Rosa Maria Montereali

Active stripe waveguides produced by electron beam lithography in LiF single crystals

Abstract Photo-luminescent colored stripes have been fabricated by electron beam lithography in lithium fluoride crystals, and their guiding properties are demonstrated for the first time. An increase of refractive index Δn larger than 5×10−3, suitable for light confinement, was achieved by irradiation with a 12 keV electron beam. This single-step process, that induces at the same time the waveguiding structure and the photo-luminescent color centers, offers good perspectives for the development of color-center integrated optical lasers tunable in the visible.

Soft x-ray submicron imaging detector based on point defects in LiF

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...

LIF FILMS : PRODUCTION AND CHARACTERIZATION

Abstract LiF films were produced by thermal evaporation onto amorphous substrates kept (during evaporation) at temperatures ranging from ambient up to 350°C. The evolution through two polycrystalline states was analysed by X-ray diffraction techniques, which also revealed that the LiF film lattice parameter is equal to that of LiF crystal. The refractive index of the produced films was measured by transmission optical interferometry. Talisurf-6 profilometer measurements on the film surface show sharp edges and very smooth surfaces, which reflect the substrate surfaces.

New perspectives of coloured LiF for optoelectronic devices

Colour centre lasers (CCL) based on LiF crystals have been operating at room temperature (RT) in the visible and in the near infrared since many years. On the other hand little is known about LiF films treated by low energy electrons, which are promising candidates for the realisation of miniaturised broad band active optical devices, like waveguide and microcavity amplifiers and lasers. The high concentration of defects produced in these optically confined active structures does not seem to affect appreciably the numerical value of the gain coefficients, although complex mechanisms of losses should be taken into account. Interactions among several types of defects in extremely coloured samples should be further investigated in crystals and films. The formation of different kind of centres could be controlled by a careful choice of irradiation conditions and growth parameters in the polycrystalline films.

Optical characterization of low-energy electron-beam-colored LiF crystals by spectral transmittance measurements

The complex refractive index of a LiF crystal surface layer irradiated by low-energy electrons is modified by the stable formation of color centers embedded in it. A simplified dipole-electromagnetic field interaction model has been adopted in order to estimate the dispersion curves of colored LiF from a single optical transmittance measurement. The excellent agreement with the corresponding experimental curves (obtained by means of spectrophotometry and ellipsometry) demonstrates this to be a promising approach for LiF-based optical waveguide characterization.

LiF films: absorption and luminescence of colour centres

Abstract LiF films 1 μm thick have been produced by thermal evaporation on amorphous silica substrates. X-ray diffraction has shown the existence of a polycrystalline structure. The films were subjected at room temperature to electron beam irradiation of energy 3 keV and current 120 μA on anea of about 0.5 cm2. This irradiation produced F2 and F3+ centres stable at room temperature that were observed by light absorption measurements. Although the concentration of the centres was much higher than that obtained usually in bulk crystals, luminescence was also observed.

Thermal and fast neutron detection in chemical vapor deposition single-crystal diamond detectors

Recently, a compact solid-state neutron detector capable of simultaneously detecting thermal and fast neutrons was proposed [M. Marinelli et al., Appl. Phys. Lett. 89, 143509 (2006)]. Its design is based on a p-type/intrinsic/metal layered structure obtained by Microwave Plasma Chemical Vapor Deposition (CVD) of homoepitaxial diamond followed by thermal evaporation of an Al contact and a L6iF converting layer. Fast neutrons are directly detected in the CVD diamond bulk, since they have enough energy to produce the C12(n,α)B9e reaction in diamond. Thermal neutrons are instead converted into charged particles in the L6iF layer through the L6i(n,α)T nuclear reaction. These charged particles are then detected in the diamond layer. The thickness of the L6iF converting layer and the CVD diamond sensing layer affect the counting efficiency and energy resolution of the detector both for low- (thermal) and high-energy neutrons. An analysis is carried out on the dynamics of the L6i(n,α)T and the C12(n,α)B9e reactio...

Point defects in thin insulating films of lithium fluoride for optical microsystems

Publisher Summary This chapter describes the interesting characteristics of polycrystalline lithium fluoride (LiF) films and the peculiar properties of point defects induced by low-penetrating ionizing radiation in this material, with the aim of developing miniaturized optical systems. LiF is unique among all known materials because of its large spectral transparency. Wide band-gap materials are special in many aspects. First of all, they are transparent to visible light, and this peculiarity allows control of color. The control over the refractive index on a scale comparable with the wavelength of light, combined with the ease of integration of conventional passive and active optical functions in LiF film-based structures—as in optical microcavities—makes it possible to be optimistic about the perspectives of application in integrated optical circuits. Among alkali halide (AH) crystals, LiF stands apart as far as its physical and optical properties are concerned. Despite its typical ionic structure, the crystal is relatively hard and resistant to moisture.

Surface nanostructuring and optical activation of lithium fluoride crystals by ion beam irradiation

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.

Reflectance and transmittance of a slightly inhomogeneous thin film bounded by rough, unparallel interfaces

Inhomogeneity of a thin film, rough interfaces of the film with air and substrate and non-parallelism of these interfaces severely affect reflectance and transmittance. Neglecting these film imperfections could give rise to erroneous optical characterisation of the film from measured spectra. Each one of these imperfections, considered alone, has its own characteristic influence on the interference fringes of the reflectance and transmittance spectra. In this work, a model of an inhomogeneous thin film bounded by rough, unparallel interfaces is introduced for calculating reflectance and transmittance at normal incidence. The effects on the spectra due to roughness as predicted by the proposed model are compared with those evaluated by an approach involving the effective-medium approximation (EMA). In this latter approach, a thin transition layer replaces the rough interface, but it proves inadequate for reproducing the transmittance spectrum. Finally, the model is successfully applied to the characterisation of a LiF film thermally evaporated on a glass substrate.