F. Somma

Mode analysis in He+-implanted lithium fluoride planar waveguides

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.

Diamond photoluminescence spectra: Dependence on excitation energy and microstructure

Abstract Photoluminescence data are presented for diamond films grown by different techniques (hot-filament CVD, microwave PECVD, DC arc-jet) and excited in the range 450–650 nm by different laser sources. In large-grain textured samples, sharp vibronic, structures, related to nitrogen and silicon impurity centers, are clearly observed, whereas in small-grain, randomly oriented films spectra are dominated by broad luminescence bands, whose maxima move with excitation energy. In the former case a careful deconvolution of emission spectra allows us to obtain vibronic parameters of the centers and their fine structure details, while excitation spectra give the energy location of higher electronic excited states. On the other hand, the line-shape and the excitation-dependent shift of broad luminescence bands are related to a continuous distribution of gap states, able to trap photogenerated electron-hole pairs.

Color-Center Waveguides in Low-Energy Electron-Bombarded Lithium Fluoride

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.

Increase of refractive index induced by absorbing centres

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.

NaF films: Growth properties and electron beam induced defects

Abstract 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.

F-aggregate centres in KCl films

Abstract 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.

Photoluminescence of LiF : NaF films at room temperature

Abstract In this paper we present the results of the photoluminescence spectra of multilayer LiF : NaF films colored by low energy (3 keV) electron beams. We show that dielectric multilayered films can be used in the design of new materials with optical properties that cannot be matched by bulk materials.

Defect generation in low-energy ion-assisted thermal deposited lithium fluoride films

We present first results concerning stable formation of primary electronic defects and lithium nanometer sized clusters in LiF thin films grown by ion-assisted thermal deposition. The optical and morphological properties of the as grown LiF films, dependent on the deposition conditions, such as ion-beam energy and ion species (Xe, Ar), are reported. The experimental results show a larger efficiency of low-energy Xe ions in inducing the formation of lithium nano-clusters. To analyse the role of the deposition conditions, a preliminary interpretation of the lithium nano-cluster formation mechanism based on the spherical and/or cylindrical spike thermal model is given.

Crystalline properties and colour centres in alkali halide thin films

Abstract 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.

Optical properties of FA and (F2)a centres in doped KI

Abstract Crystals of additively coloured KI doped with lithium or sodium exhibit, after irradiation with F-light, new absorption and emission bands due to associated and perturbed centres. The defects responsible for these bands have been identified using excitation spectra and polarization measurements. Fa(I) and (F2)a centres are found to be present in both KI:Li+ and KI:Na+ crystals. A weak emission in KI: Li+ is ascribed to FB(II) centres. The possibility of using these systems as laser active media is discussed.