature routinely produces nanostructured surfaces with useful properties, such as the self-cleaning lotus leaf, the colour of the butterfly wing, the photoreceptor in brittlestar and the anti-reflection observed in the moth eye. Scientists and engineers have been able to mimic some of these natural structures in the laboratory and in real-world applications. Here, we report a simple aperiodic array of silicon nanotips on a 6-inch wafer with a sub-wavelength structure that can suppress the reflection of light at a range of wavelengths from the ultraviolet, through the visible part of the spectrum, to the terahertz region. Reflection is suppressed for a wide range of angles of incidence and for both s- and p-polarized light. The antireflection properties of the silicon result from changes in the refractive index caused by variations in the height of the silicon nanotips, and can be simulated with models that have been used to explain the low reflection from moth eyes. The improved anti-reflection properties of the surfaces could have applications in renewable energy and electro-optical devices for the military.     

Polarized pulse waves in random discrete scatterers

In recent years there has been increasing interest in the use of polarization for imaging objects in a cluttered environment. Examples are optical imaging through clouds, optical detection of objects in a biological medium, and microwave detection of objects in clutter. We extend previous studies of continuous-wave scattering to pulse-polarization scattering in discrete scatterers. We solve the time-dependent vector radiative transfer equation for a plane-parallel medium by using Mie scattering and the discrete ordinates method. The time-dependent degree of polarization and cross-polarization discrimination are calculated and verify the advantages of circular over linear polarization in maintaining greater copolarized components rather than cross-polarized components.     

Color holography to produce highly realistic three-dimensional images

The 1964 publication by Emmett Leith and Juris Upatnieks [J. Opt. Soc. Am. 54, 1295 (1964)] introduced the possibility of using holograms to record three-dimensional (3D) objects. Since then, there has been an interest in creating display holograms, i.e., holograms primarily produced to show objects in 3D. More recently, full color holography has become a reality, which was predicted in the 1964 paper. To record a hologram in which both the 3D shape and the color of the object are accurately reproduced, at least three laser wavelengths are needed. By computer simulation of the holographic color rendering process, the required amount of laser wavelengths and their distribution within the visible electromagnetic spectrum have been investigated. The quality of a color hologram also depends on the properties of the recording material. The demand on a panchromatic material for color holography is described. Recording techniques for color holograms are presented as well as the future of color holography as the perfect 3D imaging technique.     

Phase Shift Effects in Fabry-Perot Interferometry

A method is demonstrated for utilizing in Fabry-Perot interferometry the data on reflection phase shift dispersion obtained from fringes of equal chromatic order. Unknown wavelengths can be calculated from the Fabry-Perot patterns obtained with a large etalon spacing, even without prior knowledge of the phase shift of the reflecting surfaces. When the theoretical phase shift as a function of wavelength is known approximately, then the correct orders of interference can be determined for both the Fabry-Perot fringes and fringes of equal chromatic order. From the wavelengths of the latter the phase shift dispersion can be measured to an accuracy of about 10 A. The method is especially useful for reflectors with large dispersion of phase shift, such as multilayers. Results in the visible spectrum are reported for aluminum films and a pair of dielectric 15-layer broadband reflectors.     

Measurements and simulations of polarization states of underwater light in clear oceanic waters

Polarization states of the underwater light field were measured by a hyperspectral and multiangular polarimeter and a video polarimeter under various atmospheric, surface, and water conditions, as well as solar and viewing geometries, in clear oceanic waters near Port Aransas, Texas. Some of the first comprehensive comparisons were made between the measured polarized light, including the degree and angle of linear polarization and linear Stokes parameters (Q and U), and those from Monte Carlo simulations that used concurrently measured water inherent optical properties and particle volume scattering functions as input. For selected wavelengths in the visible spectrum, measured and model-simulated polarization characteristics were found to be consistent in most cases. Measured degree and angle of linear polarization are found to be largely determined by an in-water single-scattering model. Model simulations suggest that the degree of linear polarization (DoLP) at horizontal viewing directions is highly dependent on the viewing azimuth angle for a low solar elevation. This implies that animals can use the DoLP signal for orientation.     

Polarization-dependent vacuum-ultraviolet reflectometry using elliptically polarized synchrotron radiation

In the laboratory of the Physikalisch-Technische Bundesanstalt (PTB) at the Berlin electron-storage ring BESSY II, a procedure has been developed to investigate the dependence of vacuum-ultraviolet reflection on polarization. It is based on characterizing the elliptically polarized synchrotron radiation at PTB's normal-incidence monochromator beamline for reflectometry by means of polarization-sensitive photodetectors. For this purpose, the polarization dependency in the detector responsivity was determined at a small, low, solid angle of acceptance for the synchrotron radiation, i.e., within the orbital plane of the storage ring where the degree of linear polarization is known to be almost 100%. Our method allows the polarization dependence of reflection samples to be measured with relative standard uncertainties ranging from 2.4% to 11% in the spectral range between 60 and 160 nm. The method has been applied to the optimization of polarizing mirrors at the Lyman-alpha wavelength of 121.6 nm.     

Focused Light Study of Surface Plasmons on Ag and the Anomalous Skin Effect

Abstract

The optical response of thin Ag films has been studied using surface plasmon-polaritons excited via a novel focused light attenuated total reflection technique, which is described in detail. The results show that the anomalous skin effect is important in room temperature condensed polycrystalline Ag films at visible wavelengths, especially with the removal of defects on annealing the films. Incorporation of the ASE theory and surface scattering into the reflectivity model provides better agreement between theory and experiment.