Fluorescence and absorption of polystyrene exposed to UV laser radiation

We show that when polystyrene is exposed (for 15–60 sec) to a UV laser light beam (λ = 248 nm), its absorption and luminescent properties change significantly. In the irradiated polymer, optical centers are formed with absorption bands in the 280–460 nm region and fluorescence bands in the 330–520 nm region. We have established the chemical structure of the optical centers for fluorescence of polystyrene. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 1, pp. 54–58, January–February, 2006.     

The nature of radiative centers formed by nonbridge oxygen in especially pure sol-gel glasses

We carry out measurements of the optical absorption of sol-gel quartz glasses that have been exposed to γ-radiation. The dependence of optical density on irradiation dose is analyzed, and two types of centers formed by nonbridge oxygen are distinguished. We revealed a difference in the peak wavelengths of absorption bands with an energy of 2.0 eV for various types of centers. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 4, pp. 600–603, July–August, 1998.     

Germania/ormosil hybrid materials derived at low temperature for photonic applications

We report on germania/organically modified silane (ormosil) hybrid materials produced by the sol–gel technique for photonic applications. Acid-catalyzed solutions of γ-glycidoxypropyltrimethoxysilane mixed with germanium isopropoxide have been used as precursors for the hybrid materials. Planar waveguide films with a thickness of about 2 μm have been prepared by a single spin-coating process and low-temperature heat treatment from these high germanium content hybrid materials. Atomic force microscopy, thermal gravimetric analysis, UV–visible spectroscopy, and Fourier-transform infrared spectroscopy have been used to investigate the optical and structural properties of the films. The results have indicated that a dense, low absorption, and high transparency (in the visible range) waveguide film could be achieved at a low temperature. A strong UV-absorption region at short wavelengths ∼200 nm, accompanied by a shoulder peaked at ∼240 nm, has been noticed due to the neutral oxygen monovacancy defects. The propagation mode and loss properties of the planar waveguide films have also been investigated by using a prism-coupling technique. Received: 5 November 2002 / Revised version: 27 December 2002 / Published online: 19 March 2003 RID="*" ID="*"Corresponding author. Fax: +65-67909081, E-mail: ewxque@ntu.edu.sg     

Fluorescence properties of Bi4Ge3O12(BGO) single crystals under laser excitation: Excited state dynamics and saturation effects

Laser-excited techniques were used to investigate the optical properties of bismuth germanate crystals. Absorption, reflectivity, excitation, emission, lifetime, time-resolved fluorescence, photoconductivity, thermally stimulated conductivity measurements were performed at various temperatures on single crystals of different origins.The absorption is shown to occur in bismuth and germanate centers while both intrinsic and perturbed Bi³⁺ ions together with impurities contribute to the total fluorescence.The emission mechanism at room temperature involves a thermally activated energy migration, and at low temperature localized emitting centers. Formation of deep holes in the wide emission band at room temperature reveals saturation effects on various luminescent centers, promoted by energy migration. Trapped exciton models are proposed to explain the excited state dynamics occurring at low and room temperature.     

Optical nonlinearities in hyperbranched polyyne studied by?two-photon excited fluorescence and third-harmonic generation spectroscopy

The nonlinear optical properties of a hyperbranched polyyne (hb-Polyyne) have been measured at infrared wavelengths by using femtosecond and nanosecond pulsed excitation. This hyperbranched polyyne exhibited strong and intrinsic (simultaneous) two-photon absorption and upconverted blue fluorescent emission under femtosecond excitation around 800 nm. The hb-Polyyne in chloroform solution is characterized by a large two-photon absorption cross section of 9068 GM (1GM=10⁻⁵⁰ cm⁴ s) and a fluorescence quantum yield of 0.57. On the other hand, by third-harmonic generation (THG) spectroscopy with nanosecond excitation, the measured third-order nonlinear susceptibility χ ⁽³⁾ for solid films of hb-Polyyne ranged from 2.4×10⁻¹¹ to 6.1×10⁻¹¹ esu in the spectral range of 1100–1600 nm, with results comparable to the values exhibited by the well-known conjugated polymer MEH:PPV, but with a much better transparency for visible wavelengths.     

Unconventional approaches to combine optical transparency with electrical conductivity

The combination of electrical conductivity and optical transparency in the same material – known to be a prerogative of only a few oxides of post-transition metals, such as In, Sn, Zn and Cd – manifests itself in a distinctive band structure of the transparent conductor host. While the oxides of other elements with s² electronic configuration, for example, Mg, Ca, Sc and Al, also exhibit the desired optical and electronic features, they have not been considered as candidates for achieving good electrical conductivity because of the challenges of efficient carrier generation in these wide-bandgap materials. Here we demonstrate that alternative approaches to the problem not only allow for attaining the transport and optical properties which compete with those in currently utilized transparent conducting oxides (TCO), but also significantly broaden the range of materials with a potential of being developed into novel functional transparent conductors. PACS 71.20.-b; 72.20.-i     

Luminescent properties of MBE-grown Si:Er/SOI structures

Here we report on luminescent properties of multilayer Si:Er structures grown by sublimation molecular-beam epitaxy on “silicon-on-insulator” substrates. We demonstrate formation in such structures of a unique erbium-related center Er-1. This optical complex stands out among other known erbium-related centers in silicon for its record narrow luminescent line (<10 μeV) and largest absorption cross-section and, therefore, provides the best conditions to achieve practically significant amplification and stimulated emission in erbium-doped silicon structures.     

Electrical and optical analyses of Er-doped silicon grown by liquid-phase epitaxy

We have studied the deep levels present in Er-doped silicon epilayers grown by the liquid-phase epitaxy method by deep level transient spectroscopy (DLTS) and optical DLTS, in order to identify the majority and minority carrier traps and a possible correlation between these traps and the observed photoluminescence (PL) and cathodoluminescence (CL) spectra. Capacitance–voltage analyses have been performed to analyze uniformity and depth distribution of the existing traps and marked differences have been observed between the luminescent and non-luminescent materials.The PL and depth resolved CL revealed the presence of dislocation-related emission lines which can possibly be correlated to the broadened peaks observed in DLTS analyses of luminescent material.     

Influence of temperature and environment humidity on the transmission spectrum of sol-gel hybrid channel waveguides

Hybrid materials produced by the hydrolytic sol-gel process reveal strong absorption in the spectral region of 1550 nm, mainly due to the high concentration of OH groups. This characteristic constitutes an obstacle to the application of these materials, in particular to telecom integrated optical devices. A simple thermal treatment applied to the device does not contribute to increase the transparency. The real-time influence of temperature and environment humidity on the optical transmission properties of sol-gel organic-inorganic raised channel waveguides is analyzed in order to achieve an understanding of the OH group’s behaviour in the materials. We demonstrate that the level of the optical losses at 1550 nm can significantly be decreased at high temperature. Cooling the sample in dry atmosphere makes it possible to reach a better transparency, which can be useful to improve the performance of hybrid sol-gel optical devices.     

The Correlation Between <Emphasis Type="Italic">f–f</Emphasis> Absorption and Sensitized Visible Light Emission of Luminescent Pr(III) Complexes: Role of Solvents and Ancillary Ligands on Sensitivity

The electronic absorption, excitation and sensitized visible light emission studies of three praseodymium (III) complexes: [Pr(fod)₃(bpy)], [Pr(fod)₃(phen)] and [Pr(fod)₃(bpm)]_n (fod = anion of 6,6,7,7,8,8,8- heptafluoro-2,2-dimethyl-3,5-octanedione; bpy = 2,2'-bipyridyl, phen = 1,10-phenanthroline, bpm = 2,2'-bipyrimidine) in a series of non–aqueous solvents is presented. The f–f absorption transitions of Pr(III) are environment sensitive which is reflected by the change in the intensity (oscillator strength) and band shape (stark splitting) upon change in the solvent and/or the ligands. The sensitization of intense Pr(III) emission, in the visible region, of the complexes in solution upon excitation into the ligand centered π→π* absorption band is remarkable. The planar phen has pronounced impact and increases considerably the emission intensity of Pr(III) luminescence than the flexible bpy while bpm has been found least effective in promoting the emission intensity. The intensity of the f-f absorption and sensitized emission are correlated with the nature of the solvents. The donor solvent pyridine enhances the emission intensity of the [Pr(fod)₃(phen)] drastically and of [Pr(fod)₃(bpy)] marginally while the luminescent intensity of [Pr(fod)₃(bpm)]_n is decreased. The combined photophysical studies demonstrate that entry of the solvent molecule(s) to inner coordination sphere (complex–solvent interaction) is governed by the structure and basicity of the ancillary heterocyclic ligand attached to the Pr(III) complex. The strong donor DMSO transforms the three complexes into a similar species, [Ln(fod)₃(DMSO)₂], which results in similar electronic absorption and emission properties of the complexes in this solvent. The results demonstrate that highly luminescent praseodymium chelates can be designed with ligands containing suitable energy levels and their emission properties can be further modulated through suitable ancillary ligands and donor solvents, thus opening perspectives for applications in electroluminescent devices and luminescent probes.     

Femtosecond energy relaxation in suspended graphene: phonon-assisted spreading of quasiparticle distribution

Ultra-fast optical measurements of few-layer suspended graphene films grown by chemical vapor deposition were performed with femtosecond pump–probe spectroscopy. The relaxation processes were monitored in transient differential transmission (ΔT/T) after excitation at two different wavelengths of 350 and 680 nm. Intraband electron–electron scattering, electron–phonon scattering, interband Auger recombination and impact ionization were considered to contribute to ΔT/T. All these processes may play important roles in spreading the quasiparticle distribution in time scales up to 100 fs. Optical phonon emission and absorption by highly excited non-equilibrium electrons were identified from ΔT/T peaks in the wide spectral range. When the probe energy region was far from the pump energy, the energy dependence of the quasiparticle decay rate was found to be linear. Longer lifetimes were observed when the quasiparticle population was localized due to optical phonon emission or absorption.     

含稀土铕(Ⅲ)配合物光学树脂的制备及光学性能的研究

报道了将多种稀土铕（Ⅲ）的二元及三元配合物复合于苯乙烯（St)／甲基丙烯酸（HMA）的共聚体系,制备得到具有发光功能的透明光学树脂,对其光学性能进行研究。同时,考察了稀土配合物的含量对聚合物 透明性、发光性能的影响。     

Theoretical study on optical and electronic properties of bis-dipolar 1,8-naphthalimide derivatives

A series of donor–π–acceptor type of 1,8-naphthalimide derivatives with ethylene as π-conjugated bridges have been designed to explore their optical and electronic properties as luminescent materials for organic light-emitting diodes (OLEDs). The frontier molecular orbital analysis turned out that the vertical electronic transitions of absorption and emission are characteristic as intramolecular charge transfer. The calculations showed that their optical and electronic behaviours are clearly affected by the aromatic substitute groups, but not significantly to the stability of molecules. The calculated results suggest that all the selected candidates are promising as luminescent materials for OLEDs.     

Spectrally resolved light absorption properties of cooled soot from?a?methane flame

The optical properties of combustion-generated soot, crucial information for quantitative soot emission diagnostics and for climate modeling, have been determined for the particular case of cooled soot from a methane flame. Optical extinction measurements were performed over a wavelength range of 450–750 nm using a novel diffuse-light, spectrally resolved line-of-sight attenuation experiment, and quantified using extractive methods coupled with scanning and transmission electron microscopy in conjunction with a detailed uncertainty analysis. The absorption component of the total measured extinction was isolated by calculating the expected scattering contribution, according to the Rayleigh–Debye–Gans approximation for polydisperse fractal aggregates. In contrast to the large degree of scatter seen in data previously reported in the literature, a consistent trend of negligible variation of the soot absorption refractive index function E(m) with wavelength over the visible was observed (E(m)=0.35±0.03 at wavelengths of 450–750 nm). These new data are also cast in the form of dimensionless extinction, which is independent of the scatter correction, as well as mass absorption cross section, which is independent of the mass density of soot and is commonly used by atmospheric modelers.     

Emission properties of femtosecond (fs) laser fabricated microstructures in Polystyrene (PS)

We fabricated microstructures and micro craters in thin films and bulk of PS. To the best of our knowledge, this is the first report of the emission from fs laser modified regions of PS when excited at 337, 400, 458, 488 and 514 nm wavelengths with different emission peaks. We systematically studied the emission in context of formation of optical centers and analyzed the spectra of irradiated PS. Change in the excitation wavelength leads to a shift in the emission peak, whereby, we infer that the emission should be due to a myriad of optical centers. Interestingly these optical centers have a similar excitation spectrum. Diphenylbutadiene (DPBD) is probably the main optical center among other optical centers thus formed in the process of fs laser irradiation of PS.     

Study of the spectral properties of alkali halide electroluminescence with plasma electrodes

A method is given for separating the electroluminescence (EL) spectrum of alkali halide crystals (AHC) from the emission from plasma electrodes. Results are given for experimental studies of EL in AHC with electrodes in the form of a low-pressure plasma over a broad temperature range (from room temperature to 120 K) and electric fields up to 10⁹ V/m. The importance of the effect of defect formation processes on the structure of luminescent centers and the quantum yield of EL in AHC is shown. A mechanism is proposed for formation of the luminescent centers in AHC in a very strong electric field. Control Systems and Radio Electronics, Tomsk University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fiziaa, No. 9, pp. 102–106, September, 1997.     

Color and luminescence centers in LiF(U) crystals irradiated by pulsed electron beams

Characteristics of color and luminescent centers produced in LiF:U⁶⁺ (0.01M%) crystals by pulses of accelerated electrons (0.25 MeV, 15 ns, 8–200 mJ/cm²) at 77–350 K are studied using time-resolved optical spectroscopy. It is found that the time constant of the exponential rise of the cathodoluminescence intensity in the visual spectral range after completion of the irradiation pulse coincides with the decay time of a group of lines in the UV region at T = 77–280 K and with the decay time of the number of V _k -centers induced by the pulse of irradiation at T > 280 K. The mechanisms of population of the ground radiative level of the emission center and the structure of its electronic and nuclear subsystems are discussed.     

Properties of the oxygen vacancy in ZnO

As-grown ZnO bulk crystals and crystals annealed in vacuum, oxygen, or zinc vapour were characterized by electrical, optical and magnetic resonance spectroscopy. The experiments show that the residual carrier concentration is caused by residual H, Al, Ga and oxygen vacancies (V_O) in the material. Annealing the samples in O₂ at about 1000 °C (2 atm, 20 h) reduces the H and V_O donor concentration by typically one order of magnitude. The photoluminescence and deep level transient spectroscopy (DLTS) results suggest a correlation between the broad unstructured emission at 2.45 eV (“green band”) and a donor level 530 meV below the conduction band, it is attributed to the V_O ^0/++transition. By using DLTS experiments with optical excitation it is possible to observe a metastable level 140 meV below the conduction band which is assigned to the V_O ^2+/+ recharging. The results give evidence for the “negative-U” properties of the oxygen vacancy defects predicted by recent theoretical calculations. PACS 71.55.Gs; 72.20.Jv; 76.30.Mi; 76.70.Hb; 78.55.Et     

Preparation and optical properties of sol–gel neodymium-doped germania/γ-glycidoxypropyltrimethoxysilane organic–inorganic hybrid thin films

Germania/γ-glycidoxypropyltrimethoxysilane organic–inorganic hybrid spin-coating thin films doped with neodymium ions are prepared by a sol–gel technique and a spin-coating process. Acid-catalyzed solutions of γ-glycidoxypropyltrimethoxysilane mixed with germanium isopropoxide are used as matrix precursors. Thermal gravimetric analysis, UV–visible spectroscopy, and Fourier transform infrared spectroscopy are used to study the structural and optical properties of the hybrid thin films. The results indicate that films that are crack-free and have a high transparency in the visible and near-infrared range can be obtained; a strong UV absorption region at short wavelength ∼200 nm, accompanied with a shoulder peaked at ∼240 nm, due to the neutral oxygen monovacancy defects, is also identified. Upconversion emission properties of the transparent dried gel and the thin films heated at different heat treatment temperatures and doped with different neodymium ion concentrations are studied; a relatively strong room-temperature yellow to violet upconversion emission at 397 nm (⁴ D _3/2→⁴ I _13/2) is observed under a xenon lamp excitation with yellow light at the wavelength of 580 nm (⁴ I _9/2→⁴ G _5/2). The effect of Nd³⁺ doping concentration and heat treatment temperature on upconversion emission of the thin films is also studied. The mechanism of the upconversion is proposed. PACS 81.05.Kf; 81.20.Fw; 78.55.Hx     

Recombination processes in europium-doped cadmium iodide crystals

Results of comprehensive research into optical and luminescent-kinetic characteristics of europium-doped cadmium iodide crystals excited by nitrogen laser radiation, α-particles, and x-rays are presented. Crystals under study have been grown by the Bridgman–Stockbarger method. The doping EuCl₃ admixture was introduced into the charge in quantities of about 0.05 and 1.0 mol%. Impurity absorption detected in the near-edge region of the crystals is interpreted as part of the Eu²⁺ ion long-wavelength band associated with f–d-transitions. The cation impurity and matrix defects in CdI₂:Eu²⁺ crystals create complex centers responsible for emission with a maximum in the 580–600-nm region. The short component in the luminescence decay kinetics of weakly-doped crystal excited by α-particles and x-ray photons is due to the exciton emission characteristic of CdI₂. The slow component in the scintillation pulse results from recombination of charge carriers followed by creation of exciton-like states on the defect-impurity centers. Laser or x-ray excitation induces light-sum accumulation on the trapping levels at a depth of 0.2–0.6 eV that is mainly related to matrix microdefects. Trapping centers associated with the chlorine impurity are observed in the heavily-doped crystal. Photostimulated luminescence at 85 K arising at the electron stage of the recombination process is caused by recombination of electrons released from F-type centers with holes localized near the activator. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 3, pp. 358–364, May–June, 2009.