R. M. Montereali

Amplified spontaneous emission in active channel waveguides produced by electron-beam lithography in LiF crystals

In this letter we report the observation of amplified spontaneous emission of the red light from LiF:F2 centers in active channel waveguides realized by electron-beam lithography in lithium fluoride crystals. Low pumping power densities have been used in quasi-continuous-wave regime at room temperature; the appreciable values of the gain coefficients, 4.67 cm−1 with an exciting power density of 0.31 W/cm2 at 458 nm, make this material a good candidate for the realization of active integrated optical devices.

High-contrast photoluminescent patterns in lithium fluoride crystals produced by soft x-rays from a laser-plasma source

A technique using soft x-rays and extreme ultraviolet light generated by a laser-plasma source has been investigated for producing low-dimensionality photoluminescent patterns based on active color centers in lithium fluoride (LiF) crystals. Strong visible photoluminescence at room temperature has been observed in LiF crystals from fluorescent patterns obtained by masking the incoming radiation. This technique is able to produce colored patterns with high spatial resolution on large areas and in short exposure times as compared with other coloration methods.

Photoluminescence of LiF crystal colored by a focused electron beam

Abstract The absorption, emission and excitation optical spectra of F2 and F+3 color centers generated in LiF through low energy (30 keV) electron beam irradiation were measured at room temperature by pumping with an Ar+ laser the resulting thin colored layer. A quenching of the emission intensity of the F+3 centers versus the pump power was observed, with characteristics similar to bulk colored crystals. The analysis of the photoemission spectra showed a new band, which has been discovered previously also in bulk crystals colored and treated in different ways. The possibility of applications of the colored ribbon for miniaturized laser devices is commented.

Hard x-ray contact microscopy with 250nm spatial resolution using a LiF film detector and a tabletop microsource

An innovative route for deep-submicrometer spatial resolution hard x-ray microscopy with tabletop x-ray source is proposed. A film of lithium fluoride (LiF) was used as imaging detector in contact mode. We present here the x-ray images recorded on LiF films of a Fresnel zone plate with submicrometer gold structures and of an onion cataphyll. The images were read with an optical confocal microscope in fluorescence mode. The measured spatial resolution was about 250nm, i.e., close to the resolution limit of the confocal microscope. The advantages and drawbacks, and the possible improvements, of this route are discussed.

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.

Two-beam interferometric encoding of photoluminescent gratings in LiF crystals by high-brightness tabletop soft x-ray laser

Two-beam interferometric encoding of periodic lines of permanent color centers in LiF has been obtained by use of an intense and high spatially coherent soft x-ray laser beam. A spatial resolution of the lines less than 1μm is demonstrated. We have used the 46.9nm laser pulses (0.3mJ, 1.7ns at 0.2Hz) produced in compact capillary discharges. Due to the low penetration depth of their radiation, their high brightness and spatial coherence, soft x-ray lasers can represent a powerful tool to encode integrated optical devices having low dimensionality and high spatial resolution inside optically transparent dielectrics, in extremely short exposure times.

Luminescent patterns based on color centers generated in lithium fluoride by extreme ultraviolet radiation and soft X-rays

Primary and aggregate color centers in lithium fluoride (LiF) crystals and polycrystalline LiF films were produced by an innovative irradiation technique using extreme ultraviolet radiation and soft X-rays generated by a laser-plasma source. This irradiation facility allowed the efficient formation of active color centers on luminescent patterns with submicron spatial resolution on large areas and short exposure times. The method looks promising for the realization of low-dimensionality photonic devices. The optical characterization of the colored structures was performed by means of absorption and photoluminescence measurements on LiF samples colored under different irradiation conditions.

Lumen: A highly versatile spectrophotometer for measuring the transmittance throughout very long samples as well as microstructures

Commercial spectrophotometers are basically designed to measure the absorbance of solutions contained in transparent cuvettes. Although their functions can be generally extended by means of a variety of accessories, these instruments are generally not flexible and do not fit solid samples thicker than some centimeters. This paper describes a highly versatile spectrophotometer, named Lumen, composed of discrete components and suitably arranged on a large breadboard. Two noticeable instrument configurations are discussed: the first allows to measure the transmittance through long samples, like crystals of lengths up to 23 cm; the second was developed to perform the same measurement in microstructures a few tens of micrometers wide, like those obtained in insulating materials by focused radiation beams. The data acquisition software, written in LabVIEW, adopts some original solutions that make Lumen unique.

Concentration quenching of the emission of F3 + and F2 color centers in LiF

Abstract In this work the emission efficiencies of F3 + and F2 centers as a function of their concentration have been investigated in LiF crystals colored at RT with 5 MeV electrons. From a careful investigation of their absorption and emission spectra at 80 K and RT, we evaluated that the emission efficiencies η of both F3 + and F2 defects are constant up to ∼ 1016 centers/cm3. When increasing the concentration up to 2 × 1018 centers/cm3, η decreases slightly for the F2 centers and considerably more, down to a factor of 100 at the maximum concentration, for the F3 + ones. The same results have been obtained at 80 K for the F2 centers, while the concentration quenching of F3 + centers is 10 times less intense with respect to RT.

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.