Visible Wavelength Band Research Articles

Characteristics of surface plasmon-polariton waves excited on 2D periodically patterned columnar thin films of silver.

Periodically patterned thin films of slanted silver nanocolumns were deposited by directing a collimated vapor flux of silver toward square and hexagonal gratings of photoresist on glass substrates. Angle-resolved specular-transmittance measurements in the visible and near-infrared wavelength bands on these periodically patterned columnar thin films (CTFs) were carried out to investigate the excitation of surface plasmon-polariton (SPP) waves bound tightly to either the air/CTF or the photoresist/CTF interfaces. The orientation of the propagation vector of the incident p-polarized plane wave with respect to the morphologically significant plane of the CTFs was varied to reveal asymmetric (unidirectional) coupling of Floquet modes to SPP waves. The asymmetric coupling is maximal when the propagation vector of the incident plane wave lies wholly in the morphologically significant plane. Theoretical understanding based on the Bruggeman formalism to homogenize the silver CTFs into hyperbolic biaxial continua is able to explain the experimental observations very well.

Broadband waveplate lenses

We report on lenses that operate over the visible wavelength band from 450 nm to beyond 700 nm, and other lenses that operate over a wide region in the near-infrared from 650 nm to beyond 1000 nm. Lenses were recorded in liquid crystal polymer layers only a few micrometers thick, using laser-based photoalignment and UV photopolymerization. Waveplate lenses allowed focusing and defocusing laser beams depending on the sign of the circularity of laser beam polarization. Diffraction efficiency of recorded waveplate lenses was up to 90% and contrast ratio was up to 500:1.

Polarization-Insensitive Concentric Circular Grating Filters Featuring a Couple of Resonant Peaks

We demonstrate here guided mode resonance (GMR) concentric circular grating filters (CCGFs) on an HfO 2-on-silicon platform. The fabricated devices are realized with a membrane structure by completely removing the silicon substrate beneath the HfO 2 device layer and, thus, can operate across the entire visible wavelength band. Polarization-insensitive properties under normal incidence are experimentally achieved resulting from their rotational symmetry. Substantially differing from 1-D linear grating filters, CCGFs typically feature a couple of resonant peaks for normal incident arbitrary linear polarization, which, from a local point of view, can be physically attributed to the coexistence of azimuthal and radial components of the incident light electric field. The CCGFs here have potential applications, such as polarization-insensitive two-color mixing devices and polarization-insensitive color filtering devices.

Absorption spectrum of gafchromic® EBT2 film with angular rotation

Study of the absorption spectrum in film dosimetry is important because the spectral absorbance of the film is related to the film’s total absorption dose. We investigated the absorption spectra of Gafchromic EBT2 film at various rotational angles in a visible wavelength band. The film was irradiated with 6- MV photon beams for a total dose of 300 cGy. The absorption spectra for different angles of rotation were obtained 24 h after the irradiation and we fitted the spectra by using Lorentzian functions. Two dominant absorption peaks, one at approximately 586 nm (green) and the other at 634 nm (red) were observed. The measured spectrum was decomposed into peaks at 542 nm, 558 nm, 578 nm, 586 nm, 626 nm, 634 nm, and 641 nm. The maximum total area of the red absorption band occurred at 45° (225°) and the minimum at 90° (270°). As the angle of rotation changed, the intensities and integrated areas of the blue and the green peaks also changed with a 180° period, with minima at 90° and 270°, and maxima at 0° and 180°, although the overall absorbance was very low. The spectral peak wavelengths remained constant within ±1.2 nm for all angles. The absorption spectrum of the film should no hysteresis of; spectra taken at 0° and 360° were substantially the same and showed similar behaviors for all rotational angles. The change in the absorbance with the rotational angle of the film affected the dosimetric properties, resulting in rotational variations of the film’s dosimetry in each red-green-blue channel.

Embedded optics and electronics single digital micromirror device-based agile pixel broadband imager and spectrum analyzer for laser beam hotspot detection

To the best of the authors’ knowledge, proposed is the first embedded optics and electronics-based agile pixel broadband optical imager and spectrum analyzer using a single digital micromirror device (DMD) for laser beam hotspot detection. Specifically, the novel design engages both the wavelength-dependent grating structure of the DMD optical chip and its three-tilt-state micromirror programmability to form both an optical spectrum analyzer (OSA) and an agile pixel imager within one basic instrument. Also, to our knowledge, presented for the first time for a DMD chip is a complete mathematical analysis for incident laser beam operations via diffraction theory. Using off-the-shelf components and the Texas Instruments (TI) Lightcrafter DLP chip and drive electronics, embedded engineering of the proposed instrument is implemented for the visible laser wavelength band. Experimental OSA operation of the instrument is demonstrated using classic laser wavelengths of 633 and 532 nm with beam power levels of 20 and 180 mW, respectively. Experimental imaging operation of the instrument compared with a commercial scanning slit-based laser beam profiler is successfully conducted for a 633 nm test laser beam having a hotspot with ∼68% peak-to-null power drop near its center. Applications for the proposed imager-OSA instrument include laser beam testing during laser assembly and manufacturing operations.

Photoluminescence characteristics of the single-host white-light-emitting Sr3−3x/2(VO4)2:xEu (0≤x≤0.3) phosphors for LEDs

In this article, the single-host white-light-emitting Sr3−3x/2(VO4)2:xEu (0≤x≤0.3) phosphors were synthesized by solution combustion method using citric acid as a fuel. The synthesized phosphors were annealed at 850–1150°C for 10h. At first, the photoluminescence (PL) characteristics of the prepared Sr3−3x/2(VO4)2:xEu (0≤x≤0.3) phosphors emit bluish-green light with the maximum wavelength at 510nm and sharp red emission at 611nm under 360nm excitation, due to the charge transfer transitions of VO43− and f–f transitions of Eu3+ ions, respectively. The XRD results show that as the europium concentration is increased, an additional EuVO4 secondary phase is formed, which is efficient red-emitting phosphor. On the other half, the luminescence intensity of VO43−decreases when the annealing temperature increases with emerging of the 5D0–7FJ (J=1, 2) transitions of Eu3+ due to efficient energy transfer from vanadate group to Eu3+. Thus, the Sr3−3x/2(VO4)2:xEu (0≤x≤0.3) phosphors were affected by the Eu3+ contents and annealing temperature. In addition, the optimal composition is found to be Sr2.85Eu0.1(VO4)2 at 1150°C. Our results suggest that the Sr3−3x/2(VO4)2:xEu (0≤x≤0.3) phosphors may be good candidate for a new phosphor to convert the near-UV(ultraviolet) into visible wavelength bands.

Plasmon resonance of silver micro–sphere in fiber taper

We have experimentally studied the plasmon resonance phenomenon of a silver micro-sphere with a diameter of 2.3 μm in cone-shaped air cavity of a hollow fiber taper. To take insight into the plasmon resonance phenomenon, we move the micro-sphere along the fiber and observe the significant shift of the resonance peak. We also explore the light response in both infrared and visible wavelength band by finite difference time domain method. The significant variations of the magnetic and power field distribution are observed. The interesting results imply that the configuration has great potential in optical sensors and color filters.

THE 2011 JUNE 23 STELLAR OCCULTATION BY PLUTO: AIRBORNE AND GROUND OBSERVATIONS

On 2011 June 23, stellar occultations by both Pluto (this work) and Charon (future analysis) were observed from numerous ground stations as well as the Stratospheric Observatory for Infrared Astronomy (SOFIA). This first airborne occultation observation since 1995 with the Kuiper Airborne Observatory resulted in the best occultation chords recorded for the event, in three visible wavelength bands. The data obtained from SOFIA are combined with chords obtained from the ground at the IRTF, the U.S. Naval Observatory Flagstaff Station, and Leeward Community College to give the detailed state of the Pluto-Charon system at the time of the event with a focus on Pluto's atmosphere. The data show a return to the distinct upper and lower atmospheric regions with a knee or kink in the light curve separating them as was observed in 1988, rather than the smoothly transitioning bowl-shaped light curves of recent years. The upper atmosphere is analyzed by fitting a model to all of the light curves, resulting in a half-light radius of 1288 ± 1 km. The lower atmosphere is analyzed using two different methods to provide results under the differing assumptions of particulate haze and a strong thermal gradient as causes for the lower atmospheric diminution of flux. These results are compared with those from past occultations to provide a picture of Pluto's evolving atmosphere. Regardless of which lower atmospheric structure is assumed, results indicate that this part of the atmosphere evolves on short timescales with results changing the light curve structures between 1988 and 2006, and then reverting these changes in 2011 though at significantly higher pressures. Throughout these changes, the upper atmosphere remains remarkably stable in structure, again except for the overall pressure changes. No evidence of onset of atmospheric collapse predicted by frost migration models is seen, and the atmosphere appears to be remaining at a stable pressure level, suggesting it should persist at this full level through New Horizon's flyby in 2015.

Canopy-scale relationships between foliar nitrogen and albedo are not observed in leaf reflectance and transmittance within temperate deciduous tree species

Strong positive correlations between the maximum rate of canopy photosynthesis, canopy-averaged foliar nitrogen concentration, and canopy albedo have been shown in previous studies. While leaf-level relationships between photosynthetic capacity and foliar nitrogen are well documented, it is not clear whether leaf-level relationships between solar-weighted reflectance and nitrogen underlie the canopy-scale patterns. Using an integrating sphere, we measured the reflectance and transmittance (350–2500 nm) of both individual leaves and multileaf stacks. Samples were collected from 12 broadleaf species at the Harvard Forest in central Massachusetts, USA. Across all species, foliar nitrogen (both mass-based nitrogen concentration and area-based nitrogen content) and leaf mass / area ratio were negatively, rather than positively, correlated with solar-weighted reflectance and transmittance in ultraviolet–visible and near-infrared wavelength bands (350–700 nm and 700–2500 nm, respectively). Leaf-level anatomy and biochemistry, therefore, do not appear to drive the canopy-level association between increasing foliar nitrogen content and increasing canopy albedo. This suggests that interactions between leaf optical properties and structural canopy-scale traits that correlate with nitrogen content (perhaps some combination of crown shape, leaf area index, leaf angular distribution, or other structural characteristics of the canopy), may instead underlie the previously observed relationship between nitrogen and canopy-level shortwave albedo.

Photoluminescence properties of Sr 3(VO 4) 2, Sr 2Y 2/3−yEu y(VO 4) 2 and Sr 2Y 2/3−zSm z(VO 4) 2

Photoluminescence properties of Sr 3(VO 4) 2, Sr 2Y 2/3− y Eu y (VO 4) 2 and Sr 2Y 2/3− z Sm z (VO 4) 2 ( y, z = 0, 0.1, 0.2, and 0.3) samples have been investigated. The self-activated Sr 3(VO 4) 2 sample shows the activation and emission bands due to the charge transfer transition of VO 4 3 - . While Sr 2+ is partially substituted by Y 3+, Sr 2Y 2/3 (VO 4) 2 can be used as the good host material for Eu 3+ and Sm 3+ activator-ions. The Eu 3+ and Sm 3+ ions doped Sr 2Y 2/3(VO 4) 2 can be effectively excited by the near UV light with wavelengths of 300 nm, and emit red light around 617 and 604 nm, respectively. Specially, for Sr 2Y 2/3− y Eu y (VO 4) 2, Y 3+ substitution quite enhances the emission intensity and quenching concentration of Eu 3+. For example, Sr 2Y 0.56Eu 0.1(VO 4) 2 has higher emission intensity than Sr 2.85Eu 0.1(VO 4) 2 without Y 3+, and even much higher than Y 1− x Eu x VO 4 which has been extensively used. Our results suggest that Sr 2Y 2/3− y Eu y (VO 4) 2 samples may be good candidate for a new phosphor to convert the UV pump light into visible wavelength bands.

Effect of KI/AgI on the thermal and optical properties of the GeS 2–In 2S 3 chalcogenide glasses

GeS 2–In 2S 3–KI/AgI chalcohalide glasses were prepared by traditional melt-quenching method and the glass-forming region was determined. The dependence of glass properties on composition as formula of (100 − x)(0.75GeS 2–0.25In 2S 3) − x(In 2S 3–KI/AgI) was investigated. The allowed indirect transition of samples was calculated according to the classical Tauc equation. The results show that 50 mol% AgI can be introduced in the glassy matrix while only 40 mol% of KI can be incorporated in the GeS 2–In 2S 3–KI glass system. GeS 2–In 2S 3–AgI glass system has larger density, higher refractive index and better thermal stability, while GeS 2–In 2S 3–KI glass system has shorter visible cut-off wavelength and higher optical band gap.

Single-photon frequency down-conversion experiment

We report the first quantum frequency downconversion experiment. Using the difference frequency generation process in a periodically poled lithium niobate waveguide, we successfully observed the phase-preserved frequency downconversion of a coherent pulse train with an average photon number per pulse of $<$1, from the 0.7-$\mu$m visible wavelength band to the 1.3-$\mu$m telecom band. We expect this technology to become an important tool for flexible photonic quantum networking, including the realization of quantum repeater systems over optical fiber using atom-photon entanglement sources for the visible wavelength bands.

Estimation of oxygen saturation from erythrocytes by high-resolution spectroscopic optical coherence tomography

We demonstrate the feasibility of estimating hemoglobin oxygen saturation (SaO(2)) from erythrocytes over a visible wavelength band using high-resolution spectroscopic optical coherence tomography (SOCT). We take SOCT spectra from individual cells on in vitro human blood phantoms and calculate SaO(2) by fitting the spectra to an analytical model. We also use diffusive spectroscopy (DS) measurements, which is validated by direct measurement of oxygen partial pressure, as a comparison. The result shows a reliable linear association between the SaO(2) derived from SOCT and the DS measurement, indicating this methodology has potential on in vivo applications to evaluate blood oxygenation.

Effects of Spatial and Spectral Resolutions on Fractal Dimensions in Forested Landscapes

Recent work has shown that more research is needed in applying fractal analysis to multi-resolution remote sensing data for landscape characterization. The purpose of this study was to closely examine the impacts that spatial and spectral resolutions have on fractal dimensions using real-world multi-resolution remotely sensed data as opposed to the more conventional single resolution and aggregation approach. The study focused on fractal analysis of forested landscapes in the southeastern United States and Central America. Initially, the effects of spatial resolution on the computed fractal dimensions were examined using data from three instruments with different spatial resolutions. Based on the criteria of mean value and variation within the accepted ranges of fractal dimensions, it was determined that 30-m Landsat TM data were best able to capture the complexity of a forested landscape in Central America compared to 4-m IKONOS data and 250-m MODIS data. Also, among the spectral bands of Landsat TM images of four national forests in the southeastern United States, tests showed that the spatial indices of fractal dimensions are much more distinguishable in the visible bands than they are in the near-mid infrared bands. Thus, based solely on the fractal analysis, the fractal dimensions could have relatively higher chances to distinguish forest characteristics (e.g., stand sizes and species) in the Landsat TM visible wavelength bands than in the near-mid infrared bands. This study has focused on a relative comparison between visible and near-mid infrared wavelength bands; however it will be important to study in the future the effect of a combination of those bands such as the Normalized Difference Vegetation Index (NDVI) on fractal dimensions of forested landscapes.

Bend insensitive optical fiber with ultralow bending loss in the visible wavelength band

We report experimental development of a single-mode optical fiber optimized for a wavelength band of 400 nm to 700 nm, which showed negligible bending loss of 0.09 dB/loop at 400 nm and 0.23 dB/loop at 633 nm for a loop diameter of 10 mm.

Studies of stratospheric ozone layer from near-earth orbit utilizing ultraviolet polarimeter

Stratospheric aerosols (at altitudes above 30 km) play an important role in formation of the thermal condition and the ozone layer thickness. Nowadays, there are data on optical thickness of aerosols at these altitudes in the visible wavelength band received by spectrophotometric methods. At the same time, the origin of these aerosols, as well as particle distribution function, has not been completely studied (the real part of refractive exponent). Amongst remote methods, only polarization measurements (data gaining of changing light polarization reflected by the Earth atmosphere from a phase angle) allow to gain these characteristics. The principal astronomic observatory of the NAS of Ukraine proposes to carry out the polarization measurements of the stratospheric aerosol at a wavelength of approximately 260 nm from near-earth orbit. Currently, NASA is carrying out a similar experiment; however, these measurements are being carried out in the wavelength interval above 412 nm. Their data are formed basically by dispersed light of the troposphere and its reflection from the Earth surface and clouds. Our experiment makes it possible to exclude the influence of troposphere and underlying surface, and explore the light flux dispersed by the Earth atmosphere at altitudes above 30 km.

Simulation of subwavelength metallic gratings using a new implementation of the recursive convolution finite-difference time-domain algorithm

We introduce a new implementation of the finite-difference time-domain (FDTD) algorithm with recursive convolution (RC) for first-order Drude metals. We implemented RC for both Maxwell's equations for light polarized in the plane of incidence (TM mode) and the wave equation for light polarized normal to the plane of incidence (TE mode). We computed the Drude parameters at each wavelength using the measured value of the dielectric constant as a function of the spatial and temporal discretization to ensure both the accuracy of the material model and algorithm stability. For the TE mode, where Maxwell's equations reduce to the wave equation (even in a region of nonuniform permittivity) we introduced a wave equation formulation of RC-FDTD. This greatly reduces the computational cost. We used our methods to compute the diffraction characteristics of metallic gratings in the visible wavelength band and compared our results with frequency-domain calculations.

Application of narrow-band TBVI in estimating fruit yield in citrus

Recently, the two-band vegetation index (TBVI) has been developed as an alternative to the conventional normalised difference vegetation index (NDVI) for assessing various agricultural crop characteristics. The potential of the narrow-band TBVI derived from airborne hyperspectral imagery to predict fruit yield in citrus was examined in this paper. Hyperspectral images in 72 visible and near-infrared (NIR) wavelength bands (from 407 to 898 nm) were acquired three times over a citrus orchard in Japan on May 26, June 21 and July 21, 2005 by an airborne imaging spectrometer for applications (AISA) Eagle system. Spectra for individual trees were obtained by averaging reflectance values of all pixels on each citrus canopy that was recognised from the acquired images. Narrow-band TBVI involving all possible two-band combinations of 72 channels was tested in terms of its potential to predict fruit yield on individual citrus trees. Multiple linear regression (MLR) models based on several significant wavelengths were also evaluated. Results indicate that the hyperspectral image obtained on May 26 demonstrated the highest correlation with citrus yield. This suggests that canopy features at the fast vegetation growth stage provide more relevant information on the yield variability among individual trees. Moreover, the TBVI showed a greater potential than the simple combination of several significant wavelengths in providing an indication of fruit yield in citrus. Based on the hyperspectral image obtained on May 26, the TBVIs calculated for the red and NIR regions were significantly correlated with citrus yield, and the “red edge” region showed a higher relevance than the red region in calculating the TBVI that has a higher correlation with citrus yield. The TBVI based on the 823 nm (NIR) and 728 nm (red edge) wavelengths was found to provide optimal citrus yield information ( R 2 =0.5795, RRMSE=0.6636). Due to the significant effect of the alternate bearing mechanism on individual citrus trees, canopy size cannot be used as a single predictor for citrus yield ( R 2 =0.4516, RRMSE=0.8298). However, by incorporating the TBVI and canopy size into the model, the prediction result was greatly improved ( R 2 =0.6913, RRMSE=0.6071). This suggests that the yield on individual trees is determined by the combined effect of the TBVI and canopy size. This study demonstrates the potential of narrow-band TBVI derived from airborne hyperspectral imagery to predict the fruit yield in citrus. Yield estimates can provide valuable information for forecasting yields, planning harvest schedules and generating prescription maps for tree-specific application of alternate bearing control measures and other management practices.

Ground-based near infrared spectroscopy of Jupiter's ring and moons

The backscattered reflectivity of Jupiter's ring has been previously measured over distinct visible and near infrared wavelength bands by a number of ground-based and spaceborne instruments. We present spectra of Jupiter's main ring from 2.21–2.46 μm taken with the NIRSPEC spectrometer at the W.M. Keck observatory. At these wavelengths, scattered light from Jupiter is minimal due to the strong absorption of methane in the planet's atmosphere. We find an overall flat spectral slope over this wavelength interval, except for a possible red slope shortward of 2.25 μm. We extended the spectral coverage of the ring to shorter wavelengths by adding a narrow-band image at 1.64 μm, and show results from 2.27-μm images over phase angles of 1.2°–11.0°. Our images at 1.64 and 2.27 μm reveal that the halo contribution is stronger at the shorter wavelength, possibly due to the redder spectrum of the ring parent bodies as compared with the halo dust component. We find no variation in main ring reflectivity over the 1.2°–11.0° phase angle range at 2.27 μm. We use adaptive optics imaging at the longer wavelength L′ band (3.4–4.1 μm) to determine a 2- σ upper limit of 22 m of vertically-integrated I / F . Our observing campaign also produced an L′ image of Callisto, showing a darker leading hemisphere, and a spectrum of Amalthea over the 2.2–2.5 and 2.85–3.03 μm ranges, showing deep 3-μm absorption.

Room temperature synthesized GaAs quantum dot embedded in SiO 2 composite film

Nano-composite films, consisting of GaAs quantum dots embedded in SiO 2 glass matrix, were fabricated and analyzed in terms of their structural, chemical, and optical properties. In order to fabricate uniformly dispersed GaAs quantum dots in the SiO 2 matrix, composite films consisting of alternate layers of GaAs and SiO 2 were deposited in the manner of a superlattice by radio frequency magnetron sputter deposition. In order to control the size of GaAs quantum dots the thickness of GaAs in individual layers was varied at a fixed GaAs total volume fraction. Transmission electron microscopy and X-ray photoelectron spectroscopy analyses revealed that GaAs quantum dots were in crystalline phase and in the form of a Ga–As compound. From optical absorbance measurements, absorption edges at visible wavelength band were determined and compared among composite films of varying GaAs nominal thickness. The amount of blue shift of the absorption edge increased with decreasing GaAs nominal thickness, i.e. quantum dots size. Band gaps of the composite films determined from Tauc plots and photoluminescence measurements exhibited a linear decrease with increasing GaAs nominal thickness. Large values of third-order non-linear optical susceptibility χ (3) (∼ 7.6 × 10 − 14 m 2/V 2 at 488 nm), which were believed to originate due to the state filling effect, were observed.