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A theoretical model that features analytical formulas is developed for the evaluation of the light intensity that is radiated from a layered structure containing optically-active layers. The model is applied to an Alq_3-based organic light-emitting diode to study the convenience of using a LiF/Al cathode and find optimal thicknesses of some of the device layers

The photo-bleaching effects of selective ultraviolet and blue light illumination on the photoluminescence spectra of thermally evaporated tris(8-hydroxy-quinoline) aluminum (Alq_3) thin films exposed in air were investigated by fluorescence spectroscopy and advanced optical imaging techniques. The photoluminescence intensity time behavior observed in the Alq_3 films at different excitation powers allows to establish that Alq_3 molecules in their exciting state are involved in these complex phenomena. The photo-bleaching processes were exploited for direct laser patterning of the investigated Alq_3 thin films.

Point defects in lithium fluoride (LiF) have recently attracted renewed attention due the exciting results obtained in the realisation of miniaturised optical devices. Among light-emitting materials, LiF is of particular interest because it is almost not hygroscopic and can host, even at room temperature, stable color centers (CCs) that emit light in the visible and in the near infrared spectral range under optical excitation. The increasing demand for low-dimensionality photonic devices imposes the use of advanced irradiation methods for producing luminescent structures with high spatial resolution. An innovative irradiation technique to produce luminescent CCs in LiF crystals and films by using an extreme ultra-violet and soft X-ray laser-plasma source will be presented. This technique is capable to induce colored patterns with submicrometric spatial resolution on large areas in a short exposure time as compared with other irradiation methods. Luminescent regular arrays produced by this irradiation technique will be shown. Recently, the idea of using a LiF film as image detector for X-ray microscopy and micro-radiography based on optically-stimulated luminescence from CCs has been developed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

LiF films and crystals containing lithium colloids have been studied by scanning near-field optical microscopy (SNOM). Scattering phenomena and possible wave-guided modes have been observed studying the physical mechanism of the interaction between the LiF samples and an external laser source.

Lithium fluoride (LiF) films treated by low-energy electrons represent a very promising candidate for the realization of miniaturized broad band optical devices like active waveguides and microcavities. The main properties, in particular the refractive index, density, structure and microstructure, of polycrystalline LiF films grown by thermal evaporation on amorphous and crystalline substrates are discussed, together with their influence on colour centres formation induced by low-energy electron beam bombardment and on the choice­of the main irradiation conditions. Copyright­© 2001 John Wiley & Sons, Ltd.

Lithium fluoride (LiF) films treated by low‐energy electrons represent a very promising candidate for the realization of miniaturized broad band optical devices like active waveguides and microcavities. The main properties, in particular the refractive index, density, structure and microstructure, of polycrystalline LiF films grown by thermal evaporation on amorphous and crystalline substrates are discussed, together with their influence on colour centres formation induced by low‐energy electron beam bombardment and on the choice­of the main irradiation conditions. Copyright­© 2001 John Wiley & Sons, Ltd.

Polycrystalline NaF films were grown by e-beam assisted physical evaporation on amorphous silica substrates kept, during film growth, at constant temperatures ranging from 40°C to 400°C. The structural characterization of the films was performed by X-ray diffraction and by scanning electron microscopy.Irradiating the films with a 15 keV electron beam induced the formation of F and F-aggregate colour centres stable at room temperature. Absorption and photoemission measurements were performed and indicated a dependence of coloration on the deposition conditions.

The optical properties of Ni2+-ion impurities in a new crystal, the inverted fluoroperovskite BaLiF3, have been studied. In particular we have investigated the fine structure of the transition by means of absorption and emission spectra. By analogy to previous results we have assigned the structured absorption band around 1200 nm to the 3A2 → 3T1 transition. Four zero-phonon lines are clearly visible in the absorption spectrum. The emission in the region 1250-1600 nm, due to the same 3A2 →3T1 transition, consists of two sharp lines with their vibrational sidebands.

Alkali halide crystals with intrinsic color centers are suitable media for solid state tunable lasers, and among them LiF is one of the most popular because of its good physical and optical qualities. The F3+ centers in this host crystal have absorption and emission properties with unusual time evolution effects after excitation. In order to clarify this particular optical cycle we investigated a γ-irradiated LiF crystal containing, besides F centers, mainly F3+ centers. The behaviour of emission and absorption as a function of temperature and pumping power confirms the existence of a triplet state.

Thin films of KCl, KBr, RbCl have been obtained by thermal evaporation on amorphous substrates with different deposition parameters. The crystalline structure and orientation have been determined, and the films resulted to be policrystals with high uniformity of orientation. Production of colour centres, achieved by irradiation with low-energy electrons, leads to F center concentrations barely observed in large crystals. The colouration kinetics is similar to that in the bulk, and shows after a maximum an exponential decay at high doses because of centre aggregation coupled to thermal effects. The films exhibit a bleaching process of the colour centres at room temperature, whose kinetics depends on the irradiation damage.

The optical behaviour of colour centres is studied in thin films of KCl obtained by thermal evaporation onto amorphous substrates. Deposition conditions are optimized in order to minimize chemical and thermal effects on films. Concentration of colour centres even higher than in bulk crystals can be induced by low energy electron irradiation. Thermal bleaching of F centres is observed at room temperature and its kinetics, deduced from absorption measurements, consists of two exponential processes with very different time constants. Luminescence is obtained for the first time from aggregate colour centres in KCl films, and typical emissions of F2, F3, F4 defects (but not of F centres because of concentration quenching) are revealed.

Policrystalline LiF thin films were produced on amorphous substrates at different temperatures. Preliminary optical measurements on F2 and F3+ aggregate color centres produced by electron irradiation were performed.

The configuration of FA centres in KF:Na+ crystals has been studied by means of spectroscopic measurements. The FA(Na) dipoles are found to lie along the crystal axes, and this result is interpreted as the consequence of an on-centre position of the Na+ ions in the lattice of KF, in good agreement with theoretical expectations.

Des zones de centres colorés en forme de ruban, d'une centaine de nanomètres d'épaisseur et de largeur variable (100 nm à 5 μm), générées par faisceau électronique, sont pour la première fois imagées et caractérisées spectroscopiquement par microscopie en champ proche optique (SNOM) opérant en mode illumination locale avec une fibre optique et une détection en réflexion.

Eight nominally pure lithium fluoride crystals, obtained from a single LiF crystal containing less than 100 ppm impurities, were irradiated by gamma-rays from a 60Co source at room temperature, with doses from 8.4 to 2.5 x 105 Gy, but one at -60 degC. Optical density measurements were performed to investigate the radiation-induced color centres (CCs) and to evaluate their concentrations. Thermoluminescence (TL) glow curves were collected and simulated by a first-order kinetics approach, and from the best-fit procedure ten glow peaks (GPs) spanning from 100 to 450 degC were highlighted. A comparative analysis of GP intensities and CC concentrations as a function of the irradiation dose has questioned their association as obtained in previous measurements, showing the impurities, less than 1018 cm-3, still playing a predominant role in TL spectra. New measurements on LiF crystals more pure, at least an order of magnitude, are required to establish for sure the association of GPs to CCs.

Among insulating materials, lithium fluoride (LiF) has been successfully used as ionizing radiation dosemeter for more than 60 years. Thermoluminescence (TL) has been the most commonly used reading technique to evaluate the absorbed dose. Lately, optically stimulated luminescence (OSL) of visible emitting color centers (CCs) has also been explored in pure and doped LiF. This work focuses on the experimental behaviour of nominally pure LiF crystals dosemeters for 6 MV x rays at low doses based on photoluminescence (PL) of radiation induced CCs. Polished LiF crystals were irradiated using 6 MV x rays produced by a clinical linear accelerator. The doses (absorbed dose to water) covered the 1-100 Gy range. Optical absorption spectra show stable formation of primary F defects up to a maximum concentration of 2x1016 cm[?]3, while no significant M absorption band at around 450 nm was detected. On the other hand, under Argon laser excitation at 458 nm, PL spectra of the irradiated LiF crystals clearly exhibited the characteristic F2 and F+3 visible broad emission bands. Their sum intensity is linearly proportional to the absorbed dose in the investigated range. PL integrated intensity was also measured using a conventional fluorescence optical microscope under blue lamp illumination. The relationship between the absorbed dose and the integrated F2 and F+3 PL intensities, represented by the net average pixel number in the optical fluorescence images, is also fairly linear. Even at the low point defect densities obtained at the investigated doses, these preliminary experimental results are encouraging for further investigation of CCs PL in LiF crystals for clinical dosimetry.

Proton beams of 3 MeV energy, produced by the injector of a linear accelerator for proton therapy, were used to irradiate at room temperature lithium fluoride crystals and polycrystalline thin films grown by thermal evaporation. The irradiation fluence range was 1011-1015 protons/cm2. The proton irradiation induced the stable formation of primary and aggregate color centers. Their formation was investigated by optical absorption and photoluminescence spectroscopy. The F2 and F3+ photoluminescence intensities, carefully measured in LiF crystals and thin films, show linear behaviours up to different maximum values of the irradiation fluence, after which a quenching is observed, depending on the nature of the samples (crystals and films). The Principal Component Analysis, applied to the absorption spectra of colored crystals, allowed to clearly identify the formation of more complex aggregate defects in samples irradiated at highest fluences.

Optical absorption and photoluminescence spectra were measured on gamma colored lithium fluoride crystals at five different irradiation doses. By accurate Gaussian best-fit procedures of the absorption measurements the peak intensity of F and M bands and the concentrations of F, F2 and F3+ color centers in gamma irradiated LiF crystals at different doses were obtained. A quadratic relationship between F and F2 defect concentrations as a function of the irradiation dose was found. Photoluminescence measurements, excited by an Argon laser at 457.9 nm, were used to study the effect of the laser power on the F2 and F3+ emission spectra of gamma irradiated LiF crystals. The influence of the triplet state on the optical cicle of F3+ color center was highlighted for several excitation laser power and the ratio of the transitions probability W1 and W2 to and from the triplet state was derived.

Tl+-doped lithium fluoride crystals exhibit very interesting properties concerning the formation of colour centres after X-rays irradiation. The presence of Tl+ ions, even in small concentrations, increases the number of traps which stabilize the H-centre aggregates. These H-centre aggregates become smaller and their number increases, inducing two sets of trapping levels. Photoluminescence measurements after subsequent thermal bleaching of the thermoluminescence peaks show a faster decreasing of the F3+ centres, in three steps, compared with the F2 ones. The F-H recombination processes may induce the excitation-emission transitions of Tl+-ions.

LiF:Pb doped crystals were successfully grown by Kyropoulos method, starting with drying powders. The presence of Pb2+ ions in the LiF crystals were evidenced by the absorption band at 278 nm and by 375 nm photoluminescence. The presence of some other Pb structures with oxygen compounds in the as made samples was evidenced, decreasing after some annealing procedures. The local environment and valence state of Pb in LiF were studied by X-ray Absorption Spectroscopy at the Pb LIII and LI edges. XANES data reveal that Pb is present as Pb2+ whereas EXAFS data show that it is incorporated in the crystal and not forming PbF2 precipitates. Identical spectra are obtained for samples as prepared and after thermal annealing up to 650 degC demonstrating the stability of the incorporation site. Also the concentration of Pb in the crystal has no effect on the location site of the metal as the same spectrum is obtained for specimens with different dopant concentrations.

X-ray microscopy represents a powerful tool to obtain images of samples with very high spatial resolution. The main limitation of this technique is represented by the poor spatial resolution of standard imaging detectors. We proposed an innovative high-performance X-ray imaging detector based on the visible photoluminescence of colour centres in lithium fluoride. In this work, a confocal microscope in fluorescence mode was used to characterize LiF-based imaging detectors measuring CC integrated visible fluorescence signals of LiF crystals and films (grown on several kinds of substrates) irradiated by soft X-rays produced by a laser plasma source in different exposure conditions. The results are compared with the CC photoluminescence spectra measured on the same samples and discussed.

The advanced optical and electronic characterization of primary F-defects and metallic lithium colloids in lithium fluoride thin films produced by ion-assisted thermal deposition with Xe low-energy ions (< 200 eV) on transparent amorphous substrates is presented and discussed. The volume concentrations of defects and Li nano-clusters have been estimated by applying a best-fit procedure based on a comprehensive theoretical modelling to the spectro-photometric measurements. The appreciable presence of Li metal at the film surface was clearly demonstrated by X-ray photoelectron spectra. Advanced optical microscopy confirms the negligible formation of other types of more complex luminescent colour centres.