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The stable formation of several colour centres has been investigated in four lithium fluoride polycrystalline thin films irradiated in air by gamma rays with doses from 103 up to 106 Gy. Careful spectrophotometric measurements allowed estimating the concentrations of primary F defects and aggregate F 3 + and F2 laser-active centres only at the two highest doses. These visible-emitting electronic defects were identified through laser-induced photoluminescence spectra even at the lowest doses. For both the F2 and F 3 + light-emitting centres, the luminescence signal increases sub-linearly with the irradiation doses.

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.

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.

Non-linear optical response of single-wall carbon nanotubes, produced without catalyst, was investigated. Third-order susceptibility Degenerate Four Wave Mixing measurements have been performed on thick film of nanotubes deposited onto quartz substrate. Results are presented as measured in the nanosecond time scale at 532 nm. The second-order hyperpolarizability γ was estimated to be as large as 1.6 × 10 −29 e.s.u.

Lithium fluoride (LiF) films containing colour centres (CCs) are interesting because of their potential applications in the field of integrated optics as passive and active waveguides. Four LiF films, about 3 μm thick, were grown by thermal evaporation. After deposition, the films were irradiated with γ-rays (doses up to 106 Gy) so that primary and aggregate CCs would be generated in them. The optical properties of the films with CCs were studied by means of spectrophotometry. Absorption bands ascribable to primary and aggregate CCs were detected in the two films irradiated at higher doses. The measured photometric spectra were best-fitted with a unified model and the concentrations of the main CCs were estimated.

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.

Note: Sp. Iss. SI Reference LPMV-ARTICLE-2007-004doi:10.1016/j.jlumin.2006.01.190View record in Web of Science Record created on 2007-06-20, modified on 2017-12-03

Scanning near-field optical microscopy (SNOM) makes it routinely possible to overcome the fundamental diffraction limit of standard (far-field) microscopy. Recently, aperture-based infrared SNOM performed in the spectroscopic mode, using the Vanderbilt University free electron laser, started delivering spatially-resolved information on the distribution of chemical species and on other laterally-fluctuating properties. The practical examples presented here show the great potential of this new technique both in materials science and in life sciences. (C) 2004 Elsevier B.V. All rights reserved.

A new simple method to get soft X-ray micro-radiographs of biological material with a spatial resolution reaching 50 nm is described. A thin film of lithium fluoride (LiF) works as image detector, storing the micro-radiograph obtained exposing biological material to Extreme Ultra-Violet and soft X-ray radiations. Scanning near-field optical microscope (SNOM) is used to read the micro-radiograph at nanometric resolution up to λ/12, collecting the fluorescence ( λ=650 nm) excited by an Ar+ laser light ( λ=458 nm).

Point defect formation and stabilization properties, as well as their peculiar spectroscopic characteristics, locally modify the optical properties of insulating materials. Thin layers containing high concentrations of colour centres, hosted in a LiF single crystal and/or a polycrystalline matrix, offer the opportunity to develop innovative light-emitting photonic devices. Control of all the critical parameters should be required on spatial dimension comparable with the optical wavelengths. Recent developments in laser technologies, electron and particles beam methods, and novel photon sources, have opened a wide range of opportunities. An overview of the most significant advances in this field is provided, with particular emphasis on colour-centre LiF-based innovative miniaturised light sources, optical amplifiers and lasers. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Here we show a new, simple method to observe soft x-ray microradiographs of biological material. A thin film of lithium fluoride (LiF) works as image detector, storing the microradiograph obtained exposing biological samples to extreme ultraviolet and soft x-ray radiations. To read the stored image, collecting the optically stimulated visible luminescence emitted by the LiF active color centers locally produced by the x rays, a scanning near-field optical microscope is used with an optical aperture of 50 nm, i.e., L/12, where L is the wavelength of the collected photoluminescence.

The continuously shrinking of the photonic structures has stimulated the research and development of low-dimensionality photoluminescent patterns based on visible-emitting color centers (CCs) in thin films. In particular, Lithium Fluoride (LiF) treated with ionizing radiation by using versatile lithographic techniques, represents a very promising candidate for the realization of innovative photonic devices. Permanent tluorescent patterns based on CCs have been realized in LiF films by direct writing with a X ray microprobe -of the ESCA Microscopy beamline at ELETTRA synchrotron in different configuration. Luminescent nanostructures, written by scanning the LW specimen with respect to the X-ray beam, have been investigated by near-field optical microscope and a sub-wavelength spatial resolution is demonstrated.

We present a diode side-pumped Nd:YVO4 laser emitting up to 11 W CW output power c axis polarized at 1064 nm in a bounce geometry. We employed a 2x4x12 mm3, 6° wedge, 1.1% Nd3+ doped slab crystal. We set up a compact 2-mirrors cavity design using a cylindrical high reflectivity mirror (curved in the tangential plane) and several plane output coupler. The optimum total internal reflection angle of the designed bounce geometry, leading to the maximum output power, was measured to be about 3.5°. The overall length of the cavity ranges from 8 to 11 cm depending on the investigated configuration. As much as 11 W CW output power was obtained for approximately 26 W of absorbed pump power at 808 nm, with a slope efficiency up to 50% and a near diffraction limited outpu beam in the vertical direction. An overall M2 < 3 was obtained at the maximum output power. We also present a focal thermal lens characterization of the cavity in two different experimental setup. The study was based on a first order dependence of the focal thermal length on the absorbed pump power model fth∞k/Pabs. Our results are in good agreement with the model and the experimental behaviour of the system confirmed the outcome of the focal thermal lens measurements.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The use of lithium fluoride (LiF) crystals and films as imaging detectors for EUV and soft-x-ray radiation is discussed. The EUV or soft-x-ray radiation can generate stable color centers, emitting in the visible spectral range an intense fluorescence from the exposed areas. The high dynamic response of the material to the received dose and the atomic scale of the color centers make this detector extremely interesting for imaging at a spatial resolution which can be much smaller than the light wavelength. Experimental results of contact microscopy imaging of test meshes demonstrate a resolution of the order of 400nm. This high spatial resolution has been obtained in a wide field of view, up to several mm2. Images obtained on different biological samples, as well as an investigation of a soft x-ray laser beam are presented. The behavior of the generated color centers density as a function of the deposited x-ray dose and the advantages of this new diagnostic technique for both coherent and noncoherent EUV so...

Low-dimensionality photoluminescent patterns based on broad-band visible-emitting F2 and F3+ colour centres have been realised in Lithium Fluoride films by direct writing with a focused soft X-ray beam provided at the ELETTRA synchrotron radiation facility. Permanent fluorescent lines, with typical width from few hundreds of nanometers to tens of micrometers, have been prepared by scanning the LiF specimen with respect to the X-ray microprobe and imaged by confocal laser and near-field optical microscopy in fluorescence configuration. The use of this versatile lithographic technique offers the opportunity to integrate active and passive optical components, like waveguides and gratings, in this promising material, grown by simple and well assessed film deposition methods on different substrates. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Periodic luminescent patterns based on active color centers have been efficiently created in Lithium Fluoride (LiF) crystals and films by Extreme Ultra-Violet (EUV) light and soft x-rays from a laser-plasma source by masking the incoming radiation. Strong visible photoluminescence at room temperature has been measured from colored geometric microstructures, produced with high spatial resolution on large areas in short exposure times. Accurate spectrophotometric measurements allow estimating a significant increase in the real part of the refractive index, locally induced by the formation of high concentrations of stable primary and aggregate electronic defects at the surface of the LiF irradiated crystal, in a very thin layer, whose depth has been evaluated around 50 nm. On the basis of a semi-classical dipole-electromagnetic field interaction model, the contribution of different kinds of point defects to the overall refractive index change has been quantified. Promising opportunities in the fabrication of passive and active devices for integrated optics, such as gratings and distributed lasers, are offered by the use of this kind of radiation.

A new imaging detector for EUV or soft-X-ray radiation based on optically stimulated luminescence (OSL) of lithium fluoride (LiF) films or crystals is presented. The first micro-radiography images of biological samples and of meshes obtained on LiF using a laser-plasma source or an X-ray laser are shown, and (up to now) a resolution better than one micron is demonstrated. The dependence of the coloration density vs the deposited X-ray dose is considered and the advantages of this new diagnostic technique for both coherent and non-coherent EUV sources, compared with CCDs detectors, photographic films and photoresists are discussed. This new detector is extremely suitable for laser plasmas and for X-ray lasers sources.

Local fluorescent probes based on low-energy electron beam irradiated lithium fluoride (LiF) thin films were preparated and tested for their applications in Scanning Near-Field Optical Microscopy. © 2004 American Institute of Physics.

The miniaturization of optical devices is a key objective in the field of photonics, and a large set of materials and techniques are under investigation. Among the former, lithium fluoride (LiF) is of particular interest because it is almost not hygroscopic and it can host stable color centers (CCs) produced by ionizing radiation and emitting in the visible spectral range even at room temperature (RT) under optical excitation. The increasing demand for low-dimensionality photonic devices imposes the utilization of advanced lithographic techniques for producing luminescent structures with submicrometric spatial resolution. We present an innovative irradiation method producing CCs in LiF crystals and films by using an EUV and soft X-ray laser-plasma source. This technique is able to produce colored patterns with high spatial resolution on large (more than 10 cm2) areas in a short exposure time compared with other irradiation methods. The colored LiF samples have been characterized by optical absorption and photoluminescence measurements for different irradiation fluences.

LiF films have been grown on silicon substrate, irradiated with soft x-rays to create fluorescent regular micrometric-spaced arrays based on colour centres, and studied by Scanning Near-field Optical Microscope (SNOM). Strong variations in the local reflectivity have been observed in the infrared region between 6.1 and 9.2 mum and tentatively ascribed to a modulated variation of the refractive index of the coloured zone with respect to that of the uncoloured LiF matrix. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The measured room-temperature absorption spectra of LiF crystals implanted with 1.5 MeV He+ ions at different ion doses are presented. The application of a theoretical model allowed us to give an estimation of the band intensities associated with various kinds of electronic defects and to study their dependence on the ion dose. The model also considers the contribution due to narrometric lithium colloids, whose formation starts in the dose range at which the production of colour centres saturates.

We present results on simultaneous nanostructuring and optical activation of lithium fluoride crystals by 800eV off-normal Ar+ sputtering at different ion doses. The samples were studied by atomic force microscopy and optical spectroscopy. After ion irradiation smoothening of the initial random roughness is achieved and well-defined self-organized ripple structures appear, having a mean periodicity of 30nm and a mean height of 3nm. The simultaneous optical activation of the irradiated samples is due to the stable formation of electronic defects with intense photoluminescence in the visible spectral range.