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Electron-beam lithography has been used to define color center stripes of about one hundred micrometers length and variable width (100 nm to 5 μm) in lithium fluoride. These structures have for the first time been illustrated and spectrally characterized in near field optical microscopy (SNOM) operating in local illumination mode with an optical fiber probe and far field fluorescence detection.

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.

Research activities concerned with color centers in alkali halide films started recently. The use of versatile, well-assessed, and low-cost fabrication techniques consisting of physical vapor deposition of Lithium Fluoride (LiF) films combined with direct writing lithographic processes allows the realization of miniaturized structures, like broad-band emitters, channel waveguides, optical microcavities and point-light sources emitting in the visible spectral range. Promising results have been obtained in the generation, amplification and waveguiding of visible light in LiF treated by low energy electron beams, where the efficient formation of stable primary and aggregate color centers also induces a local modification of the refractive index. A brief overview of the investigated optical properties is presented together with a short discussion about their perspectives of applications in optoelectronics.

In this work the emission efficiencies of F 3 + + and F 2 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 F 3 + and F 2 defects are constant up to ∼10 16 centers/cm 3 . When increasing the concentration up to 2 x 10 18 centers/cm 3 , η decreases slightly for the F 2 centers and considerably more, down to a factor of 100 at the maximum concentration, for the F 3 + ones. The same results have been obtained at 80 K for the F 2 centers, while the concentration quenching of F 3 + centers is 10 times less intense with respect to RT.

Among alkali halide crystals, lithium fluoride is of particular interest because of its good physical properties and the thermal stability of several kinds of colour centres, which are laser active in the visible and in the near infrared at room temperature. In particular F+ 3 and F2 centres, which emit with high efficiency in the green and red spectral regions, possess almost the same absorption band. This coincidence, added to unusual optical properties of the F+ 3 centre, prompted careful studies of the optical cycle of these centres. Measurements at liquid-helium temperature (LHeT) have further clarified the role of a triplet state which acts as a trap for a sizeable fraction of excited F+ 3 centres. The non-radiative transitions connecting singlet and triplet states are now much better known and a comprehensive theoretical explanation is within reach.

Optical absorption and photoluminescence spectra of KF crystals, doped with small concentrations of Na impurities and additively colored, are measured in order to determine thermally activated processes between different excited states of the FA centers. The mechanism of transformation of FA centers from type I (at low temperature) to type II (at higher temperatures) is fully confirmed by the anticorrelated temperature dependences of the intensities of the two FA(Na) luminescences peaking at 900 and 2150 nm.

Alkali halide crystals with intrinsic color centers are suitable media for solid state tunable lasers, and among them LiF is one of the most popular because of its good physical and optical qualities. The F3 + centers in this host crystal have absorption and emission properties with unusual time evolution effects after excitation. In order to clarify this particular optical cycle we investigated a γ-irradiated LiF crystal containing, besides F centers, mainly F3 + centers. The behaviour of emission and absorption as a function of temperature and pumping power confirms the existence of a triplet state.

Policrystalline LiF thin films were produced on amorphous substrates at different temperatures. Preliminary optical measurements on F2 and F3 + aggregate color centres produced by electron irradiation were performed.

The configuration of F A centres in KF:Na+ crystals has been studied by means of spectroscopic measurements. The F A (Na) dipoles are found to lie along the crystal axes, and this result is interpreted as the consequence of an on-centre position of the Na+ ions in the lattice of KF, in good agreement with theoretical expectations.