Long-wavelength Shoulder Research Articles

Сверхлюминесценция на NV--центрах в синтетическом алмазе

The photoluminescence spectra of two synthetic diamond samples containing 1-10 ppm NV centers and 100-200 ppm substitutional nitrogen were studied under optical pumping at 532, 560, and 575 nm. At pump intensities less than 0.5-1.0 MW/cm2, a vibronic photoluminescence band of NV- centers in the negative charge state was observed. With an increase in the pump intensity above 0.5-1.0 MW/cm2, a superluminescence band was observed on the long-wavelength shoulder of the phonon wing in the range 700-760 nm. The intensity of superluminescence band increased with increasing pump intensity. It is shown that the position of the superluminescence band with maxima in the range 715-720 nm is unchanged at all of optical pump wavelengths.

Microstructure and Formation Mechanism of V-Defects in the InGaN/GaN Multiple Quantum Wells with a High In Content

In the growth of InGaN/GaN multiple quantum well (MQW) by metal organic chemical vapor deposition (MOCVD), V-defects have been observed and investigated. From cross-sectional transmission electron microscopy, we found that all V-defects are not always connected with threading dislocations at their bottom. By increasing the indium composition in the InxGa1−xN well layer, many V defects are generated from the stacking mismatch boundaries. The formation mechanism of these defects has been discussed in terms of stress induced by lattice mismatch and reduced In incorporation on the $$\left\{ {10\overline 1 1} \right\}$$ planes in comparison to the (0001) surface. The V-defect is correlated with the localized excitonic recombination centers that give rise to a long-wavelength shoulder in photoluminescence. The InGaN/GaN MQW grown on the $$\left\{ {10\overline 1 1} \right\}$$ faceted sidewalls of the V-defects gave much lower emission energies than those of the c-plane MQW.

Optical Scattering of Liquid Gallium Nanoparticles Coupled to Thin Metal Films.

We investigate experimentally and numerically the scattering properties of liquid gallium nanoparticles coupled to a thin gold or silver film. The gallium nanoparticles are excited either directly by using inclined white light or indirectly by surface plasmon polaritons generated on the surface of the gold/silver film. In the former case, the scattering spectrum is always dominated by a scattering peak at ∼540 nm with a long-wavelength shoulder which is redshifted with increasing diameter of the gallium nanoparticle. Under the excitation of the surface plasmon polaritons, optical resonances with much narrower linewidths, which are dependent on the incidence angle of the white light, appear in the scattering spectra. In this case, the scattering spectrum depends weakly on the diameter of the gallium nanoparticle but the radiation pattern exhibits a strong dependence. In addition, a significant enhancement of electric field is expected in the gap region between the gallium nanoparticles and the gold film based on numerical simulation. As compared with the gallium nanoparticle coupled to the gold film which exhibit mainly yellow and orange colors, vivid scattering light spanning the visible light spectrum can be achieved in the gallium nanoparticles coupled to the silver film by simply varying the incidence angle. Gallium nanoparticles coupled to thin metal films may find potential applications in light–matter interaction and color display.

Optical characterization of the HgCdTe-based composite structure obtained by Ag ion implantation

The results concerning the formation of nano-scale patterns on the surface of a ternary compound are presented. The evolution of surface morphology of (111) Hg1−xCdxTe (MCT) (х ~ 0.223) epilayers due to ion irradiation in the energy range of 100–140 keV was studied. Modification of the surface was performed using the method of normal (θ = 0°) and oblique—incidence (θ = 45°) bombardment by Ag+ ions. Surface characteristics were investigated using atomic force microscopy and X-ray photoelectron spectroscopy techniques. Low-temperature photoluminescence and Raman spectroscopy were used for the investigation of the recombination and vibrational properties as well as the defect-impurity states of as-grown and implanted Hg0.777Cd0.223Te epilayers. It was found that ion beam structured regions of MCT surface demonstrate photoluminescence emission in the range of 500–850 nm, with intense peaks at around 733 nm (1.69 eV) and 570 nm (2.16 eV) with a long-wavelength shoulder at 620 nm (2 eV). The observed emission lines are suggested to be mainly related to the presence of various point defects in oxide phases (CdO and Ag2O) induced by ion bombarding in the base material (HgCdTe).

Confocal Microscopy of Luminescence Inhomogeneity in LGSO:Ce Scintillator Crystal

Confocal microscopy was exploited to study spatial distribution of luminescence characteristics in Lu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> :Ce (LSO:Ce) and mixed Lu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2x</sub> Gd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2-2x</sub> SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> :Ce (LGSO:Ce) crystals. Spatial distributions of spectrally-integrated photoluminescence (PL) intensity, PL band peak and spectral center of mass positions, and band width were determined. In both crystals, the luminescence spectra at 405 nm excitation are dominated by one broad band peaked at ~ 510 nm, while a long-wavelength shoulder is also observed in LGSO:Ce. Spatial inhomogeneities of the order of 1-3 micron in diameter are observed for the band position in LGSO:Ce. A comparative analysis of the spectra shows that the inhomogeneity is caused by spatial distribution in the spectral component at 500-700 nm, which may be attributed to Ce <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> located in CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> polyhedra (Ce2), or, more probably, to some defects in the crystal. Our results indicate that the changes in luminescence spectrum shape reflect Lu/Gd ratio fluctuations on a micrometer scale in the bulk of the mixed LGSO:Ce crystals.

Photoelectric and luminescent properties of dysprosium-doped silver chloride

The influence of dysprosium doping on the photoelectric and luminescent properties of AgCl crystals is studied by methods of microwave photoconductivity and photoluminescence. Doping affects both the loss kinetics of photogenerated electrons and luminescence spectra and parameters of photostimulated burst of luminescence. It is shown that the charged [DyAg·· · V′Ag]· or neutral [DyAg·· · 2V′Ag]x complexes are responsible for a new luminescence band peaked at 470 nm, which manifests itself at weight concentrations of the doping additive >10−6%. The long-wavelength shoulder at 570 nm in the photoluminescence spectra is attributed to intracenter transitions in the Dy3+ ions. The rate constant of the reaction of electron capture into the traps forming upon introduction of the dopant, kt = (3–5) × 10−8 cm3 s−1, is evaluated. It is assumed that the traps are Dy3+ dysprosium ions.

Spectral characterization of bulk and nanostructured aluminum nitride

Spectral characteristics including photoluminescence (PL) spectra and its excitation spectra for different AlN materials (AlN ceramics, macro size powder and nanostructured forms such as nanopowder, nanorods and nanotips) were investigated at room temperature. Besides the well known UV-blue (around 400 nm) and red (600 nm) luminescence, the 480 nm band was also observed as an asymmetric long-wavelength shoulder of the UV-blue PL band. This band can be related to the luminescence of some kind of surface defects, probably also including the oxygen-related defects. The mechanisms of recombination luminescence and excitation of the UV-blue luminescence caused by the oxygen-related defects were investigated. It was found that the same PL band is characteristic for different AlN materials mentioned above; however, in the nanostructured materials (nanorods, nanotips and nanopowder) the intensity of UV-blue PL is remarkable lower than in the bulk material (ceramics). In the case of nanostructured AlN materials, excitation of the oxygen-related defect is mainly realized through the energy transfer from the host material (electron/hole or exciton processes) to the defects and this mechanism prevails over the mechanism of direct defect excitation.

Spectral features of substituted 9-(phenoxycarbonyl)-acridines and their protonated and methylated cation derivatives

The long-wavelength absorption of eight 9-(phenoxycarbonyl)-acridines and the 10- H-9-(phenoxycarbonyl)-acridinium and 10-methyl-9-(phenoxycarbonyl)-acridinium cations derived from them, substituted with an alkyl or trifluoroalkyl group at the benzene ring, occurs above 300 nm as the superposition of four bands. Three of these bands occupy comparable positions (expressed in nm) in all the compounds; the fourth one, however, changes position, appearing in neutral molecules as a long-wavelength shoulder below 400 nm, but in cations as an almost separate band above 400 nm. The weak fluorescence resulting from excitation within the long-wavelength absorption band is red-shifted relative to absorption, such that Stokes shifts are similar for both neutral molecules and cations. Stokes shifts tend to increase with the orientational polarisability of a medium. Computations predict that long-wavelength electronic transitions are accompanied by structural changes in molecules. They also indicate that such transitions are followed by roughly uniform electron density changes in whole molecules accompanied by small changes in their dipole moments, which accounts for the weak absorption in the long-wavelength region. The predicted radiative and non-radiative deactivation rate constants suggest the occurrence of efficient spin–orbital coupling in the molecules investigated, which is the cause of the relatively low fluorescence quantum yields. Apart from the cognitive significance of these investigations, the results demonstrate that absorption of radiation by 10-methyl-9-(phenoxycarbonyl)-acridinium cations above 400 nm may influence their chemiluminescence output.

P-194: Studies of Carrier Dynamics and Luminescence Mechanisms of C545T-doped Alq[sub 3

We have studied carrier dynamics and lminescence mechanisms of C545T-doped Alq3 by the material, optical as well as electrical characteristics measurements. The spectra of C545T-doped Alq3 become narrower, which shows dopants benefits color tuning and create saturated colors. Based on TRPL measurements, the mechanism of carrier transport from host states to singlet states of dopants was proposed for interpreting the early-stage fast decay, delayed slow rise, and extended slow decay of PL intensity. SEM image showed that as the dopant concentration was above 1%, the dopants tend to aggregate. The aggregations degrade the device performances such as an apparent long-wavelength shoulder of EL spectrum, higher driving voltage, low current density, high built-in voltage, low carrier mobility, and enhanced nonradiative recombination rates.

Positional effects of the hydroxy substituent on the photochemical and photophysical behavior of 3- and 4-hydroxystilbene.

The photophysical and photochemical behavior of meta- and para-hydroxy- and methoxystilbene are reported and compared to the aminostilbenes. Absorption spectra of all four studied compounds in organic solution display a featureless, intense long-wavelength absorption band around 300 nm. In basic aqueous solution, meta-hydroxystilbene displays a less intense band with a long-wavelength shoulder, a consequence of configuration interaction from the O(-) substituent. Emission from the meta-substituted stilbenes is much stronger than from the para isomers, with an especially large effect for the hydroxystilbenes in solvents that are good hydrogen bond acceptors. Emission from the hydroxystilbenes is weak in aqueous solution, and even weaker in basic solution, a consequence of facile nonradiative decay. The excited state lifetimes of the meta isomers are longer than those with para substituents. Ground and excited state acidity constants are reported for the hydroxystilbenes. In the ground state, the para derivative is an order of magnitude more acidic than the meta isomer, however, its short excited state lifetime precludes excited state deprotonation. In contrast, 3-hydroxystilbene does exhibit deprotonation in its excited state.

Demonstration of a three-dimensional simple-cubic infrared photonic crystal

This letter reports on the use of nonselective dry and selective wet etching to fabricate a gallium arsenide (GaAs) based three-dimensional infrared photonic crystal with a simple-cubic lattice structure. A two-and-one-half-period photonic crystal fabricated using this procedure demonstrated a drop in transmission of approximately 10 dB at a center wavelength of 12 μm for normally incident angles. A shift in the long-wavelength shoulder from 16 to 15 μm was observed as the incident infrared beam was varied from normal to oblique incidence. The measured drop in transmission agrees well with the band gap predicted theoretically for this simple-cubic structure.

Interstellar PAH emission in the 11-14 micron region: new insights from laboratory data and a tracer of ionized PAHs.

The Ames infrared spectral database of isolated, neutral and ionized polycyclic aromatic hydrocarbons (PAHS) shows that aromatic CH out-of-plane bending frequencies are significantly shifted upon ionization. For solo- and duet-CH groups, the shift is pronounced and consistently toward higher frequencies. The solo-CH modes are blueshifted by an average of 27 cm-1 and the duet-CH modes by an average of 17 cm-1. For trio- and quartet-CH groups, the ionization shifts of the out-of-plane modes are more erratic and typically more modest. As a result of these ionization shifts, the solo-CH out-of-plane modes move out of the region classically associated with these vibrations in neutral PAHS, falling instead at frequencies well above those normally attributed to out-of-plane bending, vibrations of any type. In addition, for the compact PAHs studied, the duet-CH out-of-plane modes are shifted into the frequency range traditionally associated with the solo-CH modes. These results refine our understanding of the origin of the dominant interstellar infrared emission feature near 11.2 microns, whose envelope has traditionally been attributed only to the out-of-plane bending of solo-CH groups on PAHS, and provide a natural explanation for the puzzling emission feature near 11.0 microns within the framework of the PAH model. Specifically, the prevalent but variable long-wavelength wing or shoulder that is often observed near 11.4 microns likely reflects the contributions of duet-CH units in PAH cations. Also, these results indicate that the emission between 926 and 904 cm-1 (10.8 and 11.1 microns) observed in many sources can be unambiguously attributed to the out-of-plane wagging, of solo-CH units in moderately sized (fewer than 50 carbon atom) PAH cations, making this emission an unequivocal tracer of ionized interstellar PAHS.

Structural origin of V-defects and correlation with localized excitonic centers in InGaN/GaN multiple quantum wells

In the growth of InGaN/GaN multiple quantum well (MQW) structures, a novel defect (called the “V-defect”) initiates at threading dislocations in one of the first quantum wells in a MQW stack. This defect is common to almost all InGaN MQW heterostructures. The nature of the V-defect was evaluated using transmission electron microscopy (TEM), scanning TEM (STEM), and low-temperature cathodoluminescence (CL) on a series of In0.20Ga0.80N/GaN MQW samples. The structure of the V-defect includes buried side-wall quantum wells (on the {101̄1} planes) and an open hexagonal inverted pyramid which is defined by the six {101̄1} planes. Thus, in cross section this defect appears as an open “V”. The formation of the V-defect is kinetically controlled by reduced Ga incorporation on the pyramid walls ({101̄1} planes). The V-defect is correlated with the localized excitonic recombination centers that give rise to a long-wavelength shoulder in photoluminescence (PL) and CL spectra. This long-wavelength shoulder has the following characteristics: (i) its intensity is correlated with the side-wall quantum wells; (ii) the temperature independence of the full width at half maximum strongly supports a localized exciton recombination process.

Detection of the overtone of the 3.3 micron emission feature in IRAS 21282+5050

The 1.6-1.8 micron spectrum of the planetary nebula, IRAS 21282+5050, a strong emitter of the unidentified interstellar bands, contains a 0.02 micron wide eimission feature centered at 1.680 micron, which is well matched by laboratory spectra of the 0-2 CH stretching mode in polycyclic aromatic hydrocarbons (PAHs). We identify the new feature as the overtone of the well-known 3.3 micron band. In view of the high excitation required for emission in this band, the identification indicates that the emission is by free molecules rather than molecular moieties in solid dust grains. Modeling of the intensity ratio of the 2-0 to 1-0 band implied that the PAHs emitting in these bands contain about 60 carbon atoms. It is inferred that the nu = 2-1 hot band of the CH stretching mode occurs at about 3.43 micron and contributes to the long-wavelength shoulder of the 3.40 micron feature. The main 3.40 micron feature probably is due to aliphatic sidegroups on PAH molecules.

Formation and optical properties of laser-active N2 centers in KCl.

Recent experimental results on the formation and optical properties of the laser-active ${\mathit{N}}_{2}$ center in additively colored pure KCl are reported. The ${\mathit{N}}_{2}$ center in KCl is most efficiently formed through a photoaggregation process in heavily colored crystals exposed to light on the long-wavelength shoulder of the F band. The observed room-temperature stability of the ${\mathit{N}}_{2}$ center indicates that it is an electrically neutral defect. Absorption and emission bands peak at 1.02 and 1.25 \ensuremath{\mu}m, respectively. The ${\mathit{N}}_{2}$-band absorbance grows proportionally with the ${\mathit{R}}_{1}$- and ${\mathit{R}}_{2}$-band absorbances. Polarized ${\mathit{N}}_{2}$-center fluorescence data is consistent with two degenerate orthogonal dipole moments lying on {111} planes of the crystal. Excited-state absorption spectra show higher transitions coincident in wavelength with the ${\mathit{R}}_{1}$ and ${\mathit{R}}_{2}$ absorption bands of the trigonal ${\mathit{F}}_{3}$ color center in KCl. Similar experiments on the ${\mathit{R}}_{1}$ band correlate well with the ${\mathit{N}}_{2}$-center data. Results are consistent with the identification of the laser-active part of the ${\mathit{N}}_{2}$ band as a transition of the trigonal ${\mathit{F}}_{3}$ color center.