Green Spectral Region Research Articles

Photoluminescence of nanosized Zn2SiO4:Mn depending upon preparation method

Nanosized Zn2SiO4:Mn powders were prepared by two different methods: a high- energy ball-milling of microcrystalline powder (so-called "top-down") and a sol-gel method ("bottom-up"). It was shown that it is possible to obtain particles of 30±10 nm by means of the ball-milling. A particle size of the Zn2SiO4:Mn synthesized by the sol-gel method ranged from 20 to 110 nm. It was found all samples exhibit photoluminescence (PL) in the green spectral region with a maximum emission wavelength from 515 to 520 nm. A nanopowder obtained by the ball-milling showed a significant decrease of the PL intensity comparing with bulk material. The PL intensity of the samples prepared by sol-gel method is much higher than that of ball-milled Zn2SiO4:Mn.

Optimal design of green InGaN/GaN LEDs mediated by surface-plasmon gratings

We propose an optimal design for enhancing the external quantum efficiency of InGaN/GaN LEDs operating in the green spectral region, by mediating surface plasmons (SP), showing that there is plenty of potential for additional improvement in this area. Coupling the spontaneous emission from quantum wells into SP modes on metallic grating structures could enhance the internal quantum efficiency of conventional LEDs by more than twofold, and the relatively long propagating length of SPs in the green region could allow a narrow radiating angle and thus yield a threefold improvement in the extraction efficiency of radiation. Thus, our design enhances external quantum efficiency by about sixfold overall, offering a practically attainable method to realize highly efficient green SP-LEDs.

Synthesis and characterization of LiNbO3 nanocrystals prepared by the aerosol assisted chemical vapor deposition method

This work reports the synthesis of LiNbO3 nanocrystals by an aerosol assisted chemical vapor deposition method, onto silicon (001) substrates. Some of the nanocrystals showed a hexagonal morphology and characteristic sizes ranging from a few tens to a few hundreds of nanometers. The nanocrystals with hexagonal structure were obtained at 773K using a carrier gas flow of 6L/min and a deposition time of 90min. Structural, morphological, photoluminescence and cathodoluminescence properties were studied by grazing incidence X-ray diffraction, scanning electron microscopy, high resolution transmission electron microscopy, fluorospectrometry, and cathodoluminescence, respectively. An extraordinary violet luminescence at 396nm with excitation in UV region (280nm) was observed in one of the samples. Cathodoluminescence (CL) imaging and spectroscopy studies of the nanocrystals exhibit stimulated emission of light in the near ultraviolet–violet–green spectral region. The emission peaks in CL spectra varied according to the nature of nanoparticles. A red-shift in the CL spectra with a decrease in the particle size was observed. Apparently the size of LiNbO3 typical nanocrystals investigated in this work has tuned the photoluminescence emission. This result encourages the use and integration of LiNbO3 nanocrystals into nanophotonics applications.

Diode-pumped Pr3+:LiYF4 visible dual-wavelength laser

A diode-pumped simultaneous continuous-wave (cw) dual-wavelength Pr3+:LiYF4 laser in the green spectral region is experimentally demonstrated. We calculate the condition of gain-to-loss balance via an uncoated glass plane to achieve the simultaneous dual-wavelength operation. The simultaneous dual-wavelength Pr3+:LiYF4 laser at 546nm in π-polarization and 550nm in σ-polarization is experimentally realized. At an absorbed pump power of 1.5W, the maximum output power obtained at 546nm and 550nm is 184mW and 158mW, respectively.

Optical properties and electronic band structure of BiMg2PO6, BiMg2VO6, BiMg2VO6:Pr3+ and BiMg2VO6:Eu3+

The luminescence properties of the yellow pigment BiMg2VO6 are revisited and those of BiMg2PO6, BiMg2VO6:Pr3+ and BiMg2VO6:Eu3+ are described. It is shown that the undoped systems exhibit broad band emission in the green or orange spectral regions, but only upon UV or near UV excitation. In contradiction with a previous report, we found that the blue, host absorbed, photons are lost non-radiatively and do not contribute to the luminescence processes in BiMg2VO6. To understand these experimental results, the optical properties of BiMg2VO6 and BiMg2PO6 are theoretically analysed on the basis of electronic structure diagrams calculated by the DFT method. It is found that the optical transitions are mostly localised within [VO4]3− units or non-regular Bi3+ ions and occur in the UV or near UV regions. The luminescence of the trivalent lanthanide dopants is weak (Eu3+) or unobserved (Pr3+) in BiMg2VO6 which is explained by inefficient energy migration in the host lattice to the impurity sites.

Coherent nanocavity structures for enhancement in internal quantum efficiency of III-nitride multiple quantum wells

A “coherent” nanocavity structure has been designed on two-dimensional well-ordered InGaN/GaN nanodisk arrays with an emission wavelength in the green spectral region, leading to a massive enhancement in resonance mode in the green spectra region. By means of a cost-effective nanosphere lithography technique, we have fabricated such a structure on an InGaN/GaN multiple quantum well epiwafer and have observed the “coherent” nanocavity effect, which leads to an enhanced spontaneous emission (SE) rate. The enhanced SE rate has been confirmed by time resolved photoluminescence measurements. Due to the coherent nanocavity effect, we have achieved a massive improvement in internal quantum efficiency with a factor of 88, compared with the as-grown sample, which could be significant to bridge the “green gap” in solid-state lighting.

Luminescence spectroscopy and electronic structure of the PbMoO4 and Pb2MoO5 single crystals

Intrinsic photoluminescence (PL) and electronic structure of Pb2MoO5 single crystal are first described and compared with corresponding properties of PbMoO4 crystal. The PL properties of PbMoO4 and Pb2MoO5 crystals grown by Czochralski method are studied under the VUV synchrotron radiation excitations (3.7–14eV region of excitation photon energies) at T=8K. The electronic band structures of perfect PbMoO4 and Pb2MoO5 are calculated by the Full-Potential Linear Augmented Plane Wave method. The Pb2MoO5 crystals reveal intensive PL band in the yellow–red spectral region peaking at ∼600nm. All studied PbMoO4 samples reveal intensive PL band in the green spectral region and this band is accompanied by additional fast-decaying (τ≈3.5ns) component in the violet when the samples are grown with 30° orientation of seed with respect to the c axis. The excitation spectra of the violet component of PbMoO4 reveal complex excitation band with main peak at 6.2eV. It is assumed that the violet PL component of PbMoO4 is generated by intra-center excitations from the ground to singlet excited states 1A1–1T1,2 of the MoO42− molybdate groups located near point defects in subsurface layer of the crystal. Strong anisotropy of reflectivity spectra of PbMoO4 below 6eV is a consequence of peculiarities of the electronic structure of the crystal and reflectivity bands at ∼3.6 and ∼4.3eV are formed by band-to-band electronic transitions.

Four emission bands from a mixed-ligand iridium complex IrQ(ppy)2 at room temperature

Four photoluminescence bands are observed from iridium complex in UV, violet, green, and red spectral regions at room temperature. Such a multiple emission is found from a mixed-ligand iridium(III) complex, (8-quinolinolato) bis(2-phenylpyridyl) iridium IrQ(ppy)2, which consists of IrQ and Ir(ppy)2 components (Q: 8-quinolinolato, ppy: phenylpyridyl). Of the four emission bands, the UV emission band with maximum at about 330nm and the red emission band are attributed to the ligand-centered (1LC) 1nπ* and metal-to-ligand charge transfer (3MLCT) 3(ππ*) states from IrQ, respectively, while the violet emission band with maximum at about 400nm and the green emission band at about 513nm are attributed to the 1LC 1(nπ*) and 3MLCT 3ππ* states from Ir(ppy). It is suggested that (1) IrQ and Ir(ppy) generate their own emissions by the inefficient Förster energy transfer between IrQ and Ir(ppy) due to the orientation factor of nearly zero, and (2) each of IrQ and Ir(ppy) gives rise to two emissions from the singlet and triplet states by the inefficient intersystem crossing.

Photophysical and Electrochemical Properties of Novel Luminescent and Photoconductive Copolymers

Photophysical and electrochemical properties of novel luminescent carbazole-based, carbazole-fluorene and fluorene-bithiophene copolymers, and photoconductive copolymers containing 4,6- bis(3-dodecylthien-2-yl)thieno[3,4-c][1,2,5]thiadiazole as electron acceptor unit and various electron donor unit (9,9-dioctylfluorene, 9,9-bis(2-ethylhexyl)fluorene, 2,5-bis(dodecyloxy)benzene, 2,5- didodecylbenzene, 3,3′-didodecyl-2,2′-bithiophene, 2,7-carbazole or dithieno[3,2-b:2′,3′-d]silole derivatives) were studied. Effects of molecular weight and side-chain nature and length on photophysical properties are reported. Luminescent carbazolebased and carbazole-fluorene copolymers exhibited an intense photoluminescent (PL) emission in blue spectral region with maxima at about 417-419 nm in tetrahydrofuran (THF) solutions and at 430-436 nm in thin films, whereas 9,9-dihexadecylfluorenebithiophene copolymers PL emission in green spectral region with maxima at about 495 nm in THF solutions and at 542 nm in thin films. The donor-acceptor copolymers absorb in a long-wavelength region up to near infrared and possess a narrow optical band gap in the range of 1.1 - 1.4 eV depending on the character of donor units and alkyl side-chain nature. They exhibited ambipolar redox properties, high electron affinity, electrochromic and thermochromic behavior.

Characterization of the solar light field within the ocean mesopelagic zone based on radiative transfer simulations

The solar light field within the ocean from the sea surface to the bottom of the mesopelagic zone was simulated with a radiative transfer model that accounts for the presence of inelastic radiative processes associated with Raman scattering by water molecules, fluorescence of colored dissolved organic matter (CDOM), and fluorescence of chlorophyll-a contained in phytoplankton. The simulation results provide a comprehensive characterization of the ambient light field and apparent optical properties (AOPs) across the entire visible spectral range within the depth range 200–1000m of the entire mesopelagic zone for varying chlorophyll-a concentration and seawater optical properties in the mixed surface layer of the ocean. With increasing depth in the mesopelagic zone, the solar irradiance is reduced by ~9–10 orders of magnitude and exhibits a major spectral maximum in the blue, typically centered around a light wavelength of 475nm. In the green and red spectral regions, the light levels are significantly lower but still important owing to local generation of photons via inelastic processes, mostly Raman scattering and to a lesser extent CDOM fluorescence. The Raman scattering produces a distinct secondary maximum in irradiance spectra centered around 565nm. Comparisons of our results with light produced by the radioactive decay of the unstable potassium isotope contained in sea salt (40K) indicates that the solar irradiance dominates over the 40K-produced irradiance within the majority of the mesopelagic zone for most scenarios considered in our simulations. The angular distribution of radiance indicates the dominance of downward propagation of light in the blue and approach to uniform distribution in the red throughout the mesopelagic zone. Below the approximate depth range 400–500m, the shape of the angular distribution is nearly invariant with increasing depth in the green and red and varies weakly in the blue. The AOPs at any light wavelength also assume nearly constant values within the deeper portion of the mesopelagic zone. These results indicate that the mesopelagic light field reaches a nearly-asymptotic regime at depths exceeding ~400–500m.

Y3Al5O12:Re3+ (Re=Ce, Eu, and Sm) nanocrystalline powders prepared by modified glycine combustion method

Yttrium aluminum garnet doped with rare earth ions (Ce3+, Eu3+ and Sm3+) was prepared by modified glycine method. Ce3+ as a dopant was used in four different concentrations (Y3-xCexAl5O12; x(%) = 1, 2, 3, 5), while doping concentration of Eu3+ and Sm3+ was Y3-xEuxAl5O12; x(%) = 3 and Y3-xSmxAl5O12; x(%) = 1, respectively. Phase composition of powders was investigated using XRD technique and expected target phase was confirmed. Photoluminescent characterization included measurements of excitation and emission spectra, as well as determination of emission decays. Y3-xCexAl5O12 shows intense broad-band emission, with maximum in green spectral region, at about 524 nm under ultraviolet or blue excitation. The origin of the luminescence is the 5d1?4f1 transition which is both parity and spin allowed. Ultraviolet and blue excitations of Eu3+ and Sm3+ doped Y3Al5O12 produce intense orange and red emissions. These emissions are phosphorescent in character and come from spin forbidden f-f electron transitions in Eu3+ and Sm3+ ions. For the case of Eu3+ doping emission comes mainly from 5D0?7F1 transitions with Stark components peaking at 590 nm and 590.75 nm, and with emission decay of 4.15 ms. In the case of Sm3+ doping, the emission spectrum, shows 4G5/2?6H5/2, 4G5/2?6H7/2, and 4G5/2?6H9/2 transitions, with the most intense stark components positioned at 567.5 nm, 617 nm, and 650 nm, respectively and for transition centered at 617 nm, emission decay is 3.12 ms.

Wide wavelength tunability and green laser operation of diode-pumped Pr^3+:KY_3F_10

We report on wide spectral tunability of a quasi-continuous wave Pr(3+):KY(3)F(10) laser under InGaN laser diode excitation at 445 nm. The total tuning range exceeded 100 nm in the visible spectral range on several intervals between 521 nm and 737 nm. The broadest continuously tunable region of almost 50 nm extended from 688 nm to 737 nm. Furthermore we present what is to the best of our knowledge the first demonstration of continuous wave laser operation in Pr(3+):KY(3)F(10) on three transitions in the green spectral region. The highest output power of 121 mW was achieved at an emission wavelength of 554 nm with a slope efficiency of 27%.

Optical and EPR Properties of Mn- and Eu-Activated LaMgAl11O19 Phosphor

LaMgAl11O19 phosphor doped with Eu and Mn ions has been prepared by the urea combustion route. The as-prepared phosphor was studied using X-ray diffraction, electron paramagnetic resonance (EPR), diffuse reflectance and photoluminescence studies. The EPR spectra of LaMgAl11O19:Eu, Mn phosphor exhibit signals with the effective g values at g = 1.98, 4.29 and 7.23. The resonance signals at g = 1.98 and 4.29 were attributed to Mn2+ ions in tetrahedral and rhombic environment, respectively. The resonance signal at g = 7.23 was attributed to Eu2+ ions. The optical spectrum of this phosphor exhibits an intense band in the visible region and this band has been attributed to spin-allowed 5Eg → 5T2g transition of Mn3+ ions. Upon excitation at 324 nm, the material displays emission in the blue, green and red spectral region.

Fraunhofer Lidar Prototype in the Green Spectral Region for Atmospheric Boundary Layer Observations

A lidar detects atmospheric parameters by transmitting laser pulse to the atmosphere and receiving the backscattering signals from molecules and aerosol particles. Because of the small backscattering cross section, a lidar usually uses the high sensitive photomultiplier and avalanche photodiode as detector and uses photon counting technology for collection of weak backscatter signals. Photon Counting enables the capturing of extremely weak lidar return from long distance, throughout dark background, by a long time accumulation. Because of the strong solar background, the signal-to-noise ratio of lidar during daytime could be greatly restricted, especially for the lidar operating at visible wavelengths where solar background is prominent. Narrow band-pass filters must therefore be installed in order to isolate solar background noise at wavelengths close to that of the lidar receiving channel, whereas the background light in superposition with signal spectrum, limits an effective margin for signal-to-noise ratio (SNR) improvement. This work describes a lidar prototype operating at the Fraunhofer lines, the invisible band of solar spectrum, to achieve photon counting under intense solar background. The photon counting lidar prototype in Fraunhofer lines devised was used to observe the atmospheric boundary layer. The SNR was improved 2-3 times by operating the lidar at the wavelength in solar dark lines. The aerosol extinctions illustrate the vertical structures of aerosol in the atmospheric boundary over Qingdao suburban during summer 2011.

GaInN light-emitting diodes using separate epitaxial growth for the p-type region to attain polarization-inverted electron-blocking layer, reduced electron leakage, and improved hole injection

A GaInN light-emitting diode (LED) structure is analyzed that employs a separate epitaxial growth for the p-type region, i.e., the AlGaN electron-blocking layer (EBL) and p-type GaN cladding layer, followed by wafer or chip bonding. Such LED structure has a polarization-inverted EBL and allows for uncompromised epitaxial-growth optimization of the p-type region, i.e., without the need to consider degradation of the quantum-well active region during p-type region growth. Simulations show that such an LED structure reduces electron leakage, reduces the efficiency droop, improves hole injection, and has the potential to extend high efficiencies into the green spectral region.

Influence of boron doping and hydrogen passivation on recombination of photoexcited charge carriers in silicon nanocrystal/SiC multilayers

The influence of boron (B)-doping and remote plasma hydrogen passivation on the photoexcited charge carrier recombination in silicon nanocrystal/SiC multilayers was investigated in detail. The samples were prepared by high temperature annealing of amorphous (intrinsic and B-doped) Si1−xCx/SiC superlattices. The photoluminescence (PL) intensity of samples with B-doped silicon rich carbide layers was found to be up to two orders of magnitude larger and spectrally red shifted in comparison with that of the other samples. Hydrogen passivation leads to an additional increase in PL intensities. The PL decay can be described well by a mono-exponential function with a characteristic decay time of a few microseconds. This behavior agrees well with the picture of localized PL centers (surface states) together with the passivation of non-radiative defects by boron. The samples with B-doped SiC layers exhibit an additional PL band in the green spectral region that is quenched by hydrogen passivation. Its origin is attributed to defects due to suppression of crystallization of amorphous SiC layers as a result of B-doping. Measurement of ultrafast transient transmission allowed us to study the initial (picosecond) carrier dynamics. It was found to be dependent of pump intensity and interpreted in terms of multiparticle electron-hole recombination.

Laser Cleaning of Candidate Diagnostic Mirrors for ITER

Practical methods to clean ITER’s diagnostic mirrors will be essential to ITER’s plasma operations. We report on laser cleaning of candidate ITER single-crystal molybdenum mirrors that were plasma coated with either carbon or beryllium films 150 to 420 nm thick. A pulsed Nd laser beam was focused to 1 to 2 J/cm2 and scanned at various speeds across the surface of a mirror. The cleaning effect was measured with a novel method that combined microscopic imaging and reflectivity measurements in the red, green, and blue spectral regions and at the H-alpha and H-beta wavelengths. No damage of the molybdenum mirror substrates was observed at the range of laser intensities used. For carbon-coated mirrors, complete removal of the film and restoration of the reflectivity were measured in some conditions. For the beryllium-coated mirrors, restoration of reflectivity has so far been incomplete. Heat transfer calculations suggest a shorter, ∼5-ns laser pulse would be optimal.

Quantitative IBBCEAS measurements of I2 in the presence of aerosols

An instrument implementing Incoherent BroadBand Cavity-Enhanced Absorption Spectroscopy (IBBCEAS) for quantitative measurements of molecular iodine (I2) at ambient pressure and in the presence of aerosols is presented. The instrument is based on a LED emitting in the green spectral region around 500–550 nm, exciting a multitude of rovibronic transitions in the I2B ← X electronic absorption spectrum. I2 was generated using a permeation device and was mixed with a dilution flow of Ar and another flow containing NaCl aerosols. Retrieval of the mirror reflectivity, necessary for quantitative IBBCEAS measurements, was not pursued in this study. Instead, calibrated I2 flows were used, and a differential optical absorption spectroscopy type of algorithm was implemented for the data analysis. This approach has several benefits, in particular when light extinction due to aerosols is substantial. Estimated detection limits are roughly 0.04 nmol/l without aerosols and 0.4 nmol/l in the presence of aerosols (1010 particles/l with 60–70 nm mean diameter, leading to I0/Iaerosols ≈ 5.4). A drop in measured I2 concentration in relation to the expected mixing ratio was observed when using the highest aerosol concentration, likely due to adsorption of I2 onto aerosols.

Tunable upconversion emission in Ba2YF7:Yb3+/Er3+ nanocrystals with different Yb3+ concentration

Ba2YF7:xYb3+,0.02Er3+ nanocrystals with different Yb3+ concentration (x=0.2, 0.4, 0.6, 0.8, 0.98) were synthesized via a solvothermal method. Their X-ray diffraction patterns reveal that all the samples are cubic phase without phase change at arbitrary Yb3+ concentration. Upon excitation of 980nm laser, these nanocrystals display upconversion emission in the green and red spectral regions. The relative emission intensity at these two regions can be tuned by Yb3+ concentration. To investigate the effect of Yb3+ ion, the decay times of 650nm emission were measured and analyzed. Ba2YF7:0.6Yb3+,0.02Er3+ nanocrystals have the longest lifetime and highest emission intensity among all the samples. We concluded the lifetime of 4F9/2 level has an effect on its own population and intensity ratio of red and green emission. Our results show Yb3+ ion can tune the upconversion emission and improve upconverison emission intensity.

Suppressing void defects in long wavelength semipolar (202¯1¯) InGaN quantum wells by growth rate optimization

We report on void defect formation in (202¯1¯) semipolar InGaN quantum wells (QWs) emitting in the green spectral region. Fluorescence and transmission electron microscopy studies indicate that this type of defect is associated with voids with {101¯1}, {101¯0}, and {0001¯} side facets in the QW region. Systematic growth studies show that this defect can be effectively suppressed by reducing the growth rate for the active region. Green light-emitting diodes (LEDs) with reduced active region growth rate showed enhanced power and wavelength performance. The improved LED performance is attributed to the absence of void defects in the active region.