Radiation-induced color centers in the near-surface layer of lithium fluoride crystals

For radiation-induced intrinsic color centers, we show that the concentrations of identical centers and the concentration ratios of different centers are quite different in the near-surface layer and within the interior volume of a lithium fluoride crystal. We have established that these differences also depend on the sign of the difference between the temperature at which the crystal was irradiated with γ photons and the vacancy mobility temperature. We provide an interpretation for the results obtained, based on the structural features of the near-surface layer and the concentration ratio of vacancies and electrons in the layer, serving as the starting components for color center formation. We found that the concentrations of centers change over the course of a few days by tens of percent in the layer “emerging” from the interior onto the surface as a result of cleavage of the crystal. We measured the luminescence lifetimes of F₃ (R2) and F₄ centers: 6.6 nsec and 11.7 nsec. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 6, pp. 775–781, November–December, 2006.     

Miniaturized structures based on color centers in lithium fluoride: Optical properties and applications

Research activities concerned with color centers in alkali halide films started recently. The use of versatile, well-assessed, and low-cost fabrication techniques consisting of physical vapor deposition of Lithium Fluoride (LiF) films combined with direct writing lithographic processes allows the realization of miniaturized structures, like broad-band emitters, channel waveguides, optical microcavities and point-light sources emitting in the visible spectral range. Promising results have been obtained in the generation, amplification and waveguiding of visible light in LiF treated by low energy electron beams, where the efficient formation of stable primary and aggregate color centers also induces a local modification of the refractive index. A brief overview of the investigated optical properties is presented together with a short discussion about their perspectives of applications in optoelectronics.     

Advanced optical spectroscopy of luminescent color centers in lithium fluoride thin films

Lithium fluoride, LiF, is a well-known dosimeter material. In thin films the direct use of optical absorption spectra to individuate the stable formation of different kinds of point defects is often precluded by the presence of interference fringes due to the refractive index difference between film and substrate. Photoluminescence measurements are more sensitive. Recently a very effective investigation method is developing: it is often called combined excitation–emission spectroscopy (CEES) in the literature. In this work we present the basic characteristics of this technique and the first results of the investigation of polycrystalline LiF films grown by thermal evaporation on fused silica substrates and gamma irradiated at several doses up to 10⁶ Gy in air.     

Aggregate color center formation processes in lithium fluoride crystals after irradiation

Lithium fluoride crystals were irradiated by different doses of gamma photons at a temperature of 77 K. We measured the aggregation kinetics for the color centers with different annealing temperatures above the temperature of anion vacancy mobility. We show that the lifetimes of the vacancies decrease while the lifetimes of the F₂⁺ F_2^{+} centers increase as the irradiation dose increases. We explain these types of dependences based on the aggregation processes for color centers in the post-radiation period. We determine the time constants and energies (analogous to activation energies in the Arrhenius equation) for the various processes involving rise and fall in the concentration of aggregate color centers. Based on the experimental data obtained, we have established the processes forming F ₂ and F₃⁺ F_3^{+} centers in the post-radiation period. The F ₂ centers are formed when vacancies ν_a add to F₁^- F_1^{-} centers. Vacancies arising during irradiation of the crystal participate in their creation in the first fast stage. In the long final stage, vacancies are used which appear in the post-radiation period on occurrence of the reaction F₂⁺ F_2^{+} + H → ν_a + fluoride ion at the lattice site, where H is an interstitial fluorine atom. The F₃⁺ F_3^{+} centers are formed both by merging F₂⁺ F_2^{+} and F ₁ centers and as a result of addition of vacancies to F ₂ centers. In this case, vacancies are used that are generated not only during irradiation of the crystal but also in the post-radiation period. The rise in the concentration of F₃⁺ F_3^{+} centers occurs faster than the rise in the concentration of F ₂ centers.     

Low-temperature optical spectra and zero-phonon transition of color centers in γ-rayed lithium fluoride crystals

The zero-phonon lines on R_2, R~ , N_1 and R′_2 centers and some new narrow lines have been observed in bivalent-metal-doped and air-grown LiF crystals irradiated by γ-ray in the temperature range of 9.5—130K. The spectral properties and thermostabilities of the lines are investigated systematically at different temperatures.     

Luminescent patterns based on color centers generated in lithium fluoride by extreme ultraviolet radiation and soft X-rays

Primary and aggregate color centers in lithium fluoride (LiF) crystals and polycrystalline LiF films were produced by an innovative irradiation technique using extreme ultraviolet radiation and soft X-rays generated by a laser-plasma source. This irradiation facility allowed the efficient formation of active color centers on luminescent patterns with submicron spatial resolution on large areas and short exposure times. The method looks promising for the realization of low-dimensionality photonic devices. The optical characterization of the colored structures was performed by means of absorption and photoluminescence measurements on LiF samples colored under different irradiation conditions.     

Method of orthogonally polarized bands for analysis of luminescence anisotropy and dichroism in lithium fluoride crystals with color centers

A method for analysis of polarization anisotropy of radiation and dichroism in crystals is proposed that is based on excitation of luminescence by the probe field having the form of a system of light bands with orthogonal polarization. The theoretical and experimental results concerning application of this method to the analysis of lithium fluoride crystals with radiation-produced color centers are presented. The specificity of optical microimage recording in crystals containing a collection of color centers is investigated.     

F L V a + a and F L F color centers in lithium fluoride crystals

Using optical spectroscopy and X-ray diffraction analysis, the nature of defects in γ-irradiated lithium fluoride crystals is found, mechanisms of their formation are proposed, and the absorption bands at 385 (F _L F centers) and 506 nm (F _L V _a ⁺ centers) are interpreted.     

Spatial structures based on color centers created by electrons in lithium fluoride crystals: Absorption and luminescence characteristics

We have shown that in spatial structures based on color centers created by electrons in a lithium fluoride crystal, the distances between centers reach 1.6 nm and 3.6 nm for F₁ and F₂ centers respectively. This suggests considerable potential opportunities for using electron technology to form structures in the crystals with spatial resolution of such an order of magnitude. We measured the decrease in fluorine content on the irradiated surface of the crystal. We found the concentrations of F₁, F₂, F₃⁺, F₃(R₂), and F₄(N₁) centers. We established that the specific characteristics of color center formation by electrons leads to an increase in the efficiency of creation of F₃ and F₄ centers. We determined the decrease in the average luminescence lifetimes of F₂ and F₃⁺ centers as a result of concentration quenching. We observed distortion of the luminescence contour for F₂ centers as a result of absorption of its short-wavelength portion by other centers and emission of radiation by the latter in its long-wavelength portion. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 1, pp. 102–110, January–February, 2008.     

Effect of impurity iron ions on the formation of photoinduced lattices and domain structures in lithium niobate

Processes and mechanisms of the formation of lattices and periodic domain structures in iron-doped lithium niobate monocrystals under the effect of laser beams are examined. The role of Jahn-Teller Fe²⁺ ions in the formation of photoinduced periodic structures is demonstrated.     

Formation and properties of metallic nanoparticles in lithium and sodium fluorides with radiation-induced color centers

The specific features of light- and temperature-induced formation of metallic nanoparticles in ??-irradiated LiF and NaF crystals have been investigated. Atomic force microscope images of nanoparticles of different sizes and in different locations have been presented. The relation between the crystal processing regimes and properties of the nanoparticles formed has been revealed. The optical properties of the processed crystals have been analyzed. The thermo- and light-stimulated processes underlying the formation of metallic nanoparticles in aggregation of the color centers and their decay due to the recovery of the crystal lattice have been studied.     

Ab initio thermodynamic characteristics of the formation of oxygen vacancies,and boron,carbon, and nitrogen impurity centers in anatase

Using the ab initio projector augmented wave (PAW) method, calculations are performed for the electronic energy-band structure of titanium dioxide having the structure of anatase doped with boron, nitrogen, and carbon. Thermodynamic characteristics are determined for the formation of impurity centers, such as the preference energy for the interstitial position, the energy of impurity oxidation, and the energy of oxygen vacancy formation. It is shown that under the conditions of thermodynamic equilibrium the interstitial position of boron atoms is stable, whereas carbon atoms, depending on the oxygen pressure, can occupy both interstitial positions and substitutional positions of oxygen atoms, and nitrogen atoms replace oxygen atoms. It is shown that the presence of oxygen vacancies promotes the thermodynamic stability of carbon and nitrogen atoms. The obtained densities of electronic states correspond to ESR spectroscopy data, which indicates the presence of spin-polarized electrons in the states of the oxygen vacancy.     

Spectroscopy of F 3 + -color centers in lithium fluoride crystals (review)

Results of investigations of radiative F ₃ ⁺ -color centers in lithium fluoride obtained by the methods of one- and two-photon absorption, polarization spectroscopy, and luminescence in singlet, triplet and triplet-singlet channels are reported. The scheme of energy levels of the F ₃ ⁺ -center is revised. The probabilities of singlet-triplet conversion and the rates of depletion of the lower triplet state in the range of 80–350 K are presented. Recommendations are given on the technology of radiative LiF coloration that provide an increase in the F ₃ ⁺ -center concentration and a decrease in the concentration of some other centers that prevent lasing on the LiF:F ₃ ⁺ active medium from being obtained. The phototransformations of color centers in LiF are analyzed. The parameters of lasers with LiF:F ₃ ⁺ active media that provide generation of radiation tunable in the green region of the spectrum are considered. Based on spectroscopic data the maximum attainable time characteristics of such lasers are discussed. The prospects for further investigations of LiF:F ₃ ⁺ lasers are outlined in brief. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 5, pp. 745–760, September–October, 1998.