Entire Visible Wavelength Research Articles

Broadband full-color monolithic InGaN light-emitting diodes by self-assembled InGaN quantum dots

We have presented broadband full-color monolithic InGaN light-emitting diodes (LEDs) by self-assembled InGaN quantum dots (QDs) using metal organic chemical vapor deposition (MOCVD). The electroluminescence spectra of the InGaN QDs LEDs are extremely broad span from 410 nm to 720 nm with a line-width of 164 nm, covering entire visible wavelength range. A color temperature of 3370 K and a color rendering index of 69.3 have been achieved. Temperature-dependent photoluminescence measurements reveal a strong carriers localization effect of the InGaN QDs layer by obvious blue-shift of emission peak from 50 K to 300 K. The broadband luminescence spectrum is believed to be attributed to the injected carriers captured by the different localized states of InGaN QDs with various sizes, shapes and indium compositions, leading to a full visible color emission. The successful realization of our broadband InGaN QDs LEDs provide a convenient and practical method for the fabrication of GaN-based monolithic full-color LEDs in wafer scale.

(Al,Ga)Ox Microwire Ensembles on Si Exhibiting Luminescence over the Entire Visible Wavelength Range

(Al,Ga)O<i>_x</i> Microwire Ensembles on Si Exhibiting Luminescence over the Entire Visible Wavelength Range

Structural and solid state properties of l-leucinium p-toluenesulfonate monohydrate: an amino acid tosylate NLO crystal

An amino acid tosylate NLO crystal, l-leucinium p-toluenesulfonate monohydrate (LLPT) was synthesized and single crystal was grown by slow evaporation solution growth technique. The single crystal XRD study was employed to evaluate the structural properties and found good agreement with the reported values. The compound crystallized in the non-centrosymmetric orthorhombic cell setting with P212121 space group. The crystal packing is dominated by N–H···O and O–H···O hydrogen bonds leading to number of chain and ring motifs. These motifs are arranged in tandem along ab-plane of the crystal leading to alternate hydrophilic and hydrophobic layers along c-axis of the unit cell. Vibrational spectral measurements (FTIR and FT-Raman) shows the different functional groups and strength of the hydrogen bonds. The optical cutoff wavelength and absorbance studies of the compound have been examined by UV–Vis–NIR absorbance spectrum. The material did not show any significance absorption in the entire visible region and cutoff wavelength at 283 nm. The emission property of the material was analyzed by Photoluminescence study and the material has ultraviolet emission. TGA/DTA studies show that the material became anhydrous around 73 °C and start melting at 169 °C. Vicker’s microhardness test show that LLPT belongs to soft category material. The dielectric behaviour and solid state parameters of the compound were analyzed at room temperature by dielectric studies with varying frequency. Kurtz and Perry SHG tests confirms the materials ability to green emission when exposed to a laser with wavelength of 1064 nm and the SHG efficiency of 0.5 times the KDP.

Lithography-free wide-angle antireflective self-cleaning silicon nanocones.

Low-cost, wideband, and wide-angle antireflective layers are of prime importance to photovoltaic and other optoelectronic applications. We report a novel fabrication methodology of random textured silicon nanocones (SiNCs) array through metal-assisted chemical etching combined with oxidation. The optical properties of the fabricated structure are studied theoretically and experimentally. The random textured SiNCs array showed very promising broadband antireflective properties through the entire visible wavelength range at different incident angles up to ±60°. In addition, the nanostructures inherently could become self-cleaning due to the high contact angle. This random cheap textured SiNCs array increases the absorption efficiency of photodetectors and reduces its cost.

Chemically Responsive Polymer Inverse-Opal Photonic Crystal Films Created by a Self-Assembly Method

The synthesis of poly-2-hydroxyethyl methacrylate inverse-opal hydrogel (IOHGPHEMA) was realized by capillary-force-induced in situ polymerization in a polystyrene colloidal crystal template. The created IOHGPHEMA films show brilliant blue-violet color when they are immersed in deionized water and reach swelling equilibrium. The stop band of the IOHGPHEMA films can be tuned within the entire visible wavelength range by immersing them into different chemical solutions, such as aldehydes, ketones, amides, dimethyl sulfoxide, and alcohols. The extent of the reflective peak shift not only depends on the number of hydrogen band donors but also on the chain length and structure of the chemicals and their concentration. Since the IOHGPHEMA films have different reflectance spectra and structural colors in response to different compounds of the same series, this provides a potential way to visually detect homologues and other compounds with similar structure and properties. This simple, yet effective, method also ...

The IAG solar flux atlas: Accurate wavelengths and absolute convective blueshift in standard solar spectra

We present a new solar flux atlas with the aim to understand wavelength precision and accuracy in solar benchmark data. The atlas covers the wavelength range 405--2300 nm and was observed at the Institut f\"ur Astrophysik, G\"ottingen (IAG) with a Fourier Transform Spectrograph. In contrast to other FTS atlases, the entire visible wavelength range was observed simultaneously using only one spectrograph setting. We compare the wavelength solution of the new atlas to the Kitt Peak solar flux atlases and to the HARPS frequency-comb calibrated solar atlas. Comparison reveals systematics in the two Kitt Peak FTS atlases resulting from their wavelength scale construction, and shows consistency between the IAG and the HARPS atlas. We conclude that the IAG atlas is precise and accurate on the order of $\pm 10$ m s$^{-1}$ in the wavelength range 405--1065 nm while the Kitt Peak atlases show deviations as large as several ten to 100 m s$^{-1}$. We determine absolute convective blueshift across the spectrum from the IAG atlas and report slight differences relative to results from the Kitt Peak atlas that we attribute to the differences between wavelength scales. We conclude that benchmark solar data with accurate wavelength solution are crucial to better understand the effect of convection on stellar RV measurements, which is one of the main limitations of Doppler spectroscopy at m s$^{-1}$ precision.

Organic–inorganic hybrid inverted photodiode with planar heterojunction for achieving low dark current and high detectivity

In this study, the strategy of using an organic–inorganic hybrid planar heterojunction consisting of polymeric semiconductors and inorganic nanocrystals is introduced to realize a high-performance hybrid photodiode (HPD) with low dark current and high detectivity. To prevent undesired charge injection under the reverse bias condition, which is the major dark current source of the photodiode, a well-defined planar heterojunction is strategically constructed via smart solution process techniques. The optimized HPD renders a low dark current of ∼10−5 mA cm−2 at −5 V and ∼10−6 mA cm−2 at −1 V, as well as a high detectivity ∼1012 Jones across the entire visible wavelength range. Furthermore, excellent photocurrent stability is demonstrated under continuous light exposure. We believe that the solution-processed planar heterojunction with inverted structure can be an attractive alternative diode structure for fabricating high-performance HPDs, which usually suffer from high dark current issues.

Two-dimensional metal-dielectric hybrid-structured film with titanium oxide for enhanced visible light absorption and photo-catalytic application

Two-dimensional design based on ultrathin TiO2 film for enhanced visible light absorption and photo-catalytic applications is reported, which mainly consists of three layers of gold film, TiO2 film, and gold nanoparticles (Au NPs). The Au and TiO2 films are produced by e-beam evaporation and atomic layer deposition, respectively, in a carefully controlled way to minimize surface roughness. As compared with bare TiO2 film, the Au NPs/TiO2/Au film significantly increased the photoactivity over the entire UV and visible wavelength range. The Au film increases the light absorption in the UV region with TiO2 acting as an impedance-matching layer, while the Au NPs increase the light absorption in the visible region due to the plasmonic resonance effects, increasing the photocurrent under visible light. 3D numerical simulation results suggest that the Au film also plays an important role in enhancing the electrical field intensity at the TiO2 film in contact with Au NPs, by efficient excitation of localized surface plasmon resonances, thereby, contributing to the enhanced photoactivity of the film in the visible range. This simple system may serve as an efficient platform for solar energy conversion utilizing the whole UV-visible range of solar spectrum based on two-dimensional plasmonic photoelectrodes.

Tri-color composite volume H-PDLC grating and its application to 3D color autostereoscopic display.

A tri-color composite volume holographic polymer dispersed liquid crystal (H-PDLC) grating and its application to 3-dimensional (3D) color autostereoscopic display are reported in this paper. The composite volume H-PDLC grating consists of three different period volume H-PDLC sub-gratings. The longer period diffracts red light, the medium period diffracts the green light, and the shorter period diffracts the blue light. To record three different period gratings simultaneously, two photoinitiators are employed. The first initiator consists of methylene blue and p-toluenesulfonic acid and the second initiator is composed of Rose Bengal and N-phenyglycine. In this case, the holographic recording medium is sensitive to entire visible wavelengths, including red, green, and blue so that the tri-color composite grating can be written simultaneously by harnessing three different color laser beams. In the experiment, the red beam comes from a He-Ne laser with an output wavelength of 632.8 nm, the green beam comes from a Verdi solid state laser with an output wavelength of 532 nm, and the blue beam comes from a He-Cd laser with an output wavelength of 441.6 nm. The experimental results show that diffraction efficiencies corresponding to red, green, and blue colors are 57%, 75% and 33%, respectively. Although this diffraction efficiency is not perfect, it is high enough to demonstrate the effect of 3D color autostereoscopic display.

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.

The Pillars of Creation revisited with MUSE: gas kinematics and high-mass stellar feedback traced by optical spectroscopy

Integral field unit (IFU) data of the iconic Pillars of Creation in M16 are presented. The ionisation structure of the pillars was studied in great detail over almost the entire visible wavelength range, and maps of the relevant physical parameters, e.g. extinction, electron density, electron temperature, line-of-sight velocity of the ionised and neutral gas are shown. In agreement with previous authors, we find that the pillar tips are being ionised and photo-evaporated by the massive members of the nearby cluster NGC 6611. They display a stratified ionisation structure where the emission lines peak in a descending order according to their ionisation energies. The IFU data allowed us to analyse the kinematics of the photo-evaporative flow in terms of the stratified ionisation structure, and we find that, in agreement with simulations, the photo-evaporative flow is traced by a blueshift in the position-velocity profile. The gas kinematics and ionisation structure have allowed us to produce a sketch of the 3D geometry of the Pillars, positioning the pillars with respect to the ionising cluster stars. We use a novel method to detect a previously unknown bipolar outflow at the tip of the middle pillar and suggest that it has an embedded protostar as its driving source. Furthermore we identify a candidate outflow in the leftmost pillar. With the derived physical parameters and ionic abundances, we estimate a mass loss rate due to the photo-evaporative flow of 70 M$_{\odot}$ Myr$^{-1}$ which yields an expected lifetime of approximately 3 Myr.

Shape-dependent light scattering properties of subwavelength silicon nanoblocks.

We explore the shape-dependent light scattering properties of silicon (Si) nanoblocks and their physical origin. These high-refractive-index nanostructures are easily fabricated using planar fabrication technologies and support strong, leaky-mode resonances that enable light manipulation beyond the optical diffraction limit. Dark-field microscopy and a numerical modal analysis show that the nanoblocks can be viewed as truncated Si waveguides, and the waveguide dispersion strongly controls the resonant properties. This explains why the lowest-order transverse magnetic (TM01) mode resonance can be widely tuned over the entire visible wavelength range depending on the nanoblock length, whereas the wavelength-scale TM11 mode resonance does not change greatly. For sufficiently short lengths, the TM01 and TM11 modes can be made to spectrally overlap, and a substantial scattering efficiency, which is defined as the ratio of the scattering cross section to the physical cross section of the nanoblock, of ∼9.95, approaching the theoretical lowest-order single-channel scattering limit, is achievable. Control over the subwavelength-scale leaky-mode resonance allows Si nanoblocks to generate vivid structural color, manipulate forward and backward scattering, and act as excellent photonic artificial atoms for metasurfaces.

Blackening of metals using femtosecond fiber laser.

This study presents an unprecedented high throughput processing for super-blackening and superhydrophobic/hydrophilic surface on both planar and nonplanar metals surfaces. By using a high pulse repetition rate femtosecond (fs) fiber laser, a light trapping microstructure and nanostructure is generated to absorb light from UV, visible to long-wave infrared spectral region. Different types of surface structures are produced with varying laser scanning conditions (scanning speed and pitch). The modified surface morphologies are characterized using scanning electron microscope and the blackening effect is investigated through spectral measurements. Spectral measurements show that the reflectance of the processed materials decreases sharply in a wide wavelength range and the decrease occurs at different rates for different scanning pitches and speeds. Above 98% absorption over the entire visible wavelength region and above 95% absorption over the near-infrared, middle-wave infrared and long-wave infrared regions range has been demonstrated for the surface structures, and the absorption for specific wavelengths can go above 99%. Furthermore, the processing efficiency of this fs fiber laser blackening technique is 1 order of magnitude higher than that of solid-state fs laser and 4 times higher than that of picosecond (ps) laser. Further increasing of the throughput is expected by using higher repetition and higher scanning speed. This technology offers the great potential in applications such as constructing sensitive detectors and sensors, solar energy absorber, and biomedicine.

A model for estimating size-fractioned phytoplankton absorption coefficients in coastal and oceanic waters from satellite data

One of the central goals of using satellite ocean color remote sensing is to detect and identify different phytoplankton groups (size classes) and describe their variability continuously and synoptically for various applications including marine ecosystem dynamics, carbon and biogeochemical cycles, and related fields in oceanography. Taking the advantage of phytoplankton communities having distinct optical properties, this study presents a new model to explicitly detect and differentiate between three phytoplankton size-classes namely, picophytoplankton, nanophytoplankton, and microphytoplankton, based on distinct differences in the optical signatures of these phytoplankton groups in a wide variety of coastal and oceanic waters. The model is based on the assumption that there is a significant relationship between chlorophyll-a concentration, and total as well as the size-fractioned absorption coefficients of phytoplankton. The new model is validated using three different in-situ datasets collected from a wide variety of locations in the global and regional oceans (including turbid coastal and eutrophic waters) and its results are further compared with those of the existing two- and three-component models. The new model performs better than other models in terms of yielding more accurate estimates of the total and size-dependent phytoplankton absorption coefficients across the entire visible wavelengths. Since satellite observation of ‘ocean color’ as detected by a remote sensor provides an estimate of the chlorophyll-a concentration, commonly used as an index of phytoplankton biomass, the new model is also applied to regional and global images of seasonal climatology over a decade of satellite ocean color observations provided by the MODIS-Aqua sensor. When applied to the MODIS-Aqua images of the Arabian Sea dominated by spatially intense algal blooms, the present model is generally excellent at predicting and describing the spatial distribution of these phytoplankton groups within cyclonic eddies and adjacent regions in the Arabian Sea. Conversely, size-fractioned phytoplankton absorption coefficients derived from global images of seasonal climatology are found to vary depending on the season and ocean basin. These global images imply that when phytoplankton abundance increases, larger size-classes are added incrementally to a background of smaller cells. Further examination of these data showed that picophytoplankton population is generally low, although dominating a major part of the surface ocean during summer and winter. Nanophytoplankton and microphytoplankton populations are high in surface waters of the North and South Atlantic, North and South Pacific Oceans, Arabian Sea, and equatorial region, showing an increasing trend in summer and a decreasing trend in winter in each hemisphere. These results suggest that the new model is an important tool which will inspire further research to investigate different phytoplankton size classes and their variability on regional and global scales.

Modelling the inherent optical properties and estimating the constituents׳ concentrations in turbid and eutrophic waters

Retrieval of the inherent optical properties and estimation of the constituents׳ concentrations from satellite ocean colour data in turbid and eutrophic waters are important as these products provide innovative opportunities for the study of biological and biogeochemical properties in such optically complex waters. This paper intends to develop models to retrieve absorption coefficients of phytoplankton, suspended sediments and coloured dissolved organic matter and describe vertical profiles of chlorophyll and suspended sediments from satellite ocean colour data. These models make use of the relationships between remote sensing reflectance ratios Rrs (555)/Rrs (443) and Rrs (620)/Rrs (490) versus aph (443) and aph (555), and acdom (443), and ad (443) to derive the model parameters. Validation with the in-situ data obtained from coastal waters around India and other regional waters (e.g., NASA bio-Optical Marine Algorithm Data-Set, NOMAD) shows that the new models are more accurate in terms of producing the spectral absorption coefficients (aph, ad, acdom across the entire visible wavelengths 400–700nm) in a wide variety of waters. Further comparison with existing models shows advantage of the new models that have important implications for remote sensing of turbid coastal and eutrophic waters. The retrieved absorption coefficients of phytoplankton and suspended sediments (non-algal matter) are also found to relate better to chlorophyll and total suspended sediments. Taking advantages of this, we derive models to determine and describe the vertical profiles of chlorophyll and suspended sediment concentrations along the depth. The model parameters are derived empirically. These new parameterizations show potential in estimating the vertical profiles of chlorophyll and suspended sediments with good accuracy. These results suggest robustness and suitability of the new models for studying the ecologically important components of optically complex turbid and eutrophic waters using remote sensing data.

New class of NCS-free cyclometalated ruthenium(II) complexes with 6-phenylpyridine-2-carboxylate for use as near-infrared sensitizers in dye-sensitized solar cells

Three examples, FT102, FT90, and FT117 of a new class of NCS-free cyclometalated ruthenium(II) complexes, Ru(tctpy)(O^N^C) (where O^N^C is a tridentate 6-phenylpyridine-2-carboxylate), were synthesized for use as near-infrared (IR) sensitizers in dye-sensitized solar cells (DSSCs). A tridentate donor ligand, 6-phenylpyridine-2-carboxylate was introduced in order to enhance the light harvesting efficiency in the longer wavelength region for the first time. Modifying the ligand improved the photovoltaic performance, and DSSCs sensitized with FT117 exhibited efficient panchromatic sensitization over the entire visible wavelength, extending into the near-IR region. The highest incident photon-to-current conversion efficiency (68%) was found at 600nm, and the action spectrum onset was near 920nm.

Nematic liquid crystal nanocomposite with scattering-free, microsecond electro-optic response

We report a microsecond electro-optic response in an anisotropic-polymer/liquid-crystal composite, which forms a homogeneously mixed structure in the nanoscale range owing to the high miscibility between them. The nanocomposite was fabricated by photopolymerizing a nematic liquid crystal (NLC) mixture doped with a cross-linkable mesogenic monomer at a concentration of 30 wt%. Our system is inherently different from polymer-dispersed liquid crystals in that the LC molecules are almost miscible in the anisotropic polymer matrix and do not form observable domains. When an electric field is applied to such a nanocomposite, the molecular alignment of the polymer matrix is retained, while the non-polymerizable NLC reorients along the electric field, leading to a shift in the birefringence. Furthermore, the reorientation of the NLC molecules in a space sufficiently smaller than the wavelength of visible light results in scattering-free characteristics over the entire visible wavelength range and a short decay response time of 15 μs.

An Evaluation of Models for the Satellite-Estimation of Phytoplankton Absorption Coefficients in Coastal/ Oceanic Waters

Several semi-analytical models for the satellite-based estimation of absorption coefficients of phytoplankton a <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ph</sub> (λ) have been used to routinely produce a <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ph</sub> (λ) product from satellite ocean color data. However, these models are generally applicable for clear ocean waters where they produce a <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ph</sub> (λ) values only at a few wavelengths in the blue-green domain; this causes the main difficulty in making these models more usable with any suite of wavelengths. Further, recent studies have shown the performance of these models to be highly questionable in optically complex waters. This emphasized the need for developing a more accurate model for the satellite-based estimation of a <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ph</sub> (λ) in a wide range of oceanic waters. In our previous study, we developed an empirical model (hereafter referred as Tiwari-Shanmugam model - “TS model”) based on the relationship of the in situ remote sensing reflectance ratio R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rs</sub> (670)/R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rs</sub> (490) and in situ a <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ph</sub> (λ) which is best fit to a third order polynomial. In the present study, we rigorously test this model along with three global inversion models (e.g., Constrained Linear Matrix (LM) model, Quasi-analytical algorithm (QAA), and GSM semi-analytical model which are often used by the ocean color community) using three independent in situ data sets from clear to turbid coastal waters and satellite match-ups data from global waters. When applied to these data sets, the TS model produces more accurate a <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ph</sub> values across the entire visible wavelengths (400-700 nm) in all these waters, whereas LM, QAA and GSM models yield significant errors in addition to being restricted to produce a <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ph</sub> values only in the blue-green wavelengths (LM and GSM). Though the TS model is mathematically simple, it overcomes such limitations and yields excellent results in terms of reproducing measured a <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ph</sub> spectra that are highly desired by the ocean color community for inputting in various bio-optical models and studying the spatial structure of the different phytoplankton communities from satellite remote sensing observations in global waters.

Tunable hyperchromatic lens system for confocal hyperspectral sensing

A new approach for confocal hyperspectral sensing based on the combination of a diffractive optical element and a tunable membrane fluidic lens is demonstrated. This highly compact lens system is designed to maximize the longitudinal chromatic aberration and select a narrow spectral band by spatial filtering. Changing the curvature of the fluidic lens allows the selected band to be scanned over the whole given spectrum. A hybrid prototype with an integrated electro-magnetic micro-actuator has been realized to demonstrate the functionality of the system. Experimental results show that the spectrum transmitted by the system can be tuned over the entire visible wavelength range, from 450 to 900 nm with a narrow and almost constant linewidth of less than 15 nm. Typical response time for scanning the spectrum by 310 nm is less than 40 ms and the lens system shows a highly linear relationship with the driving current.

Electronic Structure of Spinel-Type Nitride CompoundsSi3N4,Ge3N4, andSn3N4with Tunable Band Gaps: Application to Light Emitting Diodes

In this Letter using experimental and theoretical methods, we show that the solid solutions of group 14 nitrides having spinel structure (γ-M3N4 where M=Si, Ge, Sn) exhibit mainly direct electronic band gaps with values that span the entire visible wavelength region, making these hard and thermally stable materials suitable for optoelectronic devices and, in particular, lighting applications. Using the simulated band structure, we also calculate the exciton binding energy. The combination of large exciton binding energies and the tunable electronic band gaps in the visible range makes these binary spinel nitrides and their solid solutions a new class of multifunctional materials with optoelectronic properties that can be engineered to suit the desired application.