T. Marolo

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...

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.

Scanning near-field optical microscopy images of microradiographs stored in lithium fluoride films with an optical resolution of λ∕12

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 50nm, i.e., λ∕12, where λ is the wavelength of the collected photoluminescence.

Optical properties of rough, inhomogeneous lithium fluoride films with colour centres

Abstract 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.

Spectroscopic measurements and thermoluminescence of γ-ray colored LiF crystals

Abstract 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.

POINT DEFECTS IN LITHIUM FLUORIDE FILMS INDUCED BY GAMMA IRRADIATION

Pure and doped lithium fluoride (LiF) crystals are well-known dosimeter materials. In this work we report the preliminary results about a careful optical characterisation of polycrystalline LiF films thermally evaporated on fused silica substrates and gammairradiated at several doses up to 10 Gy in air. Gamma irradiation of LiF films gives rise to stable formation of primary and aggregate defects. Among them, F centres give rise to the absorption band peaked at 245 nm, while F2 and F3 centres are responsible for the absorption in the blue region and for stable and intense green (F3) and red (F2) photoluminescence at room temperature. Photoluminescence spectra were measured with both a commercial and a laboratory apparatus. A simplified set-up for optically stimulated luminescence reading was tested. The results are encouraging to propose LiF film on fused silica substrate as dosimeter for gamma irradiation.

Lithium fluoride coloration by laser-plasma soft x-rays: a promising tool for x-ray microscopy and photonics

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.

Micro-Radiographs Stored in Lithium Fluoride Films Observed by Scanning Near-Field Optical Microscopy

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).

Thermal lens characterization of a side-pumped Nd:YVO4 laser

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.

Refractive index change in photoluminescent patterns based on color centers generated in lithium fluoride by EUV radiation

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.