Constant fluorescence during phase grating formation and defect band bleaching in optical fibres under 5.1 eV laser exposure

The UV laser-induced bleaching of the 5.1 eV defect absorption band in optical fibres is correlated with photosensitive grating formation. No significant change in the 3.1 eV fluorescence is observed on this same time scale.<>     

Nanostructured semiconductor block copolymers: π–π Stacking,optical and electrochemical properties

The structural and optical properties of semiconductor block copolymers containing triphenylamine as hole transport material and perylene bisimide as dye and electron transport material are reported. The polymers were prepared by nitroxide mediated controlled radical polymerisation and characterized with GPC, DSC, and TGA. The electrochemical properties as determined by cyclic voltammetry show the HOMO and LUMO values of the block copolymers to be −5.23 eV and −3.65 eV, respectively. The perylene bisimide units aggregate by π–π stacking which could be analyzed with wide-angle X-ray scattering. The absorption and fluorescence properties of the perylene bisimide polymers and monomers in solution and film were investigated. It could be shown that they are strongly influenced by intramolecular coupling between different perylene bisimide units in polymers. The block copolymers exhibit a microphase separation on a nanometer scale with a constant perylene bisimide domain width of 13 nm and lengths of up to several micrometers.     

Magnetron sputtering synthesis of silicon–carbon films: structural and optical characterization

This study deals with the growth control and characterization of wide band gap silicon–carbon films obtained by reactive hydrogen plasma sputtering. The films were grown in a pure hydrogen plasma with different values of the carbon-to-silicon sputtered area ratio, r_C. During deposition, the substrate temperature was maintained at 730°C. Infrared absorption, Raman scattering and X-ray photoelectron spectroscopy, in addition to optical absorption, were used for the investigations. For C-poor samples (r_C≤30%), Si nanocrystals were formed, together with a small fraction of amorphous SiC. Further increase of sputtered carbon (r_C≥35%) led to a drastic change, resulting in SiC crystallization at the expense of Si and a near-stoichiometric composition of the layers (C/Si atomic ratio of 1.04). Excess carbon in the layers segregates in graphitic-like configuration, being likely located in the intergrain regions. The abrupt structural change observed for 30%≤r_C≤35% is accompanied by a consistent widening of the optical band gap. This is observed by a significant blue shift of the optical absorption towards the values reported for single crystal SiC. The energy gap at which optical absorption is 5×10⁴ cm⁻¹ shifts from 2.2–2.3 eV to about 4.1 eV. This structural change also correlates with a significant decrease of the refractive index.     

Effect of cobalt substitution on the optical properties of bismuth ferrite thin films

Effect of cobalt substitution on the band gap and absorption coefficient of the BiFeO₃ thin films formed on quartz substrate by low cost spin coating method have been investigated. BiFe_1−xCo_xO₃ (x=0, 0.03, 0.06 and 0.10) thin films are polycrystalline and it retains the rhombohedral distorted perovskite structure up to 10 mole % of Co substitution. Smooth and compact surface morphology with uniform size particles are observed in SEM micrographs. Narrowing and broadening of band gap is observed as a function of Co content. Two strong emission peaks at ~2.51 eV and ~2.38 eV are recorded for all films with noticeable change in intensity. Results obtained from the optical absorption and photoluminescence spectroscopy experiments have shown that there exists an inverse correlation between the variation in the band gap and the concentration of oxygen vacancies. Band gap decreased by ~100 meV and absorption coefficient increased by 28% at the wavelength of 375 nm in 6 mole % Co substituted thin film and these observations are necessary requirements to improve the efficiency of photovoltaic devices.     

Structural and optical studies on PVA capped SnS films grown by chemical bath deposition for solar cell application

Tin monosulphide (SnS) thin films capped by PVA have been successfully deposited on glass substrates for cost effective photovoltaic device applications by a simple and low-cost wet chemical process, chemical bath deposition (CBD) at different bath temperatures varying in the range, 50–80 °C. X–ray diffraction analysis showed that the deposited films were polycrystalline in nature, showing orthorhombic structure with an intense peak corresponding to (040) plane of SnS. These observations were further confirmed by Raman analysis. FTIR spectra showed the absorption bands which corresponds to PVA in addition to SnS. The scanning electron microscopy and atomic force microscopy studies revealed that the deposited SnS films were uniform and nanostructured with an average particle size of 4.9 to 7.6 nm. The optical investigations showed that the layers were highly absorbing with the optical absorption coefficient ~10⁵ cm^–1. A decrease in optical band gap from 1.92 to 1.55 eV with an increase of bath temperature was observed. The observed band gap values were higher than the bulk value of 1.3 eV, which might be due to quantum confinement effect. The optical band gap values were also used to calculate particle size and the results are discussed.     

Optical properties of photochromatic sulfur-doped chlorosodalite

Synthetic.photochromic sulfo-chlorosodalite, 6(NaAlSiO₄) ·2 NaCl(S), has been thoroughly investigated by measurements of optical absorption, photo-luminescence and cathodoluminescence. Depending on the sulfur ion form and concentration, the doped sodalite exhibits either sensitive tene-brescence or photoluminescence with long wavelength UV excitation. The photo-induced color absorption peaks at 5260A at 300°K with absorption coefficient, Δα_max >200 cm⁻¹ . This is by far the highest photo-induced absorption observed for synthetic chlorosodalite. At 80°K, the peak position of the absorption does not show significant shift within instrumental accuracy. In photoluminescence, the emission spectra as well as the excitation spectra are studied at both 300 and 78°K. Four characteristic spectral bands (IR, blue, red, and a band with oscillation in wavelength) are observed. The oscillatory S₂ ^- ion emission band starting about 2.35 eV and extending to lower energy and the IR band peaked at 1.4 eV are most efficiently excited by 3660A (3.4 eV), whereas the blue luminescence peaked at 2.7 eV has an excitation threshold of 3.9 eV. The red band is often masked by the oscillatory band and can be observed by higher energy excitation. The red and blue bands are also observable in the cathodoluminescence measurements of the sulfur-doped samples but not the undoped samples. Correlating the absorption, luminescence, and excitation spectral results, a quantitative model is derived to interpret the nature and the role of sulfur ions in the photochromic chlorosodalite material.     

Specific features of the spectra of nonlinear optical absorption in nonstoichiometric and Ni doped GaSe single crystals

The method of two radiation sources is used to measure the spectra of nonlinear optical absorption in the GaSe, GaSe:Ni, Ca_1.05Se_0.95, and Ca_0.95Se_1.05 single crystals. The levels with the depths of 1.38, 1.45, 1.55, 1.65, and 1.80 eV are detected. Absorption by nonequilibrium charge carriers, induced absorption, and induced transmission caused by a variation in the occupation of impurity centers due to laser radiation are observed in the spectra of nonlinear optical absorption.     

Electrical and optical properties of copper indium ditelluride crystals grown from near-stoichiometric compositions

CuInTe₂ crystals grown by zone-leveling of nominally stoichiometric material are highly p-type. The net acceptor density is reduced by in-diffusion of zinc which acts as a donor impurity. Zinc-annealed crystals transmit in the infrared and exhibit an absorption edge at 0.92 eV.     

Effect of thermal annealing on optical and photoelectric properties of microcrystalline hydrogenated silicon films

The effect of thermal annealing in the temperature range T _a=300–600°C of films of microcrystal-line hydrogenated silicon (μc-Si:H) lightly doped with boron on the spectral dependences of the absorption coefficient (α) at photon energies hν=0.8–2.0 eV, dark conductivity (σ_d), and photoconductivity (Δσ_ph) was studied at room temperature. With increasing annealing temperature, a nonmonotonic variation of α (at hν<1.2 eV), σ_d, and Δσ_ph was observed. The data obtained are attributed to a change in the concentration of electrically active impurities and formation of defects, caused by hydrogen effusion and bond restructuring at high annealing temperatures.     

a-Si_3N_4纳米粒子的能级结构研究

用激光化学气相沉积法制备出平均粒径为１０ｎｍ的 ａ－Ｓｉ_３Ｎ_４纳米粒子,对不同浓度的 ａ－Ｓｉ_３Ｎ_４纳米粒子溶液进行了紫外吸收及荧光光谱测试,并首次在３．０～４．１ ｅＶ范围发现６个荧光峰．根据吸收及荧光光谱研究其能级结构,描述了ａ－Ｓｉ_３Ｎ_４纳米粒子的物理结构图像,验证了硅悬挂键Ｓｉ~Ｏ在ａ－Ｓｉ_３Ｎ_４纳米粒子光谱性质中的主导作用．     

The optimization of the optical cavity of infrared Schottky-barrier detectors

In this work, we numerically solve the parameters governing the photo absorption of the back-surface of Schottky-barrier detectors. A reflectance as low as 19% is predicted if the refractive indices and thicknesses of the compositional layers are properly selected. The quantum efficiency can be improved to a value of 6% by an implantation of positive ions into the silicon interlayer for a silicide thickness of 80 Å and a barrier height of 0.18 eV where the optical absorption is optimized at 4 μm.     

Quantifying spatial localization of optical mapping using Monte Carlo simulations

Optical mapping techniques used to study spatial distributions of cardiac activity can be divided into two categories. 1) Broad-field excitation method, in which hearts stained with voltage or calcium sensitive dyes are illuminated with broad-field excitation light and fluorescence is collected by image or photodiode arrays. 2) Laser scanning method, in which illumination uses a scanning laser and fluorescence is collected with a photomultiplier tube. The spatial localization of the fluorescence signal for these two methods is unknown and may depend upon light absorption and scattering at both excitation and emission wavelengths. We measured the absorption coefficients (micro a), scattering coefficients (micro s), and scattering anisotropy coefficients (g) at representative excitation and emission wavelengths in rabbit heart tissue stained with di-4-ANEPPS or co-stained with both Rh237 and Oregon Green 488 BAPTA 1. Monte Carlo models were then used to simulate absorption and scattering of excitation light and fluorescence emission light for both broad-field and laser methods in three-dimensional tissue. Contributions of local emissions throughout the tissue to fluorescence collected from the tissue surface were determined for both methods. Our results show that spatial localization depends on the light absorption and scattering in tissue and on the optical mapping method that is used. A tissue region larger than the laser beam or collecting area of the array element contributes to the optical recordings.     

The optical and detector properties of the PbS-Si heterojunction

Related optical and detector properties of the PbS-Si heterojunction (HJ) are presented and discussed. A direct optical bandgap at 300 K of 0.44 eV has been measured for PbS-Si versus 0.42 eV for PbS-sapphire. The HJ photocurrent frequency response has been determined to consist of two waveform components: Type A, a "spike" present only during the turn-on and turn-off stages of a radiation pulse, is bias independent; Type B, a continuous response, which increases with reverse bias. The temperature characteristic of the photocurrent has been observed to be strongly dependent on which waveform type dominates. The HJ noise current is Johnson noise limited at zero bias and 1/fnoise limited under reverse bias. The HJ dark current is postulated to have four main generation mechanisms depending on temperature: Si-generated current, diffusion current from the PbS, PbS depletion or surface-generation current, and surface leakage. The PbS-Si HJ spectral response combines the effect of intrinsic absorption in Si and PbS and transitions between the Si and PbS valence bands. A detectivity of 1 × 10¹¹cm . Hz^1/2/W has been obtained at 3 µm.     

On the electrical and optical properties of oxide nanolayers produced by the thermal oxidation of metal tin

Thin SnO_2–x layers, 30 nm in thickness, are produced by the thermal oxidation of metal tin nanolayers at a temperature of 450–750°C. The electrical and optical properties of the layers are studied. During the thermal oxidation of tin nanolayers, an unsteady variation in their conductivity is observed. For the oxide films produced at 450 and 550°C, an absorption band at 340 nm (3.65 eV) is detected in the optical spectra. The conductivity-activation energy is determined for samples oxidized to different degrees. On the basis of experimental data and the data reported in publications, an oxidation mechanism controlling the properties of Sn nanolayers is proposed.     

Fabrication and properties of polymer optical fibers containingNd-chelate

Graded-index poly(methyl methacrylate) optical fibers (GI POF) containing a Nd-chelate have been fabricated. The absorption spectrum of the fiber exhibited several strong bands in the visible and infrared regions. We have observed infrared fluorescence (0.90, 1.06, and 1.3 μm) of the Nd³⁺ ion of the fiber at room temperature when it was pumped with an Ar⁺-pumped dye laser at 580 nm     

Fabrication and optical properties of neodymium-, praseodymium- anderbium-chelates-doped plastic optical fibres

Neodymium, praseodymium and erbium ions are successfully incorporated into the core of deuterated polymer-based optical fibres. The spectra of three fibres have several strong absorption bands in the visible and infrared regions. The fluorescence lifetime (6.24 μs) at 1060 nm of Nd-chelate doped plastic optical fibre with an uncooled photodiode is obtained     

Anisotropic optical properties of single crystalline PTCDA studied by spectroscopic ellipsometry

We report anisotropic spectra of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) in the optical energy range 1.5–3.7 eV, measured at room temperature by spectroscopic ellipsometry. The results were obtained on single crystals of α-PTCDA grown by temperature gradient sublimation. Spectra were measured at highly symmetric positions of the plane of incidence relative to the sample crystallographic axes and are presented as pseudo-values of refractive index 〈n〉 and extinction coefficient 〈k〉, which reflect a very large anisotropy of these optical functions. The spectroscopic properties of the excited states in the molecular plane (1 0 2) are analyzed; in particular, we identify clear Davydov splittings proving the formation of coherent excitons in the crystal. Out-of-plane, we observe a relatively narrow absorption peak at ∼2.9 eV that is ascribed to a charge-transfer transition.     

Brush electrodeposited silver indium selenide films and their optical characteristics

Silver indium selenide films were brush electrodeposited on tin oxide coated glass substrates at different substrate temperatures. The films were single phase with chalcopyrite structure. Optical absorption measurements indicated a band gap in the range of 1.20–1.30 eV with decrease of substrate temperature. Transmission spectra exhibited interference fringes. Using the envelope method, calculated values of refractive index at 850 nm decreased from 3.53 to 2.62 with decrease of substrate temperature. From the refractive index data, the value of N/m^⁎ was estimated to be in the range of 0.89–1.22. Optical data were analyzed by the single-effective oscillator model, and the single oscillator energy as well as the dispersion energy was estimated. The single oscillator energy decreased from 1.83 eV to 1.68 eV with the increase of substrate temperature. The dispersion energy increased from 5.42 eV to 12.25 eV with the increase of substrate temperature.     

Growth mechanism and optical properties of semiconducting Mg2Si thin films

Reactive deposition of magnesium onto a hot silicon substrate (200–500°C) does not result in any accumulation of magnesium or its silicide — the condensation coefficient of magnesium being zero. On the other hand, codeposition of magnesium with silicon at 200°C, using a magnesium-rich flux ratio, gives a stoichiometric Mg₂Si film. The number of magnesium atoms which condense is equal to twice the number of silicon atoms which were deposited. The Mg₂Si layers are polycrystalline with a (111) texture. These stoichiometric silicide films still show a tendency to sublimate; whereas capping with an oxide results in extensive intermixing during annealing. The Mg₂Si films thus obtained exhibit optical transparency at sufficiently long wavelength, and an absorption edge. Extraction of the absorption coefficient from the data, and analysis of its energy dependence suggest an indirect bandgap of 0.74 eV, plus direct transitions at 0.83 and 0.99 eV.     

The electronic and optical properties of amorphous silica with hydrogen defects by ab initio calculations

Hydrogen can be trapped in the bulk materials in four forms:interstitial molecular H2,interstitial atom H,O-H+(2Si=O-H)+,Si-H-(4O-SiH)-to affect the electronic and optical properties of amorphous silica.Therefore,the electronic and optical properties of defect-free and hydrogen defects in amorphous silica were performed within the scheme of density functional theory.Initially,the negative charged states hydrogen defects introduced new defect level between the valence band top and conduction band bottom.However,the neutral and positive charged state hydrogen defects made both the valence band and conduction band transfer to the lower energy.Subsequently,the optical properties such as absorption spectra,conductivity and loss functions were analyzed.It is indicated that the negative hydrogen defects caused the absorption peak ranging from 0 to 2.0 eV while the positive states produced absorption peaks at lower energy and two strong absorption peaks arose at 6.9 and 9.0 eV.However,the neutral hydrogen defects just improved the intensity of absorption spectrum.This may give insights into understanding the mechanism of laser-induced damage for optical materials.