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Optically transparent lithium fluoride (LiF) thin films, thermally evaporated on Si(100) substrates, are under investigation as novel radiation detectors based on radiophotoluminescence for imaging of the full Bragg curves of proton beams produced by a linear accelerator for proton therapy under development at ENEA C.R. Frascati. Proton irradiation induces the formation of stable colour centres in LiF, amongst which the broadband light-emitting F2 and F3+ aggregate defects, whose concentrations are locally proportional to the energy deposited in the material. Their spatial distributions in the irradiated LiF thin films and crystals are carefully measured by acquiring the latent two-dimensional visible fluorescence images with an optical microscope under blue lamp excitation. Several LiF films grown on silicon substrate were irradiated in air at increasing proton energies up to 35 MeV with their surface parallel to the particle beam and a cleaved edge perpendicularly facing it; for each sample, the fluorescence image acquired from the top surface side of the film allows to obtain the depth profile of the energy released by protons. Differences in colour centre distributions detected in LiF films with respect to LiF crystals are presented and discussed. Accurate Monte Carlo simulations allow to fully explain their experimental behaviours, paving the way towards using LiF film radiation detectors on silicon for the advanced diagnostics of proton beams at typical particle energies used for proton therapy.

Lithium fluoride (LiF) crystals and thin films have been successfully investigated as X-ray imaging detectors based on optical reading of visible photoluminescence emitted by stable radiation-induced F2 and F3 + colour centres. In this work, the visible photoluminescence response of optically-transparent LiF film detectors of three different thicknesses, grown by thermal evaporation on Si(100) substrates and irradiated with monochromatic 7 keV X-rays at several doses in the range between 13 and 4.5× 103 Gy, was carefully investigated by fluorescence optical microscopy. For all the film thicknesses, the photoluminescence response linearly depends on the irradiation dose in the investigated dose range. The lowest detected dose, delivered to the thinnest LiF film, only 0.5 μm thick, is estimated 13 Gy. Edge-enhancement imaging experiments, conducted by irradiating LiF film detectors at the same energy placing an Au mesh in front of them at a distance of 15 mm, allowed estimating a spatial resolution of (0.38 ± 0.05) μm, which is comparable to the microscope one. This very high spatial resolution in LiF film radiation detectors based on colour centres photoluminescence is combined with the availability of a wide field of view on large areas.

Proton irradiation of lithium fluoride (LiF) crystals and thin films causes the formation of electronic defects, known as color centers, in the crystal lattice, some of which show photoluminescence in the visible range under blue-light excitation. With a suitable irradiation geometry, the energy density that protons deposit in the material can be recorded as a spatial distribution of these light-emitting color centers, from which a luminous replica of the proton Bragg curve can be thereafter extracted and analyzed in a fluorescence microscope. In this paper, the cases of two LiF thin films deposited on silicon substrates and of a LiF crystal, all of them transversally irradiated with a nominal 35 MeV proton beam, are considered. A comparison of the measured photoluminescent Bragg curves with Monte Carlo simulations demonstrates (i) that the Bragg peaks in the films are located at the very same positions that would be expected in the underlying silicon substrates rather than in LiF and (ii) that an even small grazing angle of the impinging proton beam is able to significantly modify the shape of the Bragg curve in the films. Both of these findings are ascribed to the effects of multiple Coulomb scattering in both the film and the substrate. The coincidence of the Bragg peak positions with those expected in the silicon substrates and the possibility of visualizing such peaks in the Bragg curves stored as latent fluorescence images in the LiF films allow one to regard them as information transducers for proton beam diagnostics and dosimetry.

Irradiation of lithium fluoride (LiF) crystals by low energy (few keV) electron beams gives rise to the efficient formation of stable laser active colour centres, mostly F2 and F3+ aggregate defects, on their surface. The presence of primary and aggregate point defects induces a local increase of the refractive index of the thin irradiated layer in the green-red spectral range, where the F3+ and F2 visible emissions are located. The use of electron-beam lithography techniques allows for the definition of coloured strips, so miniaturised optical active channel waveguides may be fabricated by a direct-writing process. We report on the observation of amplified spontaneous emission of the red light from LiF:F2 centres in active waveguides realised with different irradiation doses in a LiF crystal, in order to determine the influence of this fundamental parameter on the amplification properties of these optical confining structures. Low pumping power densities have been used in quasi-continuous-wave regime at room temperature; the appreciable values of the gain coefficient, several cm−1, with an exciting power density of few hundreds mW/cm2, make this material a good candidate for the realisation of active integrated optical devices.

We present broadband X-ray lensless projection imaging experiments performed at the TopoTomo beamline of the ANKA synchrotron facility by using innovative solid-state detectors based on the formation of stable photoluminescent colour centres (CCs) in lithium fluoride (LiF) films and crystals. The peculiar features of these detectors, like very high spatial resolution (standard 250 nm) across a large field of view (> 1 cm2), wide dynamic range (> 103), versatility and easiness of use combined with the high sensitivity of the optical fluorescence microscopy reading technique, make them very attractive as recording plates for X-ray imaging. High-resolution X-ray images of a metallic test pattern were recorded and acquired with good contrast both in LiF crystals and thin LiF films. The stored CCs fluorescence images show edge-enhancement effects ascribable to diffraction processes occurring during the X-ray beam propagation after its interaction with the samples. The experimental data concerning films and crystals based LiF-detectors will be discussed and compared with computer simulations of X-ray intensity distributions. The LiF film performances are very encouraging for the exploitation of these solid-state detectors for X-ray lensless imaging applications.

Fluorescent lithium fluoride detectors for X-ray projection imaging F. Bonfigli, M.A. Vincenti, R.M. Montereali ENEA, C.R. Frascati, Photonics Micro- and Nano-structures Lab., UTAPRAD-MNF, Via E. Fermi 45, 00044 Frascati (Rome), Italy E. Nichelatti ENEA, C.R. Casaccia, Optical Devices Laboratory, UTTMAT-OTT, Via Anguillarese 301, 00123 S. Maria di Galeria, Rome, Italy F. Somma, S. Heidari Bateni Università degli Studi Roma Tre, Dip. di Fisica E. Amaldi, Via della Vasca Navale 84, 00146 Rome, Italy A. Cecilia, T. Baumbach Institute for Photon Science and Synchrotron Radiation (IPS)/ANKA, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany. Abstract The possibility of detecting high quality images of materials, devices and biological samples in the soft and hard X-ray spectral ranges with high spatial resolution and contrast and by using simple exposing configuration is a topical task nowadays. For this purpose, we propose the use of versatile imaging detectors based on the radiation sensitivity of lithium fluoride (LiF) to extreme ultraviolet, soft and hard X-rays [1, 2]. X-rays generate stable point defects in LiF, known as colour centres (CCs), which emit broad-band photoluminescence at visible wavelengths under optical pumping. The high dynamic response of the material to the received dose together with the atomic scale of the CCs make LiF plates, in form of thin films or crystals, extremely attractive as high-spatial-resolution radiation-imaging detectors both in absorption and phase contrast imaging configurations [1, 3]. The latent images are subsequently read by using optical fluorescence microscopes, which in the case of advanced techniques can reach spatial resolutions well below 100 nm [4]. We present lensless imaging experiments in projection mode at the TOPO–TOMO beamline of the synchrotron light source Anka (Karlsruhe, Germany) by using LiF crystals and thin films irradiated in the energy range 6–40 keV. Stable fluorescent images were formed in LiF detectors by scattered X-rays after an object had been positioned between the X-ray source and the detectors. The object used in this work is the commercial test pattern X500-200-30 (Xradia, Pleasanton, CA, USA) consisting of a gold mask (thickness 3 m) deposited on a (500500) μm2 Si3N4 window (thickness 330 nm). The LiF detectors were crystals and 1 m thick polycrystalline films deposited on glass substrates. To imprint the X-ray image of the sample on the LiF detectors, the test pattern was irradiated with several exposure times between 1 s and 60 s. The X-ray micro-radiographies were optically read with a confocal laser scanning microscope (CLSM, Nikon Eclipse 80i-C1) operating in fluorescence mode. It is worth pointing out that a 1 m LiF film was able to store high-quality and well contrasted fluorescence images of the test pattern, although X-ray attenuation length in LiF varies between 0.02-12 mm for X-ray energies in the investigated range. The stored images show edge-enhancement effects that are ascribable to diffraction processes occurring during the X-ray beam propagation after its interaction with the sample. A computer simulation was performed to calculate the incoming X-ray intensity distribution across the detector. A fairly good agreement with experimental data evidences a linear optical fluorescence response of LiF-film based detectors under the investigated conditions. This linear behaviour has been confirmed by measurements of the PL signals of F2 and F3+ CCs detected with a CLSM system for several X-ray irradiation times. Further investigations are in progress to study the exploitation of solid-state LiF detectors for X-ray lensless projection imaging experiments and applications. References [1] G. Baldacchini, F. Bonfigli, A. Faenov, F. Flora, R.M. Montereali, A. Pace, T. Pikuz, L. Reale, J. Nanoscience and Nanotechnology 3, 6 483-486, (2003). [2] S. Almaviva, F. Bonfigli, I. Franzini, A. Lai, R.M. Montereali, D. Pelliccia, A. Cedola, S. Lagomarsino, Appl. Phys. Lett. 89, 54102, (2006). [3] F. Bonfigli, A. Cecilia, S. Heidari Bateni, E. Nichelatti, D. Pelliccia, F. Somma, P. Vagovic, M.A. Vincenti, T. Baumbach and R.M. Montereali, Radiation Measurements 56, 277-280, (2013). [4] A. Ustione, A. Cricenti, F. Bonfigli, F. Flora, A. Lai, T. Marolo, R.M. Montereali, G. Baldacchini, A. Faenov, T. Pikuz, L. Reale, Appl. Phys. Lett. 88, 141107 (2006).

The depth refractive index profiles of broadband visible-emitting planar waveguides produced in LiF crystals with 1.5- and 2-MeV He+ ions at different doses have been derived from mode analysis. They show that there are two competitive mechanisms responsible for positive and negative modifications of the refractive index in the irradiated volume associated with different processes of energy deposition of the incident ions, so as to induce a complex coloration profile along the penetration direction, which is strongly dependent on the irradiation dose.

The general requirements for a plasma facing mirror (PFM) of a Thomson scattering system (TS) for a burning plasma experiment are (i) high and approximately constant reflectivity in the wavelength spectral range 400–800 nm; (ii) low sputtering yield and low erosion; (iii) high power damage threshold; (iv) good thermo-mechanical properties to preserve quality imaging. Rhodium-coated mirrors are chosen because they meet these requirements. Rhodium coated mirror were realized with substrates of copper and vanadium. The detailed optical characterization of these mirrors is presented: i.e., surface planarity measurements as well as roughness and reflectivity figures are presented. These data can be used for the choice of the PFM of a TS system for international thermonuclear experimental reactor.

The knowledge of the reflectivity and phase profiles of graded reflectivity mirrors (GRMs) is important for studying the performance of lasers equipped with optical unstable resonators that use a GRM as output coupler. This paper deals with a low cost home-made interferometer which permits simultaneous measurement of GRM reflectivity and phase profiles at CO2 wavelengths. The instrument works without any beam expander and infrared camera. The reflectivity and phase profiles are measured by performing a discrete number of local measurements along the sample diameter. To minimize the data acquisition time a fringe scanning interferometric technique based on the serrodyne technique is developed. According to this technique, the reference wave is phase modulated with a sawtooth-vibrating mirror. Reflectivity and phase are directly measured with a two phase lock-in amplifier. The RMS repeatibility for reflectivity and phase profile measurements is 0.8% and lambda /200, respectively.

Fabrication of a coated graded reflectance mirror (GRM) is based on a multilayer design where one or more layers have a radially variable thickness. We describe and experimentally validate a method that can be used to predict the shape of a profiled layer deposited by masking techniques. Deposition by masking techniques is modelled in the framework of the linear system theory. We also suggest an experimental procedure for estimating the Green function that rules particle gas diffusion, and we apply it to the case of RF-sputtering deposition of ZnSe monolayers on BK7 glass. Comparison of predicted and measured reflectivity profiles at lambda =1.064 mu m confirms the model validity.

A multilayer coating on a glass pane, specifically designed for artwork protection, has been designed. The materials selected for the coating are SiO2 (silicon oxide), TiO2 (titanium oxide) and ITO (indium tin oxide). Prototype layers were deposited by thermal evaporation on glass substrates. Neutron reflectometry consists of measuring the amount of neutrons reflected by a specimen at angles close to total reflection and can be successfully used to evaluate thickness, surface and interfacial roughness, and material inhomogeneity of each layer in the coating. Neutron reflectometry has demonstrated that some layers are likely to be affected by inhomogeneity that should be controlled to optimize the final performance of the device.

Many diffraction calculations have been carried out up to now, both numerical and analytical, to determine the transversemode properties of laser resonators. Generally these calculations have been based on simplifying assumptions to reduce the problem complexity and, consequently, have been limited to only a few configurations, while more extensive analyses have considered only the eigenmodes of conventional, empty resonators. In particular, little attention has been paid to investigate the transverse-mode properties of optical cavities equipped with interferometric frequency filtering elements (IFFEs), such as etalons, or multipass interferometric reflectors, which are utilized for achieving single-longitudinalmode operation, narrow-band frequency tuning, and shot-to-shot output frequency stabilization. One can expect that the presence of these optical elements modifies both the spatial and spectral features of the oscillating modes, since it has been demonstrated that their insertion in an optical cavity induces nonlinear variations of the phase of the cavity's electric field that is equivalent to an actual change of the cavity length.1

Planar active waveguides, produced in lithium fluoride crystals by implantation with 2 MeV He/sup +/ ions at several doses, have been characterized by optical absorption, photoluminescence and m-lines spectroscopy. All the colored samples support several modes, whose analysis allows reconstruction of the depth refractive index profiles of the waveguides as a function of dose. Two competitive physical mechanisms, associated with different processes of energy deposition along the ion track, are responsible for positive and negative modifications of the refractive index in the irradiated volume.

The effect of reflective multipass interferometers on the transverse modes of laser cavities equipped with these devices is investigated. We demonstrate, both theoretically and experimentally in a pulsed CO(2) laser, how information on the characteristics of these modes can be directly derived by simply describing these cavities in terms of suitable equivalent geometric parameters. We also discuss and explain the transverse-mode selective properties of these multimirror cavities.

Planar active waveguides were produced in lithium fluoride crystals by implantation with 1.5 MeV He + ions at several doses. The colored samples have been characterized by optical absorption, photoluminescence and m-line spectroscopy. By comparing the measured guided-mode effective indices with the ones calculated by means of the Chandler–Lama approach, the depth profile of refractive index was derived, showing that there are two competitive physical mechanisms, associated with different processes of energy deposition along the ion track, responsible for positive and negative modifications of the refractive index in the irradiated volume.

An analytical expression of the complex permittivity is derived for absorbing centres featuring inhomogeneous absorption-line broadening. Such an expression gives the dispersion law of the real part of the permittivity when the imaginary part has a Gaussian lineshape. Our mathematical approach starts from an overlap integral of Lorentzian-type dielectric susceptibilities weighted by a Gaussian probability distribution of the resonance absorption energies. The analytical solution found is consistent with the Kramers–Kronig relation. We demonstrate that, like in the case of homogeneous absorption-line broadening, the refractive index increases at photon energies lower than the resonance absorption energy also for inhomogeneous absorption-line broadening; if the absorbing centres emit Stokes-shifted radiation, such an increase can be exploited for passive and active waveguiding applications. An example is reported regarding active waveguides based on colour centres in a lithium fluoride crystal.

The use of efficient wavelength-shifters from the vacuum-ultraviolet to the photo-sensor's range of sensitivity is a key feature in detectors for Dark Matter search and neutrino physics based on liquid argon scintillation detection. Thin film of Tetraphenyl-butadiene (TPB) deposited onto the surface delimiting the active volume of the detector and/or onto the photosensor optical window is the most common solution in current and planned experiments. Detector design and response can be evaluated and correctly simulated only when the properties of the optical system in use (TPB film + substrate) are fully understood. Characterization of the optical system requires specific, sometimes sophisticated optical methodologies. In this paper the main features of TPB coatings on different, commonly used substrates is reported, as a result of two independent campaigns of measurements at the specialized optical metrology labs of ENEA and University of Tor Vergata. Measured features include TPB emission spectra with lineshape and relative intensity variation recorded as a function of the film thickness and for the first time down to LAr temperature, as well as optical reflectance and transmittance spectra of the TPB coated substrates in the wavelength range of the TPB emission.© 2013 IOP Publishing Ltd and Sissa Medialab srl.

A thin film of Tetraphenyl-butadiene (TPB) deposited onto the surface delimiting the active volume of the detector and/or onto the photosensor's optical window is the most common solution to down convert argon VUV scintillation light in current and planned liquid argon based experiments for dark matter searches and neutrino physics. Characterization of the main features of TPB coatings on different, commonly used substrates is reported, as a result of measurements at the specialized optical metrology labs of ENEA and University of Tor Vergata. Measured features include TPB emission spectra with lineshape and relative intensity variation recorded as a function of the film thickness and for the first time down to LAr temperature, as well as optical reflectance and transmittance spectra of the TPB coated substrates in the wavelength range of the TPB emission.© 2013 IOP Publishing Ltd and Sissa Medialab srl.

We employ a differential version of m-line spectroscopy through grating coupling in order to measure the refractive index of low-energy electron irradiated lithium fluoride crystal channel waveguides for broadband emission. Using photoresist films and a holographic setup for grating fabrication, we perform an accurate characterization of bidimensional structures and a direct quantitative comparison between treated and blank materials.

For the first time active planar optical microcavities with a thin colored lithium fluoride film as spacer have been fabricated by ion implantation. Irradiation has been realized through the top Bragg mirror of the passive microcavity, already fully deposited, to produce stable visible-emitting F2 and F 3 + color centers in the LiF film. The spectral and spatial emission features of the microcavities demonstrate the feasibility of such an irradiation method and the possibility of controlling the radiative properties of compact light sources based on point defects in this interesting material.

Planar optical microcavities based on a lithium fluoride (LiF) spacer thin film activated by proton implantation have been fabricated and characterised by photoluminescence for the first time. Irradiation has been performed through the top Bragg mirror to produce the stable formation of visible-emitting F2 and F3+ colour centres in the LiF film. The spectral and spatial emission features of the microcavities demonstrate the possibility of controlling the radiative properties of luminescent sources.

Recent investigations have demonstrated the relevance of the emission properties of laser active color centers (CCs) in lithium fluoride (LiF) films to design new miniaturized solid state sources. Among them, optical microcavities hold technological promises for the realization of all-solid, low-threshold, high-directional emitters and lasers. The peculiar structure of the microcavities changes the characteristics of local electromagnetic field and consequently the spatial, spectral and temporal properties of the active center emissions. In this work we will show significant changes in the emission behavior of F2 CCs in LiF films placed inside a microcavity, i.e. light intensity enhancement, spectral narrowing, spatial redistribution and alteration of spontaneous decay time, as compared to a no-cavity structure.

Lithium fluoride (LiF) films irradiated by low energy electrons were employed as active spacers in all-solid, dielectric optical microcavities emitting in the visible spectral range. We present the results of optical characterization of the spontaneous emission from F 2 color centers embedded in a LiF layer confined inside a planar microcavity. These structures seem promising for the realization of novel kinds of solid-state miniaturized emitting devices.

LiF films and crystals containing lithium colloids have been studied by Scanning Near-Field Optical Microscope (NSOM). 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