Centers In LiF Crystals Research Articles

Simulation of the dynamics of single F3+ color centers in LiF crystals

Simulation of the dynamics of single F3+ color centers in LiF crystals

Influence of Elevated Temperature on Color Centers in LiF Crystals and Their Photoluminescence

The radiation-induced photoluminescence (PL) of LiF has found its way into many applications for the detection and imaging of ionizing radiation. In this work, the influence of thermal treatment at temperatures up to 400 °C on absorption and PL emission spectra as well as fluorescent nuclear tracks in irradiated LiF crystals was investigated. It was found that carrying out PL measurements with the crystals kept at the temperature of about 80 °C leads to a considerable increase in luminescence emission of F3+ color centers at 525 nm. This enhancement of PL intensity allows for the microscopic imaging of the fluorescent nuclear tracks using only F3+ emission, which is not possible at room temperature. It was also found that heating the irradiated crystals before measurement at temperatures from 100 °C to 200 °C increases the concentration of F3+ centers. However, the related enhancement of PL emission is insufficient in terms of enabling the observation of the fluorescent tracks in this part of the spectrum. In the case of the main PL emission at 670 nm related to F2 centers, the thermal treatment at around 290 °C substantially increases the intensity of fluorescent tracks. This effect, however, was found to occur only at low fluences of alpha particles (up to about 109 cm-2); therefore, it is barely visible in the emission spectrum and not noticeable in the absorption spectrum.

Transformation of Radiation-Induced Molecular Point Defects and Color Centers in LiF Crystals under Influence of Light

The influences of UV light and integral light pulses from lamps on γ-irradiated LiF crystals with impurities of hydroxyl and magnesium ions have been investigated. The optical absorption and luminescence of color centers in different bleaching stages are studied depending on the crystal exposure time under UV light or on the number of light pulses. IR spectra of molecular hydrogen-bonded complexes and hydroxyl ions in different bleaching stages are presented. The transformation of strong hydrogen-bonded complexes under illumination into complexes with weak hydrogen bond (and vice versa) is studied. It is shown how molecular hydrogen-bonded complexes involving hydroxyl affect the transformation of color centers during the crystal bleaching.

Visible photoluminescence of color centers in LiF crystals for advanced diagnostics of 18 and 27 MeV proton beams

Solid-state radiation detectors based on the visible photoluminescence of radiation-induced F2 and F3+ color centers in lithium fluoride crystals have been used for advanced diagnostics of 18 and 27 MeV proton beams, produced by the TOP-IMPLART linear accelerator, which is under development for protontherapy applications at ENEA C.R. Frascati, Italy. Visible fluorescence microscopy was successfully used to read the latent proton beam images stored in the lithium fluoride crystals, thanks to high emission efficiency of the F2 and F3+ color centers obtained by simultaneous optical excitation in the blue spectral range. High intrinsic spatial resolution and wide dynamic range of these novel LiF detectors allow obtaining two-dimensional images of both the beam transverse intensity distribution and of the Bragg curve. By using a simple defect formation model that takes into account the energy released in the material and the saturation of defect concentration, not only the two-dimensional dose map was obtained from the beam transverse intensity distribution image, but also the full Bragg curve was reconstructed, allowing the thorough characterization of proton beams produced by the accelerator.

Depth profiles of aggregate centers and nanodefects in LiF crystals irradiated with 34 MeV 84Kr, 56 MeV 40Ar and 12 MeV 12C ions

Depth profiles of nanohardness and photoluminescence of F2 and F3+ centers in LiF crystals irradiated with 12 MeV 12C, 56 MeV 40Ar and 34 MeV 84Kr ions at fluences 1010–1015 ions/cm2 have been studied using laser scanning confocal microscopy, dislocation etching and nanoindentation techniques. The room temperature irradiation experiments were performed at DC-60 cyclotron (Astana, Kazakhstan). It was found that the luminescence intensity profiles of aggregate color centers at low ion fluences correlate with electronic stopping profiles. The maximum intensity of aggregate center luminescence is observed at fluence around 1013 ions/cm2 and rapidly decreases with further increase of fluence. At the highest ion fluences, the luminescence signal is registered in the end-of-range area only. The depth profiles of nanohardness and chemical etching have shown remarkable ion-induced formation of dislocations and increase of hardness which in the major part of the ion range correlate with the depth profile of electronic energy loss. An exception is the end-of-range region where strong contribution of nuclear energy loss to hardening at high fluences is observed.

Photoluminescence of magnesium-associated color centers in LiF crystals implanted with magnesium ions

In the present paper, the effect of magnesium nanoparticles implanted in a LiF crystal on the optical properties of color centers is studied. The transmittance spectra and AFM images demonstrate effective formation of the color centers and magnesium nanoparticles in an implanted layer of ∼ 60–100 nm in thickness. Under thermal annealing, a periodical structure is formed on the surface of the crystal and in the implanted layer due to self-organization of the magnesium nanoparticles. Upon excitation by argon laser with a wavelength of 488 nm at 5 K, in a LiF crystal, implanted with magnesium ions as well as in heavily γ-irradiated LiF: Mg crystals, luminescence of the color centers at λmax = 640 nm with a zero-phonon line at 601.5 nm is observed. The interaction of magnesium nanoparticles and luminescing color centers in a layer implanted with magnesium ions has been revealed. It is shown that the luminescence intensity of the implanted layer at a wavelength of 640 nm is by more than two thousand times higher than that of a heavily γ-irradiated LiF: Mg crystal. The broadening of the zero-phonon line at 601.5 nm in the spectrum of the implanted layer indicates the interaction of the emitting quantum system with local field of the surface plasmons of magnesium nanoparticles. The focus of this work is to further optimize the processing parameters in a way to result in luminescence great enhancement of color centers by magnesium nanoparticles in LiF.

Fluorescent detection of single tracks of alpha particles using lithium fluoride crystals

Lithium fluoride single crystals were successfully used for fluorescent imaging of single tracks of alpha particles. This was realized with a standard wide-field fluorescent microscope equipped with a 100× objective. Alpha particles create F2 and F3+ color centers in LiF crystals. The subsequent illumination with the blue light (wavelength around 445nm), excites these centers and produces fluorescence with a broad band peaked at 670nm. The observed tracks of alpha particles have diameter of about 500nm. Focusing of the microscope at different depths in a LiF crystal, enables imaging changes of shape and position of tracks, allowing for visualization of their paths. These encouraging results are the first step towards practical application of LiF as fluorescent nuclear track detectors.

Multiple filamentation of femtosecond laser pulses

The formation of fluorescent channels with color centers in LiF crystals under the action of the multiple filamentation of femtosecond laser pulses is studied experimentally and theoretically for pulse powers around four orders of magnitude higher than the critical self-focusing value.

Study of the fluorescence blinking behavior of single F2 color centers in LiF crystal

Using confocal fluorescence microscopy technique, we observed experimentally the luminescence of single F2 color centers in LiF crystal. It is disclosed that the fluorescence shows blinking behavior. It is shown that this phenomenon is caused by the F2 center reorientation occurring during the experiment. The ratio of luminescence intensities of differently oriented centers is assessed theoretically for two different experiment configurations. The calculated ratios are in fine agreement with experimental result.

Generation of color centers when annealing LiF crystals irradiated by Kr ions with an energy of 150 MeV

The effect thermal annealing has on F and Fn centers in LiF crystals irradiated by Kr ions with an energy of 150 MeV is studied with allowance for fluence and ionic currents. It is found that annealing at a temperature of 400 K using crystals irradiated at a fluence of ≥1013 iom/cm2 reduces the concentration of F centers (due to annihilation with H centers) and raises the concentration of complex Fn centers.

LiF crystals irradiated with 150 MeV Kr ions: Peculiarities of color center creation and thermal annealing

Color centers in LiF crystals are studied under irradiation at room temperature with 150MeV Kr ions in the fluence (Φ) range of 1010–1014ions/cm2 with a beam current density of 10, 50, and 100nA/cm2, corresponding to flux of 4.46×109, 2.23×1010 and 4.46×1010ions/(cm2×s), respectively. At Φ⩾3×1012ions/cm2 besides F and Fn centers also charged F3+ centers are created.Thermal annealing of irradiated LiF crystals with Φ⩾1013ions/cm2 at 400K leads to a decrease of F centers (due to annihilation with H centers) and an enhancement of complex Fn color centers (neutral and charged) due to interaction with thermally activated anion vacancies. Annealing LiF crystals at higher temperature (T⩾400K) leads to a decrease both of F centers and Fn centers. In the annealing process up to 480K the F2 and F3+ centers are dominating until their reduction above 590K. At temperatures above 740K mainly small Li and trace element (Mg, Na) colloids are present in the absorption spectrum. The role of anion vacancies on color center creation and annealing is discussed.

Effect of irradiation parameters on defect aggregation during thermal annealing of LiF irradiated with swift ions and electrons

Absorption spectroscopy were performed to study the effects of thermal annealing on the aggregation of color centers in LiF crystals irradiated with different ions between carbon and uranium of megaelectron volt-gigaelectron volt energy. The beam parameters such as energy, energy loss, and fluence have a pronounced influence on the initial defect composition and concentration as well as their evolution upon thermal annealing. A distinct phenomenon was observed, viz., the enhancement of ${\text{F}}_{n}$ centers for annealing temperatures between 500 and 700 K, followed by Li colloid formation above 700 K. The phenomenon requires specific irradiation conditions whereas the formation of Mg colloids from Mg impurities occurs in all irradiated crystals. The mechanisms of annealing and colloid formation are discussed.

Modulation frequency doubling in the axially periodic dependence of the luminescence of F 3 + centers in LiF crystals

A spatially periodic distribution of the luminescence intensity of F2 and F3+ color centers is observed in LiF crystals with induced anisotropy. The period of spatially periodic modulation of the luminescence intensity generally coincides with the period of the change in the polarization state of excitation light. For F2 centers, the experimental axial spatially periodic dependence of the luminescence intensity is close to the calculated dependence obtained in the description of the absorption and radiative transitions in the centers by linear oscillators oriented along six C2 axes of the cubic crystal. However, for F3+ centers, the same experiment shows a spatially periodic distribution of luminescence intensity with a modulation period shorter by a factor of 2. It is shown that this effect of modulation frequency doubling in the spatially periodic pattern of the luminescence of color centers arises due to the nonlinear dependence of the luminescence intensity on the excitation light intensity. For F3+ centers, upon luminescence excitation in the absorption band with λmax = 452 nm, the modulation frequency doubling is due to the saturation of the metastable triplet state of such a center.

Femtosecond-laser-encoded distributed-feedback color center laser in lithium fluoride single crystal

Focused infrared femtosecond laser pulses (wavelength ∼800nm, emission pulse duration 100fs) were employed to fabricate optoelectronic devices such as waveguides, micro-gratings and laser active centers in LiF crystals. F2 color centers of about 2×1018cm−3 and refractive index change of about 1% at 633nm were induced by the fs-laser irradiation. This technique was applied to fabricate a distributed-feedback (DFB) F2 color center laser structure inside LiF single crystal. The LiF DFB laser exhibited laser oscillation at 707nm at room temperature. The slope efficiency of ∼10% and beam divergence of ∼20mrad were achieved.

Space-selective laser function defects induced by ultra-fast intense laser in LiF crystal

In an attempt to form space-selective color centers in LiF crystal, photon-induced color centers were continuously created by focusing an ultra-fast intense laser through a microscope objective and translating the LiF crystal sample perpendicular to the axis of the laser beam. The resulting stable color centers region was induced inside the LiF crystal along the path traversed with the focal point of the laser. The measured absorption spectra of color center regions express that it is possible to form space-selective lasing color centers in LiF crystal by the irradiation of ultra-fast intense laser.

Transversely pumped laser using F 3+ color centers in LiF crystal at room temperature

A very stable LiF:F 3 + color center laser at room temperature using a transversely pumped cavity has been constructed. The laser is pumped both with a pulse dye laser and the harmonic light of a Ti:sapphire laser. The optical–optical conversion efficiency is about 3.5%. The maximum single pulse output of the laser is more than 0.3 mJ with a divergence of about 2.5 mrad. The laser output extends from 525 to 550 nm and peaks at 540 nm. The laser operates very stably for several hours under the present pump conditions at room temperature without using any cooling equipment.

Energy transfer among color centers in LiF crystals

In colored LiF crystals there are many absorption and emission bands which cover a wide region of the spectrum from 200 up to 1 300 nm without breaking the continuity. In the frame of these favourable conditions we have performed some experiments of energy transfer among various color centers. The results indicate the existence of efficient exchanges of radiative energy among several bands by using only one exciting wavelength. In particular, the emissions of the F3- and F2- color centers centered at 900 and 1 100 nm, respectively, have been observed by pumping at 672 nm completely outside their absorption bands.

Two-photon polarization spectroscopy of F3+ and F2 color centers in LiF crystals

We identified energy levels, and their corresponding symmetries, of F3+ and F2 color centers in LiF crystals, using the Feofilov method of polarized luminescence as applied to two-photon excitation. For this purpose, we measured the azimuthal dependencies of the polarization degree, P(ϕ), and the intensity, I(ϕ), of the F3+ and F2 color center luminescence in LiF crystals under two-photon excitation. To increase the reliability of the results we measured the polarization dependencies of three LiF crystal plates cut parallel to the three main crystallographic planes (100), (110) and (111), respectively. The measured polarization dependencies were seen to be in good agreement with the theoretical polarization dependencies derived for P(ϕ) and I(ϕ). We established that the new energy level of the F2 center has the same parity as the ground state. Consequently, the one-photon electric-dipole transitions are forbidden between the states. The symmetry of the new electronic state of the F3+center was found to be 1A1.

Laser oscillation by the F2+ and F2− color centers in LiF crystals

A free-running mode LiF:F2+ color center laser oscillation is achieved at room temperature in an electron-beam-irradiated LiF crystal with the F2 color centers but no F2+ ones, by producing the F2+ defects through the two-photon excitation of the F2 centers and pumping the F2+ defects with the same wavelength. Simultaneous laser oscillation by the F2 center is obtained at the initial stage of the F2+ lasing but it disappears 100 s after the pumping. The LiF:F2+ laser operating at 930 nm is used to pump an F2− color center laser based on a different electron-beam-irradiated LiF crystal with high concentration of the F2− centers. A free-running F2− pulse laser oscillation with a peak at 1145 nm and a half width of 400 cm−1 is achieved.

Emission decay times of F 3+ and F 2 color centers in LiF crystals

F 3 + and F 2 centers in LiF display efficient broad emissions in the green–red spectral region when excited in their absorption bands located around 450 nm. Recently it has been shown that color centers, impurities and low-dimensionality structures in heavily colored samples influence the optical cycles and in particular the lifetimes of the emitting states. In order to determine these effects we measured the decay times, which are reported in the literature with values scattered by more than a factor 4. We obtained the radiative lifetime τ 0=(8.1±1.2) ns for F 3 + and τ 0=(15.5±0.8) ns for F 2 centers and we did not observe any temperature dependence in the range 15–300 K.