F. Menchini

Spontaneous emission properties of color centers based optical microcavities

Abstract Optical microcavities based on lithium fluoride films treated by low-energy electrons to create visible-emitting F2 color centers have been fabricated, and their radiative properties characterized for the first time. By tuning the photon cavity mode to the maximum of the luminescence band for the F2 centers (∼670 nm), spectral narrowing, peak-intensity enhancement of the emission band as well as a highly directional radiation pattern have been observed comparatively for experiments performed on a half-cavity and a full microcavity. Spontaneous emission decay times have been measured, and a shortening of lifetime by the cavity has been observed.

Fluorescence imaging of submicrometric lattices of colour centres in LiF by an apertureless scanning near-field optical microscope

We report fluorescence imaging of colour centres in Lithium Fluoride (LiF) using an apertureless Scanning Near Field Optical Microscope (SNOM). The sample consists of periodically spaced submicrometric coloured areas F2 laser-active colour centres produced by low-energy electron beam lithography on the surface of a LiF thin film. A silicon Atomic Force Microscope (AFM) tip is used as an apertureless optical probe. AFM images show a uniform surface roughness with a RMS of 7.2 nm. The SNOM images of the red fluorescence of colour centres excited at lambda = 458 nm with an argon ion laser show that the local photon emission is unambiguously related to the coloured areas and that topographic artefacts can be excluded.

Selective production of aggregate centers in LiF crystals by ionizing radiations

Abstract Optical absorption and photoluminescence spectra have been measured in LiF crystals colored for the first time with 5 MeV electrons. The measurements show that this kind of irradiation reduces (much more than γ-rays) the production of complex aggregate defects with respect to that of F + 3 and F 2 centers. Moreover, the laser active F + 3 centers can be preferentially created in place of F 2 centers by decreasing the irradiation temperature.

Behavior of optical thin-film materials and coatings under proton and gamma irradiation

Optical materials and coatings are exposed to the flux of energetic particles when used in either space applications or nuclear energy plants. The study of their behavior in such an environment is important to avoid failure of the optical components during their operation. The optical performance of several thin-film materials ((HfO2, Ta2O5, Nb2O5, TiO2, SiO2) and coatings, under irradiation with high-dose gamma rays (5.8 MGy) and exposure to low-energy (60 keV) protons, has been investigated. Some variations of optical properties have been detected in silicon oxide after irradiation, while the other materials are stable in such conditions.

High concentrations of aggregate colour centres in heavily irradiated LiF crystals

Abstract The stable formation of several colour centres (CCs) has been investigated in lithium fluoride (LiF) single crystals irradiated at room temperature (RT) and at 213 K by 5 MeV electrons with doses from 1019 to 1023 eV/cm3. The temperature during irradiation influences the production of aggregate defects, in particular the ratio between the F3+ and F2 laser active centres and the amount of parasitic complex defects. Optical absorption and photoluminescence spectra allow clarifying the role of different aggregate defects on the emission properties of the F3+ and F2 centres at concentrations up to 1018 cm−3.

Control of F2 color centers spontaneous emission in LiF thin films inside optical microcavities

Abstract An experimental investigation on spontaneous emission properties of Fabry–Perot microcavities consisting of quarter-wavelength ZnS/Cryolite Bragg mirrors and a lithium fluoride (LiF) thin film colored with low-energy electron beam lithography is reported for the first time. Angular-resolved photoluminescence measurements show the modifications of F2 color centers visible spontaneous emission. The resonator induces a narrowing of the emission spectrum and a related increase in the directionality and intensity of the emission along the cavity axis.

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.

Effects of F2 color center spatial distribution on the emission properties of optical microcavities based on LiF films

Highly stable F2 color centers are very efficiently produced in lithium fluoride (LiF) by electron beam irradiation at room temperature. We have fabricated optical microcavities in which the active medium is a low-energy electron beam irradiated LiF film, whose optical thickness is comparable with the peak wavelength (∼668nm) of the F2 broad photoluminescence band. By selecting the proper electron beam energy, one can control the F2 color center depth distribution. This distribution influences the photoemission angular distribution of the microcavity, whose resonance properties are determined by the coupling of the depth profile of the defects with the pump electromagnetic field and microcavity modes.

Emission dynamics of heavily coloredLiF crystals

SummaryLiF crystals colored with ionizing radiation have been used in recent times as active materials for pulsed laser emissions in the visible and near infrared region of the electro-magnetic spectrum. Among the various color centers which possess high emission efficiencies, the F3+ center displays a peculiar optical cycle whose properties depend on temperature and pumping intensity. However a few effects discovered recently seem to imply the existence of interactions among the centers, which is a topic of paramount importance for applied and basic research. Preliminary results on samples containing high concentration of color centers have shown new dynamical effects, which in part can be explained in the frame of nonlinear optical processes and in part require a different approach in describing the optical cycle and its parameters.

Concentration Quenching of Photoluminescence The Case of F 3 + and F 2 Color Centers in LiF

The relative emission efficiency of F + 3 and F 2 centers as a function of their concentration has been investigated in LiF crystals colored at room temperature and -60°C with 3 MeV electrons. From a careful study of their absorption and emission spectra, we discovered that the emission efficiency η of both F + 3 and F 2 defects is constant up to ∼10 16 centers/cm 3 . When increasing the concentration up to 10 18 centers/cm 3 , η decreases slightly for the F 2 centers and considerably for the F3 ones, a difference which is bigger at room temperature with respect to liquid nitrogen temperature. A thermally activated process is taking place, where the F 2 center and other aggregated defects play an important role.