Concentration Of Color Centers Research Articles

Radiation-induced vacancy aggregation modes and radiation changes in optical and scintillation properties of insulator crystals

The mechanism of radiation changes involves both the formation of vacancy clusters (pores) from photoproduced vacancies and the optical manifestations of pores. The growth of pores is controlled by dislocations (serving as sinks for interstitials generated simultaneously with vacancies). Depending on dislocation density, the total surface of growing pores can vary with radiation dose in quite different ways. This permits one to control crystal properties via a chosen regime of irradiation. Optically, radiation-produced pores manifest themselves as follows: 1) in excitonic absorption, which strongly exceeds the absorption of usual color centers of the same concentration and can be used for estimating the concentration of pores and 2) via trapping excitons inside the exciton band, thus enhancing intrinsic luminescence and diminishing the luminescence of the dope impurity. This paper presents a joint review of the above aspects of the vacancy cluster physics which were developed separately in recent publications of the authors.

Origin of the radiation-induced 420 nm color center absorption band in PbWO 4 crystals

Polarized absorption spectra experiments were carried out to investigate the structural symmetries of color centers of PbWO 4 crystals. It was found that the intensity of the electrical vector E⊥ c of the radiation-induced 420 nm absorption band equals that of the E∥ c, indicating the non-dichroic character of the band. Based on the analysis of the energy band structure and the crystal structure of PbWO 4, the origin of the radiation-induced 420 nm absorption band was ascribed to the V F 0 di-hole center.

Photoreactions of small molecules at the surface of alkali metal halides

The photostimulated reactions of small molecules, i.e., O 2 photoadsorption and photodesorption, H 2 and CO photooxidation, and CO 2 photodecomposition on alkali halides (alkali halides) surface are discussed in this work. Most of 19 alkali halides show the activity in the reactions mentioned above. Three spectral regions of alkali halides stimulation by light are distinguished in relation to kinetics of oxygen photoadsorption and reactivity of adsorbed oxygen species. They are in correspondence with: (i) bulk exciton absorption bands; (ii) extrinsic and intrinsic bulk and surface absorption bands and (iii) color centers absorption bands. In the latter case the activity of alkali halides arises after pre-excitation in exciton bands, which leads to sensitization of alkali halides to visible light. The H 2 and CO photooxidation on alkali halides is two-step photoprocess involving some excited states of adsorbed oxygen. The latter is quenching by gaseous oxygen as it is shown for hydrogen oxidation over KBr. The photodecomposition of carbon dioxide is treated as CO 2 dissociative adsorption with CO evolution into the gas phase.

Luminescence, intrinsic photoeffect, and color-center conversion in anisotropic crystals under femtosecond laser excitation

The spatial-temporal dynamics of the photochemical conversion of color centers with varying multipolarity and orientation of quantum transitions in irradiated anisotropic crystals exposed to pulsed polarized femtosecond optical radiation and the resulting spatial distribution of the residual and newly-formed color centers are studied. The properties of the concurrent luminscence and photoconductive processes closely related to the photochemical transformation of light-absorbing quantum systems are also examined. A femtosecond laser technique is proposed, which allows extended spatial-periodic structures or narrow profiles of color-center concentrations, including single thin layers or dips, to be formed at a specified depth from the crystal surface through photochemical transformations.

Effect of energetic treatments on the structure and resistivity of evaporated MoO 3 films on cadmium sulfide substrates

We report the relationship between structure and morphology, and the resultant resistivity of amorphous molybdenum oxide MoO 3 evaporated onto polycrystalline cadmium sulfide (CdS) substrates. Cadmium sulfide layers were synthesized on glass slide substrates using the chemical bath deposition technique (CBD). The obtained CdS films were yellowish, transparent, and homogeneous, of about 100 nm in thickness. Molybdenum oxide films were deposited onto glass and glass/CdS substrates by vacuum thermal evaporation, with film thickness of about 350 nm. X-ray diffraction studies showed that the chemically deposited CdS films were polycrystalline, and the MoO 3 films deposited were amorphous, when annealing treatments were below 225°C. Atomic force microscopy showed that the CdS and MoO 3 topographies were uniform. When the glass/MoO 3 and the glass/CdS/MoO 3 films were irradiated using a tungsten lamp for 30, 60, 120, and 160 min they maintained their amorphicity. Energetic treatments did not significantly modify the resistivity; however, the presence of a CdS layer lowers the resistivity by approximately one order of magnitude. Higher degrees of amorphicity increased the resistivity in both systems, and were marked by an increase in the color centers concentration.

Enhancement of the photochromic and thermochromic properties of molybdenum oxide thin films by a cadmium sulfide underlayer

We investigated the photochromic and thermochromic behavior of amorphous MoO3 films prepared by thermal evaporation of MoO3 powder on glass and glass/CdS substrates. We used the cadmium sulfide as a carrier charge injector to produce higher color center concentrations in the MoO3 film. The semiconductor CdS (cadmium sulfide) film was synthesized using the chemical bath deposition technique. The glass/MoO3 and glass/CdS/MoO3 films were subjected to tungsten lamp (100 W) exposure times of 45 to 180 min to study the photochromic sensitivity of the films. To study the thermochromic properties, samples were thermally annealed at temperatures ranging from 100 to 250 °C during 2 h. X-ray diffraction studies show that the CdS films were polycrystalline, and the oxide films were amorphous. Optical absorption measurements showed the presence of an absorption band centered around 850 nm due to the formation of color centers. Concentrations of these were calculated using the Smakula equation. Results indicate that the addition of CdS films enhances the photochromic and thermochromic properties of MoO3. Photochromism is more effective than thermochromism in generating color centers when using CdS as a charge carrier.

Emission dynamics of heavily coloredLiF crystals

LiF crystals colored with ionizing radiation have been used in recent times as active materials for pulsed laser emissions in the visible and near infrared region of the electro-magnetic spectrum. Among the various color centers which possess high emission efficiencies, the F3 + center displays a peculiar optical cycle whose properties depend on temperature and pumping intensity. However a few effects discovered recently seem to imply the existence of interactions among the centers, which is a topic of paramount importance for applied and basic research. Preliminary results on samples containing high concentration of color centers have shown new dynamical effects, which in part can be explained in the frame of nonlinear optical processes and in part require a different approach in describing the optical cycle and its parameters.

Color-center generation in silicate glasses exposed to infrared femtosecond pulses

The optical properties of silicate glasses under high-power, 850-nm femtosecond laser irradiation have been studied. Photoinduced processes occurred at irradiances well below the threshold for laser-induce damage. Laser spectral line broadening leading to supercontinuum generation in the visible and UV spectral regions was observed in all the glasses studied. Color-center generation and intrinsic luminescence were found in boro-silicate and alkali silicate glasses. It is believed that these processes result from linear and two-photon absorption of the short-wavelength component of the supercontinuum, causing ionization of the glass matrix. No color-center absorption in the visible region was observed in fused silica at irradiances up to the laser-damage threshold.

Color-center powder laser: the effect of pulverization on color-center characteristics

Stimulated emission is obtained and characterized in a powder of LiF crystal with color centers. Absorption, luminescent, and kinetics properties of color centers are studied in powder samples of LiF. They are found to be rather different from those in a bulk material. The difference is explained by dislocations mechanically created during the pulverization of the material. The characteristics of stimulated emission in LiF powder samples were found very different from those in scattering Nd-doped luminophosphors. This difference can be ascribed to a much higher emission cross section and a much smaller lifetime of emitting centers in LiF.

Visible and infrared photoluminescence of low-energy electron irradiated LiF:KCl thin films

In this work we report the visible and infrared emissions of the color centers created in bilayer LiF:KCl thin films, obtained by pumping them with appropriate Ar + and Ti:Sapphire laser lines. The large concentration of color centers induced by low-energy electron irradiation in a thin dielectric layer of controlled depth allows to investigate the spectroscopic properties of point defects which cannot be found in mixed crystals.

Structural and optical properties of alkali halide multilayer LiF:NaF films

Alkali halide multilayer films containing a large concentration of color centers created by low energy (3–12 keV) electron beams in a thin dielectric layer of controlled depth, represent a good and simple way to obtain new optically active materials. This work presents a study of multilayer LiF:NaF films, grown by physical evaporation. The structural and morphological characterization of the samples have been performed using x-ray diffraction, scanning electron microscopy, atomic force microscopy, and x-ray photoemission spectroscopy depth profile techniques. The visible photoluminescence of optically active defects was measured by excitation with an Argon laser. Controlling the deposition and coloration parameters it is possible to obtain new photoluminescent materials with predesigned optical features.

Electron migration in BaFCl:Eu2+ phosphors

Here we report the electron migration by photo- or thermostimulation in BaFCl:Eu2+. Electrons released from F centers may be trapped by other defect sites to form F aggregates or another type of F center and vice versa. This migration reduces the photostimulated luminescence efficiency, lowers the imaging plate sensitivity, and causes the difference between the optical absorption and photostimulation spectra of color centers.

Index structure of fiber Bragg gratings in Ge–SiO_2 fibers

The behavior of the concentration of photoinduced color centers in Ge-SiO(2) optical fibers was compared with that of the index modulation associated with fiber Bragg gratings (FBG's) written in the same fibers. We find that the fluence dependence of the photoinduced Ge E? center, its thermal annealing behavior, and its reaction with H(2) are similar to that of the index modulation generated in both H(2)-loaded and unloaded Ge-SiO(2) fibers. The much higher photosensitivity of H(2)-loaded Ge-SiO(2) fibers is attributed to the much higher formation efficiency of Ge E? centers, with an additional contribution from GeH. A diamagnetic structure, possibly densification, is also found to contribute to the index modulation of FBG's.

Spectroscopic characteristics of color centers produced in a LiF crystal surface layer by microwave discharge

AbstractWe have studied the absorption and luminescence spectra of color centers in a LiF crystal surface thin layer which are produced by low energy electrons from a secondary electron emission microwave discharge. The intensityratio of the absorption band due to the F2 color center to the band due to the F center was observed to be much higher than the ratio of these color centers produced in the whole bulk crystal. It was found that the F−2 center is produced in a thin layer through a selective two‐step photoionization of F2 color center by irradiation with the 514.5 nm Ar+ laser line. A mechanism of colorcenter formation in the thin layer is discussed.

Deep-ultraviolet damage to fused silica

Several experiments using 213-nm radiation have been performed with the goals of characterizing and understanding better the mechanisms behind UV damage to fused silica. A novel method of monitoring compaction in real time was discovered which measures the amount which the damaged fused silica depolarizes the incident beam. Refractive index changes of less than one part in ten million were resolved. Compaction induced stress was observed to extend well beyond the irradiation site with a maximum at the edge of the irradiation site. Detection of compaction occurred much earlier than the detection of color center absorption when fused silica was irradiated. The rates of both compaction and of color center formation depend superlinearly on pulse energy density and can be fitted by a quadratic function, suggesting a two-photon damage process. In the same energy density range, the absorption coefficient depends linearly on energy density, from which we extract a 213-nm two-photon absorption coefficient of 5×10−4 (cm/MW). This value is a factor of 4 smaller than the value published for 193 nm [R. S. Taylor et al., Appl. Opt. 27, 3124 (1988)].

Laser‐Induced Coloration and Discoloration Processes in Alkali Halides

AbstractThe coloration kinetics of KCI, KBr, KI, and NaCl crystals under XeCl excimer laser irradiation (hv = 4.02 eV) at room temperature as well as laser‐induced discoloration of preliminarily X‐ray coloured crystals, including samples with M and R centres are studied. It can be concluded from the results that the achieved colour centre concentrations are determined by a dynamical equilibrium between laser two‐quantum coloration and a concomitant one‐quantum bleaching of the created centres. Most probably the bleaching proceeds via the laser‐light‐induced destruction of the F centre complementaries, V centres and annihilation of some produced mobile products (H interstitials?) with F centres. The X‐ray‐created F centres seem to bleach more rapidly than the laser‐created ones.

Zero-phonon line broadening in ion bombarded LiF

LiF crystals have been bombarded with H+ and He+ ions of energy 1.0 MeV at ambient temperature. The concentration of F and F aggregate centres as a function of ion fluence displays irregular growth behaviour. The width and lineshape of the F4(1) zero-phonon line (ZPL) has been used as a probe to determine the nature of dominant internal strains in the crystal. At relatively low ion fluences, the growth of major colour centre concentrations F and F2 is well correlated with the increase in width of the ZPL; here the lineshape is largely Lorentzian in accord with point defects being the dominant cause of internal strain. At higher fluences the ZPL shows an increasingly Gaussian shape indicating the possible presence of dislocation structures. At high fluences the linewidth and shape of the ZPL is largely decoupled from variations in point defect behaviour. Rutherford backscattering experiments have also been carried out in an effort to provide additional information regarding the halogen and lithium disorder. The results are discussed in relation to the current ideas on defect production in alkali halides.

Optical losses in ion-implanted waveguides

In this paper we first introduce complex refractive indices to describe the optical losses in the buffer layer of optical waveguides obtained by ion implantation. It is shown that the color-center absorption in the buffer layer can change significantly the mode-spacing function. We carried out a detailed analysis of the influence of the imaginary component of the buffer layer refractive index on the propagation characteristics.

Effects of implantation dose on the inhomogeneous broadening of zero phonon lines in LiF

The inhomogeneous broadening and line shape of the F 4 (1) zero phonon line in LiF at 2.366 eV, measured at 4 K, has been used to probe the environment of these defects and the presence of internal strain as a function of implantation dose. Measurements on LiF implanted with 1 MeV protons at room temperature show substantial broadening of the zero phonon line. Studies of low and high dose γ-irradiated samples are used for comparison purposes. The observed broadening in the proton implanted specimens shows irregularities which is compared with the growth and collapse of colour centre concentrations previously reported in the literature. The results suggest two regimes: a low dose region where broadening results from point defects and small point defect aggregates and a high dose region where extended defects are, to a significant extent, responsible. Line shape analysis is consistent with these conclusions.

Displacement damage efficiency in sapphire

The efficiency of point defect production in sapphire by a variety of ion beams has been estimated by monitoring the oxygen vacancy colour centre concentration. Results are presented for H, D, He, B, N, Ne, Ar and Kr bombardment at both room temperature and 77 K. Experimentally determined values of the mean number of oxygen vacancies produced per incident ion are compared with theoretical values. The latter have been computed by solving the Lindhard integral equations to determine the partitioning of damage between the oxygen and aluminium sublattices. The stopping powers and cross sections of Ziegler, Biersack and Littmark have been used in these calculations and give significantly improved estimates of the damage energy. A significant Z1 variation in the damage efficiency is noted for low mass ions. The variation of damage efficiency with incident ion mass is discussed in terms of the primary knock-on spectrum.