Blue Cooperative Luminescence In Yb3+-Doped Barium Gallogermanate Glass Excited At 976 nm

The Yb³⁺-doped barium gallogermanate glass has been prepared via the conventional melt method. The absorption spectra, the near infrared emission spectra, the upconversion emission spectra and the differential scanning calorimetry have been measured. Bright blue upconversion emission centered at 476 nm has been observed under the 976 nm laser diode excitation at room temperature. The blue upconversion emission mechanism has been discussed on the blue emission intensity and the measured lifetime. The slope of the log-log plot of the blue emission intensity versus the pump power is equal to 1.98, and the blue luminescence decay time is half of the near infrared fluorescence decay time, confirming that the blue emission comes from Yb³⁺-Yb³⁺ pairs cooperative upconversion mechanism. The result of differential scanning calorimetry suggests that this type of glass is suitable as a potential candidate for fiber drawing.     

Life-time resolved emission spectra in CdI2 crystals

The emission spectrum of CdI₂ is composed of ultraviolet (UV), green (G) and yellow (Y and Y′) bands peaking at 3.38, 2.50, 2.16 and 2.25 eV, respectively. In order to determine the initial states of the Y- and G-luminescence, decay curves have been measured at 6 and 80 K by varying emission energy. The observed decay curves are composed of two or three exponential components. These decay components were named τ₁, τ₂, τ₃, τ_3′ and τ₄. The emission spectrum for each decay component, i.e., the life-time resolved emission spectrum, was constructed from the observed decay curves. At 6 K, three bands at 2.12, 2.49 and 2.64 eV are obtained for τ₁, τ₂ and τ₃ components, respectively. At 80 K, a dominant band for the τ₄ component and a weak band for the τ_3′ component appear on the same energy position at 2.25 eV. The origin of each emission band in the life-time resolved emission spectra will be briefly discussed.     

Time-resolved photoluminescence study of Ga In P alloys

The time-resolved photoluminescence spectra of ordered and disordered Ga_0.52In_0.48P alloys were studied at room temperature and at 77 K liquid nitrogen, respectively. The ordered samples have well fitted two exponential processes decay curves and the time constants are sample dependent and have little relationship with the ordering degree. The decay curve of disordered sample shows that it has single exponential process and its lifetime has a tendency of reduction with the decrease of excitation intensity. The photoluminescence spectra with different delay time at 77 K show that the ordered samples exhibit about 6 ∼ 10 meV blue-shift of PL peak energy with the delay time. Received 13 December 2001 Published online 19 July 2002     

Pico-second transient luminescence of resonantly excited excitonic molecules in CuCl

Time-resolved luminescence spectra of excitonic molecules in CuCl created by the two-photon resonance excitation are studied at 4.2 K with a time resolution of ～ 60 psec. Two sharp emission lines observed are ascribed to the radiative decay of “cold” excitonic molecules with k ～ 2K₀. Stimulated emission process gives a considerable influence on luminescence properties.     

Structure in the emission from pyrene evaporated films

Emission spectra and decay times of fluorescence of pyrene thin films prepared by evaporation onto substrates at liquid nitrogen temperature were measured. Structure in the emission from films warmed slowly up to higher temperature is found to the higher energy side of the excimer emission band. The decay time is about 410 ns at temperatures between 110 K and 210 K. But the emission from the film warmed up to a temperature above 210 K shows only the excimer emission. These results are discussed in relation to an amorphous structure in the pyrene evaporated thin film.     

Picosecond time-resolved luminescence of hydrogenated amorphous silicon-carbon

Photoluminescence decay curves and time-resolved luminescence spectra of amorphous Si_0.15C_0.85 : H films are measure. The decay curves are nearly exponential with a time-constant of about 200 psec at room temperature. The peak of the emission spectrum is shifted from 440 nm immediately after the excitation to 490 nm at 400 psec after the excitation.     

Native Fluorescence and Time Resolved Fluorescence Spectroscopic Characterization of Normal and Malignant Oral Tissues Under UV Excitation—an In Vitro Study

The present work aims to investigates the native fluorescence and time resolved fluorescence spectroscopic characterization of oral tissues under UV excitation. The fluorescence emission spectra of oral tissues at 280 nm excitation were obtained. From the spectra, it was observed that the alteration in the biochemical and morphological changes present in tissues. Subsequently, the Full width at Half Maximum (FWHM) of every individual spectra of 20 normal and 40 malignant subjects were calculated. The student’s t-test analysis reveals that the data were statistically significant (p?=?0.001). The fluorescence excitation spectra at 350 nm emission of malignant tissues confirms the alteration in protein fluorescence with respect to normal counterpart. To quantify the observed spectral differences, the two ratio variables R₁?=?I₂₇₅/I₃₁₀ and R₂?=?I₃₁₀/I₃₂₈ were introduced in the excitation spectra. Among them, the Linear Discriminant Analysis (LDA) of R₁ reveals better classification with 86.4 % specificity and 82.5 % sensitivity. The fluorescence decay kinetics of oral tissues was obtained at 350 nm emission and it was found that the decay kinetics was triple exponential. Then the ROC analysis of fractional amplitudes and component lifetime reveals that the average lifetime shows 77 % sensitivity and 70 % specificity with the cut off value 4.85 ns. Briefly, the average lifetime exhibits better statistical significance when compared to fractional amplitudes and component lifetimes.     

Luminescence of dense, octahedral structured crystalline silicon dioxide (stishovite)

It is obtained that, as grown, non-irradiated stishovite single crystals possess a luminescence center. Three excimer pulsed lasers (KrF, 248 nm; ArF, 193 nm; F₂, 157 nm) were used for photoluminescence (PL) excitation. Two PL bands were observed. One, in UV range with the maximum at 4.7±0.1 eV with FWHM equal to 0.95±0.1 eV, mainly is seen under ArF laser. Another, in blue range with the maximum at 3±0.2 eV with FWHM equal to 0.8±0.2 eV, is seen under all three lasers. The UV band main fast component of decay is with time constant τ=1.2±0.1 ns for the range of temperatures 16-150 K. The blue band decay possesses fast and slow components. The fast component of the blue band decay is about 1.2 ns. The slow component of the blue band well corresponds to exponent with time constant equal to 17±1 μs within the temperature range 16-200 K. deviations from exponential decay were observed as well and explained by influence of nearest interstitial OH groups on the luminescence center. The UV band was not detected for F₂ laser excitation. For the case of KrF laser only a structure less tail up to 4.6 eV was detected. Both the UV and the blue bands were also found in recombination process with two components having characteristic time about 1 and 60 μs. For blue band recombination luminescence decay is lasting to ms range of time with power law decay ∼t⁻¹.For the case of X-ray excitation the luminescence intensity exhibits strong drop down above 100 K. such an effect does not take place in the case of photoexcitation with lasers. The activation energies for both cases are different as well. Average value of that is 0.03±0.01 eV for the case of X-ray luminescence and it is 0.15±0.05 eV for the case of PL. So, the processes of thermal quenching are different for these kinds of excitation and, probably, are related to interaction of the luminescence center with OH groups.Stishovite crystal irradiated with pulses of electron beam (270 kV, 200 A, 10 ns) demonstrates a decrease of luminescence intensity excited with X-ray. So, irradiation with electron beam shows on destruction of luminescent defects.The nature of luminescence excited in the transparency range of stishovite is ascribed to a defect existing in the crystal after growth. Similarity of the stishovite luminescence with that of oxygen deficient silica glass and induced by radiation luminescence of α-quartz crystal presumes similar nature of centers in those materials.     

Time-resolved luminescence spectroscopy of pure and doped with Ce3+ ions SrAlF5 crystals

The results of a study of time-resolved photoluminescence (PL) and energy transfer in both pure and doped with Ce³⁺ ions SrAlF₅ (SAF) single crystals are presented. The time-resolved and steady-state PL spectra in the energy range of 1.5–6.0 eV, the PL excitation spectra and the reflectivity in the energy range of 3.7–21 eV, as well as the PL decay kinetics were measured at 8.8 and 295 K. The lattice defects were revealed in the low temperature PL spectra (emission bands at 2.9 and 4.5 eV) in the undoped SAF crystals. The luminescence spectra of the doped Ce³⁺:SAF crystals demonstrate a new selective emission bands in the range of 3.7–4.5 eV with the exponential decay kinetics (τ ≈ 60 ns at X-ray excitation). These bands correspond to the d-f transitions in Ce³⁺ ions, which occupy nonequivalent sites in the crystal lattice.     

Injection of the edge electroluminescence in ZnSe MOS diodes

Injection luminescence and related properties of metal-ZnO-ZnSe-metal tunnel diodes are described. The edge emission of ZnSe is studied in the temperature range 30-300 K with respect to: a) band shape b) coupling strenght parameter, c) time resolved spectra and d) temperature dependence.External quantum efficiecies of the blue band of 2.61 × 10^-5 have been observed at 77 K. The lowest voltage threshold observed for blue band emission is 1.5 V.The relation between the electroluminescence and electrical characteristics of the MOS diodes are discussed.     

Time-resolved optical spectroscopy of CsI(Tl) crystals by pulsed electron beam irradiation

Properties of the color and emission centers induced with an electron pulse beam at temperature within 80-300 K have been studied in CsI(Tl) crystals. It has been established by optical spectrometry with time resolution that initial color centers in this crystal are only Tl⁰ and V_k centers, which spontaneously recombine emitting visible light at 2.25 and 2.55 eV. It has been shown that the emission decay kinetics at 80 K include two fast exponential components with decay constants 3 and 14 μs as well as slow hyperbolic component with the power index depending on the wavelength of the emitting light. The temperature effect on the emission kinetics has been studied and it has been directly proved that the emission rise stage at the temperature above 170 K is caused by the recombination of electrons, which are thermally released from single Tl⁰ centers, with V_kA centers. The origin of scintillations in CsI(Tl) crystal is discussed in terms of the tunnel electron transitions from ground state of Tl⁰ centers to ground state of V_k centers at different distances from each other.     

N23-core losses and autoionization emissions of clean and oxygen exposed zirconium

The electron energy loss spectra associated with N₂₃-excitation and the low energy N₂₃VV Auger emission have been studied for both the clean and oxygen exposed zirconium. In the high energy side of the N₂₃VV Auger spectrum, autoionization emission of electrons of the valence band due to the decay of 4p electrons excited to states ≈9eV above the Fermi level has been identified. The excitation process can be also observed in the electron energy loss spectra. This is the first time that an autoionization feature is observed in a electron excited Auger spectrum of a 4d transition metal.     

Decay kinetics of the defect-based visible luminescence in ZnO

Photoluminescence spectra and decays under pulsed N₂ (337 nm) laser excitation were measured for hydrothermally grown bulk and liquid-phase epitaxy (LPE)-grown film ZnO samples within 9-300 K. Temperature dependence of integrated spectra over the exciton and visible spectral regions was evaluated using a model involving standard energy barrier processes. Decay curves measured within a broad time window (10 ns-1 ms) and with extreme signal/background ratio (five orders of magnitude) point to complex decay mechanism in which the exponential and inverse power-law processes can coexist. There is no straightforward interconnection between the observed temperature dependence of integrated visible photoluminescence intensity and its decay shape over the 9-300 K temperature interval.     

The luminescence properties of MgUO4

The luminescence of MgUO₄ has been investigated. Emission and excitation spectra as well as the decay time and the quantum efficiency of the emission were measured at 4.2 K. The temperature dependence of the emission spectrum and the emission intensity was studied. The results show that in MgUO₄ the emission originates from uranate centres near defects. These centres act as optical traps. Even at 4.2 K about 90% of the excitation energy is lost non-radiatively by trapping at killer sites. These are most probably due to the presence of U⁵⁺ ions.     

Optical and scintillating properties of Ce:Li(Y,Lu)F4 single crystals

We have investigated the optical and scintillating properties of Lu co-doped Ce:LiYF₄ single crystals with various Lu content. In the transmittance and absorption spectra, the absorption peaks at 243 nm get systematically red shifted in contrast to the peaks at 197 and 200 nm which get blue shifted with the increase in Lu content. At the same time, emission peaks at 306 nm and 200 nm under 295 nm excitation also get red shifted. The decay time of Ce:Li(Y,Lu)F₄ crystals under 295 nm excitation is found to be faster than that of Ce:LiYF₄ and Ce:LiLuF₄ crystals. The alpha-peak positions in the pulse-height spectra and decay times of crystals under alpha-ray irradiation are found to vary with the Lu content.     

Luminescence with short decay time in arsenic chalcogenide glasses

Luminescence with decay time of about 10 nsec or shorter was found in a?As₂S₃ at 4.2 K on the high energy side of the luminescence observed by Kolomiets et al. From the dependence of the emission spectra on the excitation energy, it is interpreted as being due to the recombination of localized excitons.     

Emerging blue‐UV luminescence in cerium doped YAG nanocrystals

Time‐resolved luminescence properties of Ce³⁺ doped Y₃Al₅O₁₂ (YAG) nanocrystals have been studied by means of vacuum‐ultraviolet excitation spectroscopy. It was discovered that additionally to the regular Ce³⁺ yellow‐green emission which is well‐known luminescence in YAG, new emission covering a broad spectral range from 2.7 eV to 3.5 eV was revealed in the luminescence spectra for all YAG:Ce nanocrystals studied. This blue‐UV emission has fast decay time about 7 ns as well as intensive well‐resolved excitation band peaking at 5.9 eV and, in contrast to green Ce³⁺ emission, practically is not excited at higher energies. The origin of the blue‐UV emission is tentatively suggested and discussed. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

The I.R. photoluminescence emission band in ZnO

Measurements of emission spectra, excitation spectra, intensity dependence of the luminescence, decay of the luminescence, and temperature dependence of the luminescence in ZnO are reported. The results for the emission at 1·70 eV, with the exception of the decay of the luminescence, were found to be similar to those of the yellow (2·02 eV) emission band in ZnO. Both bands could be excited at the band edge and directly, the intensity of both bands was found to be linear with excitation strength and the asymptotic regions of the temperature dependence of both bands could be approximated by exponential functions. It is proposed that the luminescent transition is an electron transition from the edge of the conduction band to a hole trapped in the bulk at 1·60 eV above the edge of the valence band, and that the luminescence center is an unassociated acceptor-like center.     

Latent Fingerprint Imaging Using Dy and Sm Codoped HfO2 Nanophosphors: Structure and Luminescence Properties

The development of latent fingerprints (LFPs) detected at the site of crime is considered as an imperative physical evidence in forensic investigations. Herein, a rapid and cost‐effective approach using nonhazardous Dy and Sm codoped HfO₂ nanophosphors is demonstrated to be utilized for LFP imaging. Sol–gel method can produce Dy and Sm codoped HfO₂ of monoclinic phase with crystallite size ranging from ≈10 to 25 nm. While selected area electron diffraction and lattice spacing calculated from high‐resolution transmission electron microscopy confirm monoclinic phase, Le‐Bail profile refinement of X‐ray diffraction (XRD) patterns demonstrate an exponential increase in lattice volume with incorporating various concentrations of Dy and Sm in HfO₂ lattice. Exciting Dy and Sm codoped HfO₂ with 393 nm photoluminescence spectra show emissions in blue, yellow, and near red regions emerging primarily due to energy transfer from Dy³⁺ to Sm³⁺ through multipolar interaction suggested by time resolved decay spectra. Combining excitation and emission spectra, an energy band diagram is proposed. Owing to excellent emissions, Dy and Sm codoped HfO₂ are explored for LFP imaging with good selectivity and resolution over multivariate surfaces like float glass and aluminum foil. The third‐level details like enclosure and termination–bifurcation are extracted due to nanosized nature of particles.     

Photoluminescence of polycrystalline cadmium sulfide films at 77 °K

The absorption, reflection, emission, and luminescence spectra of polycrystalline cadmium sulfide films have been studied at 77 deg K at 4500–5500 A. The films were prepared by sublimination of the powder in argon, hydrogen sulfide, or vacuum, followed by crystallization on a heated or unheated substrate.Specimens deposited on substrates below 350 deg C had simple absorption and emission spectra no matter which medium was used, but ones coated on substrates above 350 deg C had absorption, reflection, luminescence, and emission spectra with fine structure, which was due to transitions between the 5s²¹S₀ ground state and 5s5p³P₁, 5s5p³P₂, and 5s5p³P₀ excited states of atomic excess cadmium.Four maxima were found in the excitation spectrum of the blue luminescence (4545, 4605, 4670, and4740 A). The spectrum of the luminescence is independent of exciting wavelength in the range 2200–4900 A.This structure was observed only for films with hexagonal lattice symmetry.