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Optical photoluminescence of thermally evaporated Alq3 thin films has been studied for over six years in a few samples annealed and non-annealed and afterwards exposed to the laboratory atmosphere. It was found that the measured emission intensity decays with four different time-spectral behaviors, which imply the existence of four molecular aggregations, or components. Insights have unveiled their interactions with environment agents, which are useful both for optoelectronic utilizations of this ubiquitous organo-metallic molecule possessing a long emission lifetime and for basic research and application of similar organic molecules.

This contribution to ECPD2019 is dedicated to the memory of Anatoly Faenov. During a period of approximately thirteen years 1994–2006, Anatoly and his wife Tatiana Pikuz (simply “Tania” for friends), accepting the frequent invitations of the National Institute for Nuclear Physics (INFN) and of the Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA), cooperated with many Italian research laboratories dedicated to EUV and soft X-ray generation, spread in different towns (L'Aquila, Frascati, Milano, Padova, Pisa, Roma, etc.). In spite of the fact that they could stay in Italy only about one or two months per year, their activity was so intense that more than 50 peer- reviewed publications were generated from their experimental and theoretical work (just considering only the results obtained at L'Aquila and Tor Vergata—Rome Universities and at the ENEA Research Center of Frascati), without mentioning the cultural atmosphere that they stimulated in the field of Science and Humanity. The numerous experimental spectra obtained at ENEA by means of their spherically bent mica spectrometers, together with the corresponding theoretical simulations performed in Moscow, allowed to study the changing role of different excitations mechanisms for various plasma conditions, and to characterize at best the ENEA laser-plasma source for different applications: polychromatic and monochromatic micro-radiography of dried biological samples at 1 keV, soft X-ray contact microscopy (SXCM) of living cells in the water-window spectral region, spectroscopy of hollow atoms, etc. In this memorial paper, the main results of biological samples imaging on lithium fluoride (LiF) detectors, obtained with the ENEA and Tor Vergata University laser-plasma sources, are presented. In particular, the improvement of the micro-radiography and of the SXCM techniques obtained after moving from photoresist detectors and photographic films to lithium fluoride (LiF) detectors are discussed, for both dried and wet biological samples.

Lithium fluoride crystals are irradiated at various doses by gamma rays at 77 K. The time evolution of photoluminescence signals from aggregated F 2 +, F2, F3 + and F 3 color centers, and of the absorption intensity of primary F centers are measured at various annealing temperatures. The lifetimes of anionic vacancies υa and F2 + centers, the characteristic times of concentration growth of F2, F 3 + and F3 centers, and also the activation energies of diffusion of vacancies and F2 + centers together with various processes of aggregation are determined. It is found that lifetime decreases for vacancies while increases for F2 + centers by increasing the irradiation dose. It is also shown that, after irradiation during annealing, vacancies are formed as a result of the reaction F2 ++H→υa+Fl-, where Fl- is a fluorine ion in a lattice site and H is a fluorine interstitial atom. Then these vacancies participate in color centers aggregation kinetics. The presence of F- centers in the irradiated crystal is established, and the processes which lead to the formation of F2, F3 + and F3 centers after irradiation, are unveiled. © 2013 Elsevier B.V.

The vibronic relaxation process of FA centers in RbCl:Li, which have Type I emission at low temperatures and Type II emission at high temperatures, has been studied at 10 and 77 K by measuring the resonant secondary radiation (RSR) spectrum from optically excited FA centers with pico-second time-resolved spectroscopy. This study has been prompted by the quest to improve our knowledge on the de-excitation process in the FA centers. The adiabatic potential energy curves are deduced from a theoretical analysis of the measured RSR spectra and compared with those already known. It is shown that Type II relaxation observed at 77 K occurs after Type I relaxation, supporting in turn a model proposed by Baldacchini et al. based on photoluminescence measurements.

A new method is suggested for the quantitative investigation of absorption spectra by using a luminescent probe. It is particularly useful, when the absorption bands are difficult to be measured directly due to practical reasons. The probe should possess excitation or emission band overlapping an absorption band of the material under investigation. Equations relating the absorption of the investigated material with the luminescence of the selected probe are obtained for several cases. Experimental results on Er(NO3)3 by using 3-amino-N-methylphtalimide as luminescent probe confirmed the effectiveness of the method.

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 $$ F_2^{+} $$ 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 $$ F_3^{+} $$ centers in the post-radiation period. The F 2 centers are formed when vacancies νa add to $$ F_1^{-} $$ 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 $$ F_2^{+} $$ + H → νa + fluoride ion at the lattice site, where H is an interstitial fluorine atom. The $$ F_3^{+} $$ centers are formed both by merging $$ F_2^{+} $$ 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 $$ F_3^{+} $$ centers occurs faster than the rise in the concentration of F 2 centers.

Thin films of tris(8-hydroxyquinoline)aluminum ( Alq 3 ) have been subjected to thermal processes in order to verify previous reports about improved emission efficiency upon heating organic light-emitting diodes (OLEDs). It has been found that annealed films in various atmospheres display an abrupt variation of the intensity, peak wavelength and linewidth of the photoluminescence band between 150 and 180 ∘ C , which hint at phase transition phenomena. Moreover, the annealed films exhibit a much longer lifetime, which is still one of the major challenges in the OLEDs research field.

Thin-film multilayers with dielectric and semiconductor nanolayers of 200–10 nm thicknesses have been deposited by thermal evaporation onto irradiation-resistive substrates using pure crystals as evaporated targets. Some multilayers were γ-irradiated in air at room temperature with dose of 83 kGy. X-ray diffraction and microscopy studies reveal that the multilayers consist of nanometer-sized crystals with cubic structure and defined size. Film structures were oriented along the (1 1 1) plane. Absorption spectra of non-irradiated LiF nanocrystals of 100 nm size and those of initial crystals give evidence of metal colloids presence. Photoluminescence spectra of γ-irradiated nanostructures with various LiF content show the enhancement of F3+-colour centres excitation in the region of metal colloids absorption and the increase is observed between emission intensities of F3+ and F2 centers with respect to initial crystals γ-coloured in identical conditions. Emission intensities of both centers under excitation in the M band correlate with LiF content. These effects, which are related to high-quality nanocrystals, but at the same time depend strongly on the defect content, especially as far as their 1–2 ps nonlinearities are concerned, could depend on nanocrystal purity and metal excess collection in their boundaries regions.

Recently, tris(8-hydroxyquinoline)aluminum (Alq 3 ) has been diluted into various sol–gel matrices, and a strong blueshift with respect to the well-known emission band of amorphous thin films has been observed. This result has been explained by the absence of interactions among molecules that are isolated in the microscopic pores of glasses. The same explanation is also working in analogous measurements performed lately in various matrices, where the blueshift was wrongly attributed to a mixing of facial and meridianal isomers. Indeed, it is shown here that the Alq 3 meridianal isomer alone can explain the experimental results.

Organic light emitting diodes, OLEDs, have been so much improved in the last two decades that they are being currently used in commercial applications for displays and lighting. Among many improving technologies, thermal treatments affect greatly their performances. In case the small molecule Alq3 is the active material, it was never understood why and how the temperature could have played such an important role. Now, we have discovered that such molecules tend to aggregate themselves in morphological states which are very much sensible to annealing processes, with the result of improved emission efficiency and lifetime of Alq3 thin films. Moreover, the latter ones are described by a four components model, FCM, which has been validated by degradation experiments during 6 years, a world record in its own.

Thermoluminescence (TL) of nominally pure LiF crystals irradiated with gamma rays has been studied in connection with color centers (CCs) generated during ionizing irradiation. A close analysis of the experimental TL spectra unveiled the existence of 10 glow peaks (GPs) spanning from 100 to 450°C. The relatively well-resolved GPs up to 263°C are associated with F3+, F3, and F2 CCs, while the remaining tangled ones are attributed to F and F-like CCs. A first-order kinetics approach is used to simulate the TL spectra, and the appropriate parameters of the carrier traps are obtained. A critical analysis of their values showed, on the one side, the usefulness of using pure crystals to understand their basic contributions to TL, and, on the other side, the possible existence of further weak GPs and the role still played by the residual amount of impurities.

The de-excitation process of FA(type I) centers in KCl:Na has been investigated by measuring the hot luminescence spectrum from optically excited FA centers with time-resolved spectroscopy. The experimental results are analyzed by using a model that describes a time evolution of the phonon wave packet during the vibronic relaxation process from the Franck–Condon state to a relaxed excited state. From the analysis of the experimental data, information on the vibronic mixing between 2p and 2s states, whose magnitude varies during the relaxation process, and the adiabatic potential energy curves of 2s and 2p states are extracted. The present results are compared with the already known ones of the FA(type II) centers.

An innovative X-ray imaging detector based on Optically Stimulated Luminescence from color centers in lithium fluoride is presented. Regular photoluminescent patterns produced on LiF samples by different intense X-ray sources, like synchrotrons, laser plasma sources and a capillary discharge laser have been investigated by a Confocal Laser Scanning Microscope. The use of a LiF-based imaging plate for X-ray microscopy is also discussed showing microradiographies of small animals.

After some recollections of phonon and photon trapping in ruby, a critical review of this effect of renewed interest in trivalent lanthanide-doped materials is presented showing that the two independent derivations taking their roots in the work of Milne for gases and of Birks for organic scintillators are practically described by an analogous phenomenological equation when re-absorption is weak. The problem of obtaining "extrinsic" quantum efficiency larger than one is briefly discussed.

A confocal laser scanning microscope has been used in a non-conventional approach to reconstruct the depth distribution of stable, optically active color centers induced at the surface of a lithium fluoride crystal by a low energy electron beam. The formation of F2 and F 3 + aggregate electronic defects was limited to the penetration depth of the electrons in the material, as obtained from Monte Carlo simulations. This novel measurement technique could be easily used for spatial characterization of luminescent optical waveguides in transparent dielectrics.

Processes of radiation creation and annealing of Frenkel defects, as well as their transformation processes, have been studied in γ -irradiated LiF single crystals containing various impurities, by means of measurements of thermally stimulated changes in absorption and emission spectra. Important information about the density of colour centres in LiF, which depends on γ -irradiation doses, impurities and temperatures, has been obtained for small crystal contents of oxygen, hydroxyl and metal ions. A comparison of annealing parameters of the LiF crystals subjected to different doses of irradiation, but to the same thermal bleaching procedures, has been performed and the results are critically discussed. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Light emitted when heating crystalline materials previously exposed to ionizing radiation is commonly known as thermoluminescence. The resultant glow curve is related to the material, the impurities and the defect centers generated by the irradiation. Several efforts have been made in the past to associate glow peaks with specific defect centers. Recently, in the case of LiF we established a link between F3+ and F3 centers and the temperature region of the glow curve below 200 °C. This result was obtained by using especially treated samples with known concentration of color centers in first place, and annealing measurements in second place. By continuing these studies, it has been possible to establish a further link between F2 centers and a prominent feature of the glow curve near 260 °C. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical absorption and emission spectra have been studied in LiF crystals containing oxygen, hydroxyl and metal impurities, and coloured with γ -rays at room temperature with doses up to 83 kGy. Well-defined bands in absorption and emission spectra of uncoloured samples have been attributed to impurities. Colouring of the same crystals adds new spectroscopic features, which are related to various colour centres, but at the same time depends strongly on the impurity content, especially as far as their formation efficiency is concerned. The connection between impurities and colour centres will be discussed on the basis of optical spectra of uncoloured and coloured, and also pre-annealed and bleached crystals. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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 2.5 105 Gy, but one at -60 °C. 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 °C 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. © Published under licence by IOP Publishing Ltd.

The Rayleigh scattering (RS) by independent small particles is a well-known effect which also accounts for the blue color of the sky. Lately, the blue shift of the greenish emission band of thin films of Alq 3, tris(8-hydroxyquinoline)aluminum, after thermal treatments of various nature has been attributed to RS. Here, we show that RS effects cannot account for the observed spectral features, which are attributed to the onset of various molecular aggregations in the otherwise amorphous films, a morphological model already utilized to explain the optical properties of Alq 3 .

Nowadays the possibility to obtain micro-radiographs is limited in resolution by the detection properties of the used technique. Here we show a new method to create and observe micro-radiographs. A thin film of Lithium Fluoride works as image detector, storing the micro-radiograph obtained exposing biological material to Extreme Ultra-Violet and soft X-ray radiations. To read the stored image, collecting the optically stimulated luminescence of the active color centers produced by the X-rays, a Scanning Near-Field Optical Microscope (SNOM) is used, for its peculiarity to overcome the diffraction limit, showing details smaller than the wavelength of the collected light. The samples were illuminated with 458 nm laser light and an uncoated tapered optical fiber was collecting the fluorescent signal emitted by the Lithium Fluoride color centers. The SNOM images of these micro-radiographs stored in Lithium Fluoride films showed an optical resolution of 50 nm, corresponding to ∼λ /12, where λ is the wavelength of the collected light. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A lithium fluoride (LiF) crystal was irradiated with extreme ultraviolet (EUV) radiation, which generated primary and aggregate color centers (CCs) within a surface layer. The optical properties of this layer were investigated by means of spectrophotometry in the wavelength range 210–800 nm. The photometric spectra were analyzed by means of two independent approaches, which gave comparable optical constants and thickness of the colored layer. Its thickness was 46.6 ± 0.7 nm, which is approximately the same as the estimated penetration depth of EUV radiation in LiF. The stable formation of CCs induced an increase of the complex refractive index of the layer by approximately 2.8% in the visible range. The volume concentrations of the main CCs were estimated from their dielectric-susceptibility partial contributions to be in the range ∼2 × 1019–2 × 1020 cm−3 within the layer.

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.

The spectral identification of several photoluminescent defects produced in LiF by ionising radiation is still uncertain because many optical transitions are possible and their different spectral features are often overlapping. Performing a conventional spectroscopic characterization requires a lot of time and intuition, but recently a more effective investigation method has been developed: combined excitation-emission spectroscopy (CEES). In this approach a large number of excitation (or emission) spectra is recorded at a sequence of emission (or excitation) wavelengths. The resulting three-dimensional data set of excitation intensities as a function of excitation and emission wavelengths, or energies, is visualized using a contour plot, where lines of equal emission intensities are drawn. For the first time this method has been successfully applied to coloured LiF crystals and films. As an example, the case of visible-emitting colour centres, absorbing in the M band spectral region in a gamma irradiated LiF crystal is reported and discussed. Spectroscopic features of well-known active defects have been confirmed, more complex centres clearly identified and other properties better clarified. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The extreme ultraviolet radiation emitted by a laser-plasma source or by a capillary discharge laser is applied to the generation of luminescent patterns in lithium fluoride. This novel technique is able to produce colored patterns with high spatial resolution on large (more than 10 cm/sup 2/) areas in a short exposure time compared with other irradiation methods like the electron beam writing. The potentials of this technique for applications in photonics are commented. This work reviews the activity performed during the past four years at the ENEA Frascati Center and at L'Aquila University, Italy. Preliminary images of microradiography or X-ray contact microscopy using lithium fluoride as an imaging detector are presented. The advantages of this new detector compared with photographic films or with photoresists are discussed.

The advent of scanning near-field optical microscopy (SNOM) has augmented at a microscopic level the usefulness of optical spectroscopy in the region between 300 nm and 10 mum. Two-dimensional imaging of chemical constituents makes this a very attractive and powerful new approach. In this paper we show SNOM results obtained in several geometrical configurations on boron clusters in silicon, Li clusters embedded in a LiF sample and BN growth on silicon. We also show some results on the wavelength dependence of the reflectivity (R) in the near infrared (IR) of biological cells in liquid environment with the observation of the local fluorescence. The SNOM images revealed features that were not present in the corresponding shear-force (SF) images and which were due to localized changes in the bulk properties of the sample. The size of the smallest detected features clearly demonstrated that near-field conditions were reached both in the visible and infrared region. (C) 2004 Elsevier B.V. All rights reserved.

Tris(8-hydroxyquinoline)aluminum (AlQ) powder has been characterized using photoluminescence and electron spin resonance (ESR). The broad asymmetric excitation band is centered at ∼425 nm, while the emission band centered at ∼505 nm is similar to those reported in films. A significant ESR signal with a linewidth of 10 G and g-value of 2.0038 has been observed in as synthesized AlQ powder at room temperature. The signal at same position is present in AlQ powder as-received from Aldrich, as synthesized AlQ powder, and AlQ powder purified by vacuum sublimation. The appearance of the ESR signal is attributed to organic free radicals. Moreover, exposure of AlQ powder to humid air enhances the ESR signal intensity which increases linearly with exposure duration. This is in agreement with recently proposed degradation mechanism in AlQ based organic light emitting diodes, where degradation was attributed to unstable cationic AlQ species. The discovery of natural free radicals in freshly synthesized AlQ may be useful to prevent the degradation of AlQ and other analogous compounds in organic light emitting diodes (OLEDs).

The combined case of self-trapping and self-quenching in the lifetime analysis of Yb, Er, and Ho doped Y 2 O 3 single crystal fiber is studied. After deconvoluting theoretically the measured lifetime points, it is shown that self-quenching, for a rather large doping range, is well described by a limited diffusion process within the doping ion subsystem towards impurities analogous to the doping ions themselves. Fast diffusion towards intrinsic non-radiative centers cannot explain the observed results. As an application, a simple quantitative method for optimizing the gain material concentration for amplifiers and lasers is proposed and performed.

Light produced when heating materials which have been previously exposed to ionizing radiation is commonly known as thermoluminescence (TL). The structure of TL glow curves is characteristic of the material and is influenced by the impurities it contains as well as the type of defect centers generated by the irradiation. An extensive body of knowledge exists concerning TL. In the case of LiF, several efforts have been made to associate glow peaks with defect centers responsible for optical absorption, but up to now there is no certain attribution. In this work nominally pure LiF samples have been irradiated with γ rays, then treated thermally and optically to change the concentration of electron color centers in a known way, and the resulting TL has been studied in conjunction with optical absorption. By comparing the glow curves from ambient temperature to 450 °C and the optical absorption in the purple-blue spectral region, it has been possible to establish a link between F 3 + color centers and the low temperature region of the glow curve below 200 °C. Accurate spectroscopic measurements and theoretical fittings of the absorption spectrum in the UV–VIS spectral region have also been made in order to ascertain the possible role of other aggregate color centers in TL.

Stable color center production in lithium fluoride crystals has been applied to single-shot high-spatial-resolution imaging of a 2 ns long, 46.9 nm capillary discharge soft-X-ray laser pulse. Images of the unfocused beam and of the beam focused by a multi-layer mirror were recorded. The LiF crystal sensitivity was sufficient to impress high-contrast photo-luminescent patterns with single-pulse irradiation on an area up to 40 mm2. Interference fringes due to the soft-X-ray laser coherence were observed. The results demonstrate the potential of LiF crystals and films as a sub-micrometer resolution two-dimensional imaging tool for single-shot soft-X-ray laser applications.

Thermally grown thin film and bulk crystals of lithium fluoride (LiF) were irradiated by swift heavy ions to understand the color center production mechanism and their transformation to more complex defects. These samples have been characterized by optical absorption and photoluminescence (PL) under 442 nm excitation at room temperature by HeCd laser. We observed that bulk crystals show relatively very intense PL bands at around 530 and 665 nm , which correspond to the emission of F3+ and F2 centers, respectively, as compared to the thin films at the same electronic excitation and ion fluence. Detailed study on bulk LiF has been performed at different electronic excitations and ion fluences for understanding the ion matter interaction under these dense electronic excitations.

Light emitted as a function of temperature by crystalline materials, which have been exposed to ionizing radiation, is commonly known as thermoluminescence. Its structured shape, which is often resolved in narrow bands called glow peaks, is related to the material, the impurities and the defect centers generated by the irradiation. Several efforts have been made to associate these peaks to the defect centers, but up to now in the majority of the cases and in particular in LiF, there is not any certain attribution. In order to overcome this situation, pure LiF samples have been irradiated with gamma rays and treated thermally and optically in order to produce samples with different known concentrations of color centers. By comparing the glow curves from ambient temperature to 450°C, and optical absorption and emission spectra in the visible region, it has been possible to establish a link between F3+ centers and the low temperature region of the glow curve below about 200°C.

Colored LiF salt has been always considered a singular optical material among alkali halides and other dielectric crystals for its peculiar characteristics, which in due time have been applied with success in thermoluminescence and laser technology. Lately, while the two previous topics have been revived, new relevant results have been obtained in the optoelectronic field by using both bulk crystals and newly characterized thin films. In practice, miniaturized photoluminescent patterns can be produced rather easily by using low-energy electron beams and soft X-rays. So, LiF salt is becoming a new interesting photonic material with promising developments in basic reasearch and applications as well.

The stable formation of several colour centres (CCs) has been investigated in lithium fluoride (LiF) single crystals irradiated at room temperature (RT) and at 213 K by 5 MeV electrons with doses from 1019 to 1023 eV/cm3. The temperature during irradiation influences the production of aggregate defects, in particular the ratio between the F3+ and F2 laser active centres and the amount of parasitic complex defects. Optical absorption and photoluminescence spectra allow clarifying the role of different aggregate defects on the emission properties of the F3+ and F2 centres at concentrations up to 1018 cm−3.

Amorphous tris(8-hydroxyquinoline) aluminium (Alq3) thin films were deposited on fused silica substrates by thermal evaporation. Optical absorption, emission and excitation spectra were measured at room temperature and, for the first time, at 80 K, on an Alq3 film in a spectral range extending from the near ultraviolet to the visible. Spectroscopic features were carefully investigated as a function of temperature.

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.

The F 2 centers in LiF display an efficient broad emission in the red spectral range when excited in their absorption band located around 450 nm and promising results have been obtained in transferring the optical properties to low-dimensional emitting structures, where their density reaches values higher than 10 17 F 2 /cm 3 . The emission decay times measured in the entirely colored LiF crystals irradiated by 3 MeV electrons with increasing doses do not show any variation up to 10 17 F 2 /cm 3 and decrease by 20% of the value approaching 10 18 F 2 /cm 3 . The values at low concentrations practically coincide with the lifetime of the excited level. The comparison among samples irradiated at different temperatures seems to exclude relevant interactions with the F 3 + centers, while it supports an energy transfer mechanism with the F 2 + ones.

The purpose of this work is to present a simple quantitative theoretical study of the self-quenching process for resonant transitions when radiative self-trapping occurs both in samples of given concentration and shape, and in a planar cavity of known reflectivity and loss. Comparing with experimental results from literature for Cr3+ and Yb3+ for T⩾77 k, we show that self-trapping may increase the self-quenching process by reducing the active ion critical concentration. Considering that studies of effects of micro-cavities on optical transitions have generally dealt with resonant transitions, we think that another important outcome of this study is to show that, without having to rely on density of states coupling effects, a cavity of given Q may both increase or reduce the measured spontaneous lifetime of a resonant transition.