Near-infrared Wavelength Region Research Articles

Measurements of complex refractive indices of metals at several wavelengths by frustrated total internal reflection due to surface plasmon resonance

By employing a modified Otto's configuration for measuring surface plasmon resonance that has been proposed by Bliokh and his coworkers [Appl. Phys. Lett.89, 021908 (2006)APPLAB0003-695110.1063/1.2220540] we have obtained complex refractive indices of metals at several wavelengths. We demonstrate that the configuration has high potential for obtaining dispersion relations of metal conductors in bulk samples as well as in thin films from the visible to the near-infrared wavelength region. Furthermore, we show that the configuration enables us to obtain the complex refractive indices of metals or a thickness or refractive index of a dielectric layer on the metal at different points simultaneously. We have constructed a measurement system and carried out basic experiments. The experimental results agreed well with numerically simulated values or published ones.

Infrared photocurrent response of charge-transfer exciton in polymer bulk heterojunction

We study the charge-transfer exciton absorption and photocurrent response in solution-processed bulk heterojunction based on poly(3-hexylthiophene) donor and (6,6)-phenyl-C61-butyric acid methyl ester acceptor in the near-infrared wavelength region. While the exciton absorption exists only for wavelength below 650nm, direct generation of charge-transfer exciton formed between the donor and acceptor extends the absorption wavelength to 950nm. For films with micrometer thickness, the photon-to-electron conversion efficiency is about 60% at 750nm wavelength under reverse voltage bias and the photocurrent to dark current ratio is about 8.6 at 900nm and remains 3.6 even at 1000nm. Photodetector with high sensitivity covering exclusively the 650–1000nm near infrared region can therefore be made without a low bandgap material. The charge-transfer exciton absorption coefficient and photocurrent sensitivity depend on the annealing condition which controls the donor-acceptor morphology.

Prediction of Yellow-Poplar (Liriodendron Tulipifera) Veneer Stiffness and Bulk Density Using near Infrared Spectroscopy and Multivariate Calibration

This study investigated the feasibility of using near infrared (NIR) spectroscopy and multivariate calibration to predict bulk density and stiffness of 3.2 mm thick yellow poplar veneer strips. Full-range (800–2500 nm) raw NIR spectra and spectra pre-treated using the first derivative method, along with spectra from three other different wavelength windows of 1200–2400 nm, 1800–2400 nm and 1400–2000 nm were regressed against the bulk density (kg m−3) values and the dynamic modulus of elasticity (stiffness; GPa) of the veneers using the projection to latent structures (PLS) method to develop calibration models. All predictive models developed performed well in the prediction of bulk density and stiffness of new test samples that were not included in the calibration models. R2 values ranged from 0.67-0.78 and 0.56-0.72, respectively, for bulk density and stiffness. There was significant improvement in models developed with first derivative spectra over models developed with raw spectra. The models developed using the first derivative used fewer latent variables to achieve predictive models with higher R2 values, lower root mean square errors of prediction (RMSEP) and standard errors of prediction (SEP). Models developed using the full NIR spectral range (800–2500 nm) and the NIR spectral region of 1200–2400 nm performed better than models developed using the restricted NIR wavelength regions of 1800–2400 nm and 1400–2000 nm. However, there was no clear distinction between models developed using the full NIR spectral range and the NIR spectral region of 1200–2400 nm. Overall, models developed with the first derivative pre-processed spectra using the whole NIR spectrum performed best in predictability. The results of this study show the potential of using multivariate data analysis coupled with NIR spectroscopy for on-line sorting and assessment of veneer stiffness prior to the lay-up process in the manufacturing of veneer-based engineered wood products such as plywood, Parallam and laminated veneer lumber.

An Extended 3-D Radiosity–Graphics Combined Model for Studying Thermal-Emission Directionality of Crop Canopy

Radiosity-graphics combined model (RGM) has been proposed to calculate the radiation regime and bidirectional reflectance distribution function of complex 3D scene, which is limited in visible and near-infrared wavelength (0.3-3 mum) region. In this paper, RGM is extended to thermal region (named as TRGM) based on thermal-radiosity theory and thermal-emission directionality of vegetation canopy. The TRGM has been implemented on Microsoft Windows platform, and a parameterization scheme for crop canopies is introduced in this paper. It is then evaluated by comparing with two row-crop directional thermal emission models and one thermal radiative-transfer model. Field experiment data has been used to validate the TRGM for row structural wheat and maize canopies. The root mean square error of directional brightness temperature (DBT) is smaller than 1.0degC for the wheat canopy and 0.5degC for the maize canopy while the canopy DBTs vary more than 4degC. Model sensitivity analyses have also been conducted to illustrate influences of component temperature distribution, component emissivity, incident atmospheric radiation, and canopy structure on the crop canopy DBT.

Fabrication of Near-Infrared Volume Phase Holographic Grism with High Efficiency and High Dispersion, and Its Application to a Wavelength De-multiplexing Device

We designed and developed a volume phase holographic (VPH) grism with high resolution and high efficiency in the near-infrared light domain (1550 nm) to be applied as a wavelength de-multiplexing device of a photonic network. In order to achieve both diffraction efficiency of over 80% and wavelength resolution of 0.08 nm, we newly designed and fabricated a VPH grism with high efficiency and high wavelength resolution in near-infrared wavelength region by sandwiching a VPH grating between prisms. An active feedback control technique was introduced to a two-beam interference exposure system, and we also applied it to a wavelength de-multiplexing device for a photonic network. Consequently, a VPH grism was successfully fabricated with diffraction efficiency of over 80% by employing the Bragg condition of 1550 nm at the incident wavelength from 1530 to 1570 nm, and high refractive index modulation of 0.047 was confirmed.

Mapping leaf chlorophyll and leaf area index using inverse and forward canopy reflectance modeling and SPOT reflectance data

Reflectance data in the green, red and near-infrared wavelength region were acquired by the SPOT high resolution visible and geometric imaging instruments for an agricultural area in Denmark (56°N, 9°E) for the purpose of estimating leaf chlorophyll content ( C ab ) and green leaf area index (LAI). SPOT reflectance observations were atmospherically corrected using aerosol data from MODIS and profiles of air temperature, humidity and ozone from the Atmospheric Infrared Sounder (AIRS), and used as input for the inversion of a canopy reflectance model. Computationally efficient inversion schemes were developed for the retrieval of soil and land cover-specific parameters which were used to build multiple species and site dependent formulations relating the two biophysical properties of interest to vegetation indices or single spectral band reflectances. Subsequently, the family of model generated relationships, each a function of soil background and canopy characteristics , was employed for a fast pixel-wise mapping of C ab and LAI. The biophysical parameter retrieval scheme is completely automated and image-based and solves for the soil background reflectance signal, leaf mesophyll structure, specific dry matter content, Markov clumping characteristics, C ab and LAI without utilizing calibration measurements. Despite the high vulnerability of near-infrared reflectances ( ρ nir) to variations in background properties, an efficient correction for background influences and a strong sensitivity of ρ nir to LAI, caused LAI– ρ nir relationships to be very useful and preferable over LAI–NDVI relationships for LAI prediction when LAI > 2. Reflectances in the green waveband ( ρ green) were chosen for producing maps of C ab . The application of LAI–NDVI, LAI– ρ nir and C ab – ρ green relationships provided reliable quantitative estimates of C ab and LAI for agricultural crops characterized by contrasting architectures and leaf biochemical constituents with overall root mean square deviations between estimates and in-situ measurements of 0.74 for LAI and 5.0 μg cm − 2 for C ab . The results of this study illustrate the non-uniqueness of spectral reflectance relationships and the potential of physically-based inverse and forward canopy reflectance modeling techniques for a reasonably fast and accurate retrieval of key biophysical parameters at regional scales.

Near-Infrared Resonant Nanoshells for Combined Optical Imaging and Photothermal Cancer Therapy

Metal nanoshells are core/shell nanoparticles that can be designed to either strongly absorb or scatter within the near-infrared (NIR) wavelength region ( approximately 650-950 nm). Nanoshells were designed that possess both absorption and scattering properties in the NIR to provide optical contrast for improved diagnostic imaging and, at higher light intensity, rapid heating for photothermal therapy. Using these in a mouse model, we have demonstrated dramatic contrast enhancement for optical coherence tomography (OCT) and effective photothermal ablation of tumors.

Improved near-infrared transparency in sputtered In2O3-based transparent conductive oxide thin films by Zr-doping

Transparent conductive Zr-doped In2O3 (In2−2xZr2xO3) films were deposited on glasses by sputtering method. High mobility of over 80 cm2∕V s was achieved under a carrier density of 1.3−2.9×1020 cm−3 at Zr concentrations (x) of 0.014–0.022, and the film at x=0.022 showed the lowest resistivity of 2.6×10−4 Ω cm. Reflecting the high mobility and the low carrier density, the transparency extended from the visible to the near-infrared (NIR) wavelength region with reduced magnitude of the free-carrier absorption. The results indicate that Zr-doped In2O3 films have a performance advantage for applications that require high conductivity and transparency in NIR wavelength region.

Multichannel Photonic Crystal Wavelength Filter Array for Near-Infrared Wavelengths

Multichannel wavelength filters consisting of 2-D photonic crystal (PhC) for the near-infrared wavelength region (~800 nm) are demonstrated. The filter is a thin-film wavy multilayer structure and is fabricated by the autocloning method, which is based on a radio frequency bias sputtering process. Twelve long-pass-type filter regions are integrated on a common silica substrate. Sharp cutoff characteristics and almost equal channel spacing are experimentally verified. To precisely control the effective lattice constant of PhC, a modulated lattice structure on the substrate is utilized

Genetic variation in leaf pigment, optical and photosynthetic function among diverse phenotypes of Metrosideros polymorpha grown in a common garden

Coordinated variation has been reported for leaf structure, composition and function, across and within species, and theoretically should occur across populations of a species that span an extensive environmental range. We focused on Hawaiian keystone tree species Metrosideros polymorpha, specifically, 13-year old trees grown (2-4 m tall) in a common garden (approximately 1 ha field with 2-3 m between trees) from seeds collected from 14 populations along an altitude-soil age gradient. We determined the genetic component of relationships among specific leaf area (SLA), the concentrations of nitrogen (N) and pigments (chlorophylls, carotenoids, and anthocyanins), and photosynthetic light-use efficiency. These traits showed strong ecotypic variation; SLA declined 35% with increasing source elevation, and area-based concentrations of N, Chl a + b and Car increased by 50, 109 and 96%, respectively. Concentrations expressed on a mass basis were not well related to source elevation. Pigment ratios expressed covariation that suggested an increased capacity for light harvesting at higher source elevation; Chl/N and Car/Chl increased with source elevation, whereas Chl a/b declined; Chl a/b was higher for populations on younger soil, suggesting optimization for low N supply. Parallel trends were found for the photosynthetic reactions; light-saturated quantum yield of photosystem II (Phi (PSII)) and electron transport rate (ETR) increased with source elevation. Correlations of the concentrations of photosynthetic pigments, pigment ratios, and photosynthetic function across the ecotypes indicated a stoichiometric coordination of the components of the light-harvesting antennae and reaction centers. The constellation of coordinated morphological, biochemical and physiological properties was expressed in the leaf reflectance and transmittance properties in the visible and near-infrared wavelength region (400-950 nm), providing an integrated metric of leaf status among and between plant phenotypes.

Multiple Regression Analysis of the Variable Component in the Near-Infrared Region for Type 1 AGN MCG +08-11-011

We propose a new method of analyzing a variable component for type 1 active galactic nuclei (AGNs) in the near-infrared wavelength region. This analysis uses a multiple regression technique and divides the variable component into two components originating in the accretion disk at the center of an AGN and from the dust torus that far surrounds the disk. Applying this analysis to the long-term VHK monitoring data of MCG +08-11-011 that were obtained by the MAGNUM project, we found that the (H - K) color temperature of the dust component is T = 1635 ± 20 K, which agrees with the sublimation temperature of dust grains, and that the time delay of K to H variations is Δt ≈ 6 days, which indicates the existence of a radial temperature gradient in the dust torus. As for the disk component, we found that the power-law spectrum of fν ∝ να in the V to near-infrared HK bands varies with a fixed index of α ≈ -0.1 to +0.4, which is broadly consistent with the irradiated standard disk model. The outer part of the disk therefore extends out to a radial distance where the temperature decreases to radiate the light in the near-infrared.

Direct synthesis of single-walled carbon nanotubes bridging metal electrodes by laser-assisted chemical vapor deposition

Direct synthesis of single-walled carbon nanotubes (SWNTs) bridging prepatterned Mo electrodes has been achieved using laser-assisted chemical vapor deposition (LCVD). The synthesized SWNTs are found predominantly semiconducting. By controlling the spot size of the focused laser beam, synthesis of SWNTs can be achieved in a localized manner, which is governed by the thermal and optical properties of materials as well as the laser parameters. The synthesis process is fast and can be achieved in both far- and near-infrared laser wavelength regions. LCVD method provides a potential approach to in situ remove SWNTs with specific chiralities during the growth.

Optical properties of dye-doped zirconia films by spectroscopic ellipsometry

The optical properties of zirconia films doped with rhodamine 6G and oxazine 725 by the sol–gel process were investigated using spectroscopic ellipsometry (SE). Accurate refractive index n and the extinction coefficient k were determined using a three-oscillator classical Lorentz model in the wavelength range of 300–800 nm. The derived refractive index of dye-doped films exhibited anomalous dispersion in the absorption region. Wavelength tunable output lasing action yellow and near-infrared wavelength region was achieved by DFB configuration using zirconia films doped with R6G and oxazine 725.

High‐Resolution Observations of Multiwavelength Emissions during Two X‐Class White‐Light Flares

We observed two X-class white-light flares (WLFs) on 2003 October 29 (~20:40 UT) and November 2 (~17:16 UT) using the Dunn Solar Telescope (DST) and its High-Order Adaptive Optics (HOAO) system in several wavelengths. The spatial resolution was close to the diffraction limit of DST's 76 cm aperture, and the cadence was as high as 2 s. This is the first time that WLFs have been observed in the near-infrared (NIR) wavelength region. We present a detailed study in this paper comparing photospheric continuum observations during the two events with corresponding line-of-sight magnetograms from the Solar and Heliospheric Observatory (SOHO) Michelson Doppler Imager (MDI) and hard X-ray (HXR) data from the Ramaty High-Energy Solar Spectroscopic Imager (RHESSI). We also discuss several models that provide possible mechanisms to explain these continuum enhancements, especially in the NIR.

Monolithically integrated optical modulator based on polycrystalline Ba0.7Sr0.3TiO3 thin films

Good-quality polycrystalline Ba0.7Sr0.3TiO3 (BST0.7) thin films were deposited on fused silica substrates and Si substrates having a thick amorphous SiO2 layer at a relatively low temperature of 550°C by spin-coating metal organic solutions. The thin films were highly transparent to light in the ultraviolet to near-infrared wavelength regions. The optical propagation loss for a 5-μm-wide and 300-nm-thick polycrystalline waveguide based on BST0.7 was about 17dB∕cm at a wavelength of 632.8nm. An electro-optic Mach-Zehnder interferometer modulator based on the polycrystalline BST0.7 thin film was monolithically integrated on a Si substrate with standard lithography and wet etching. Optical modulation was successfully demonstrated. The estimated electro-optic coefficient (6.7pm∕V) is the highest reported so far for this kind of film deposited on a fused silica substrate or a Si substrate with an amorphous SiO2 layer.

Near-infrared electroluminescence based on perylenediimide-doped tris(8-quinolinolato) aluminum

For the purpose of exploring near-infrared emission, the photoluminescence (PL) and electro- luminescence (EL) of N,N′-bis(neopentyl)-3,4:9,10-perylenebis(dicarboximide) (BNPTCD): tris(8-quinolinolato)aluminum (Alq3) mixed films were investigated. It was found that BNPTCD:Alq3 mixed films exhibit PL in the near-infrared wavelength region and that organic light-emitting diodes using Alq3 highly doped with BNPTCD as an emitting layer exhibit near-infrared EL with a peak at 805 nm originating from BNPTCD aggregates.

Polymer solar cells with low-bandgap polymers blended with C 70-derivative give photocurrent at 1 μm

A new series of low-bandgap alternating polyfluorenes with different donor–acceptor–donor moieties have been synthesized. Electrochemical and optical absorption measurement show that onset bandgaps of these polymers range from 1.2 to 1.5 eV. These polymers, blended with a C 70-derivative as acceptor, are used for solar cell fabrication. Devices show promising photovoltaic properties, and the spectral response of photocurrent covers all visible and near-infrared wavelength regions with its onset extended to 1 μm. The best data gives a photocurrent density of 3.4 mA/cm 2, open circuit voltage of 0.58 V and power conversion efficiency of 0.7% under illumination of AM1.5 (1000 W/m 2) from a solar simulator.

Fabrication of three-dimensional woodpile photonic crystals in a PbSe quantum dot composite material

Incorporating active media into three-dimensional (3D) photonic crystals (PCs) is a useful step towards exploring the functionalities of PCs. Here we report, for the first time, on the fabrication of 3D woodpile PCs with a commercial PbSe quantum dot (QD) composite material by using the two-photon polymerization technique. The fabricated crystals possess photonic band gaps in the near-infrared wavelength region, which have a suppression rate of ~50% in the stacking direction, measured with an angle-resolved Fourier-transform infrared spectrometer. The woodpile structures fabricated under different conditions are also characterized by using a scanning near-field optical microscope, providing a useful feedback towards optimizing the fabrication of 3D woodpile PCs in QD composites.

Synthesis of nanosized Y-type TiOPc by a high gravity method

Oxotitanium phthalocyanine (TiOPc) is an ideal photoconductive material and electrophotographic photoreceptor since it exhibits remarkable absorption and photosensitivity properties in the visible and near-infrared wavelength region, which lead to the generation of free carriers with a high quantum yield [1]. It was reported that Y-type TiOPc had the most excellent photosensitive properties among five typical crystal forms of TiOPc [2–7]. In conventional routes, TiOPc of other crystal forms are usually synthesized and then transformed to Y-type TiOPc by processes such as thermomechanical, solvent, acid treatment and mechanical grinding. Solvent treatment is preferred among these processes due to its convenience and high efficiency where TiOPc is subjected to crystal transformation in the presence of water, aromatic compounds, alcohol, ketone, ether and their derivatives. Since Chen et al. [8] reported that photoconductivity of TiOPc depended on particle size, many works have been directed to obtaining nanosized Y-type TiOPc by different methods. In this paper, Y-type TiOPc is synthesized under high gravity environment created by a rotating packed bed (RPB) reactor, which can greatly intensify micromixing and mass transfer between fluids. Advantages of the high gravity method in synthesizing nanosized inorganic materials have been reported in our previous works [9, 10].

Spectroscopy and energy transfer characteristics of PLZT:Tm 3+ transparent ceramics

Thulium-doped PLZT transparent ceramic is proposed as a new material for photonic applications in the near-infrared wavelength region (1.8 and 1.44 μm). In this work, its structural and spectroscopic characteristics are investigated through ground state absorption, luminescence and excited state lifetime measurements. The results are discussed in terms of energy transfer processes among Tm 3+ ions as the cross relaxation 3H 4, 3H 6 → 3F 4, 3F 4 and the energy migrations 3H 4, 3H 6 → 3H 6, 3H 4 and 3F 4, 3H 6 → 3H 6, 3F 4. Excellent thermal and mechanical properties associated with desirable spectroscopic characteristics suggest that PLZT:Tm 3+ is a potential laser active medium, worth of further development and investigations.