R. A. Nunes

Electron-beam production of colour centres on alkali halide crystals and films

Abstract In this article we analyze the production of localized colour centres on the surface of alkali halide crystals and films produced by electron beam radiations in the 10–30 keV range. It was found that the colour centre production is restricted to electron penetration (3–10 μm depending on the material and electron energy) with a very intense local optical density (above 10 4 cm −1 ). As the refractive index variation calculated using the Kramers-Kronig relations from the observed absorption bands should be enough for light confinement (10 −3 –10 −2 ), electron beam generated colour centres should allow the production of light wave guides for several applications. Optical absorption bands can be created in ionic crystals anywhere from the ultraviolet to the infrared, preserving transparent regions of the spectrum. With an appropriate computer aided design (CAD) program and a microcomputer interface to the adapted micropositioning mechanical devices and shutter, it was possible to control an electron microprobe sample holder in order to draw precise patterns and have various controlled exposure times.

Intensity quenching of the F3+ colour centre emission in lithium fluoride

In order to evaluate the possibility of tunable F2/F3+ CW colour centre laser operation in LiF crystals at room temperature, the authors performed an investigation of the emission characteristics of gamma -irradiated LiF crystals under CW argon ion laser excitation at room temperature. They measured a strong quenching of the F3+ emission intensity that was dependent on the pumping laser power. The characteristic decay times of the F3+ luminescence bands were also investigated with different pumping powers and a model for the recombination kinetics is discussed.

Simultaneous Measurements of Chlorophyll Concentration by Lidar, Fluorometry, above-Water Radiometry, and Ocean Color MODIS Images in the Southwestern Atlantic

Comparisons between in situ measurements of surface chlorophyll-a concentration (CHL) and ocean color remote sensing estimates were conducted during an oceanographic cruise on the Brazilian Southeastern continental shelf and slope, Southwestern South Atlantic. In situ values were based on fluorometry, above-water radiometry and lidar fluorosensor. Three empirical algorithms were used to estimate CHL from radiometric measurements: Ocean Chlorophyll 3 bands (OC3MRAD), Ocean Chlorophyll 4 bands (OC4v4RAD), and Ocean Chlorophyll 2 bands (OC2v4RAD). The satellite estimates of CHL were derived from data collected by the MODerate-resolution Imaging Spectroradiometer (MODIS) with a nominal 1.1 km resolution at nadir. Three algorithms were used to estimate chlorophyll concentrations from MODIS data: one empirical - OC3MSAT, and two semi-analytical - Garver, Siegel, Maritorena version 01 (GSM01SAT), and CarderSAT. In the present work, MODIS, lidar and in situ above-water radiometry and fluorometry are briefly described and the estimated values of chlorophyll retrieved by these techniques are compared. The chlorophyll concentration in the study area was in the range 0.01 to 0.2 mg/m3. In general, the empirical algorithms applied to the in situ radiometric and satellite data showed a tendency to overestimate CHL with a mean difference between estimated and measured values of as much as 0.17 mg/m3 (OC2v4RAD). The semi-analytical GSM01 algorithm applied to MODIS data performed better (rmse 0.28, rmse-L 0.08, mean diff. -0.01 mg/m3) than the Carder and the empirical OC3M algorithms (rmse 1.14 and 0.36, rmse-L 0.34 and 0.11, mean diff. 0.17 and 0.02 mg/m3, respectively). We find that rmsd values between MODIS relative to the in situ radiometric measurements are < 26%, i.e., there is a trend towards overestimation of RRS by MODIS for the stations considered in this work. Other authors have already reported over and under estimation of MODIS remotely sensed reflectance due to several errors in the bio-optical algorithm performance, in the satellite sensor calibration, and in the atmospheric-correction algorithm.

Grazing incidence X-ray diffraction analysis of alkali fluoride thin films for optical devices

Abstract Alkali fluoride multilayered thin films are promising systems for different optical applications, including light generation and waveguide. Despite this, some of their physical properties are still not well known. The mechanical behaviour of these materials, when evaporated in the form of thin films, plays a fundamental role for the success of the related optoelectronic device. In this work, LiF, NaF and LiF/NaF polycrystalline thin films were deposited onto different substrates at temperatures varying from 50 to 200°C by physical vapour deposition (PVD). Using the X-ray diffraction technique with grazing incidence geometry (GIXRD), the residual stresses and the microstrains present in films deposited in different experimental conditions were measured. The lattice parameters and the mean domain were also estimated. The results show the dependence of the residual stresses in the films upon the cooling rate and the type of substrates and give, for the first time, an order of magnitude of these quantities for such systems.

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.

Investigation on the color center distribution in LiF thin films

The F2 center depth distribution in LiF thin films irradiated by 15 and 30 keV electron beams was obtained experimentally by means of optical absorption measurements of multilayered LiF/KBr films. Monte Carlo simulations of the incidence of electron beams in LiF were also carried out to provide the dissipated energy and the fluor ionization depth profiles for comparison with the experimental data. The ionization and the dissipated energy profiles have essentially the same shape, with their maxima at a depth of 41±5% of the electron range. The F2 distributions could be described by Gaussian distributions centered at depths of 1.5 and 1.6 μm for 15 and 30 keV irradiation, respectively.The F2 center depth distribution in LiF thin films irradiated by 15 and 30 keV electron beams was obtained experimentally by means of optical absorption measurements of multilayered LiF/KBr films. Monte Carlo simulations of the incidence of electron beams in LiF were also carried out to provide the dissipated energy and the fluor ionization depth profiles for comparison with the experimental data. The ionization and the dissipated energy profiles have essentially the same shape, with their maxima at a depth of 41±5% of the electron range. The F2 distributions could be described by Gaussian distributions centered at depths of 1.5 and 1.6 μm for 15 and 30 keV irradiation, respectively.

Color centers photomasks produced by electron-beam lithography

In this paper we discuss the feasibility of using direct electron beam lithography in the production of a new type of photomask suitable for photolithography with both the g mercury line and KrF deep UV lasers. The masking effect is produced by the presence of molecular type defects - color centers - which show strong absorption bands in the visible and deep UV. The centers are created in a superficial layer of Lithium Fluoride (LiF) crystals by direct electron beam irradiation. A simple mask is presented and the photostability of the centers when submitted to various visible and UV photo-transposition steps is studied. The visible absorption band can withstand ~ 1000 exposures while the UV band can be used - 50 times. The use of this material reduces the number of necessary steps in photomask fabrication, eliminating the need for chemical procedures.

Characterization of color centers production in polycristalline alkali halide thin films

Electron irradiation of several alkali halides films was investigated by Monte Carlo simulations of the trajectories of the electrons in these materials. The main features of the distribution of the beam’s dissipated energy in these materials were obtained. An indication of a correlation between the F2 depth distribution and the dissipated energy was observed.