Lithium Fluoride Crystals Research Articles

Photo-induced gratings in thin color center layers on lithium fluoride

We study the recording of permanent Bragg gratings on surface-colored lithium fluoride (LiF) crystals by using the interference pattern of a continuous-wave UV argon-ion laser operating at 244 nm. Gratings with spatial periodicity ranging from 400 to 1000 nm are written by using a phase-mask interferometer and are stable for several months after the writing process. Absorption and photoluminescence spectra show the bleaching of primary F and F -aggregate laser-active color centers as a result of the process. Confocal microscopy is used to determine the pitch and the profile of the fluorescent gratings. The UV laser-induced optical bleaching in highly colored LiF ultrathin layers is responsible for the periodic spatial modulation of absorption and photoemission properties that characterize the gratings. In the colored surface layer, a reduction of as much as 50% of the initial color-center-induced refractive-index increase has been estimated in the bleached areas.

The photoluminescence excitation spectra of multicomponent media

With no restrictions on the value of optical density, we derive an equation that describes the photoluminescence excitation spectra of separate components with overlapping absorption bands in a multicomponent medium. Based on the equation obtained, we propose methods for measuring the ratios of absorption optical densities of the components and quantum yields of their luminescence. The applicability of the proposed methods is demonstrated by measuring the characteristics of coumarin and porphyrin dissolved in different proportions in ethanol. We have also measured the quantum yield and luminescence decay time of the radiationinduced color centers F3+ in the lithium fluoride crystal and calculated probabilities of radiative and nonradiative transitions from the first excited singlet state of these centers.

X‐ray microscopy of plant cells by using LiF crystal as a detector

A lithium fluoride (LiF) crystal has been utilized as a new soft X-ray detector to image different biological samples at a high spatial resolution. This new type of image detector for X-ray microscopy has many interesting properties: high resolution (nanometer scale), permanent storage of images, the ability to clear the image and reuse the LiF crystal, and high contrast with greater dynamic range. Cells of the unicellular green algae Chlamydomonas dysosmos and Chlorella sorokiniana, and pollen grains of Olea europea have been used as biological materials for imaging. The biological samples were imaged on LiF crystals by using the soft X-ray contact microscopy and contact micro-radiography techniques. The laser plasma soft X-ray source was generated using a Nd:YAG/Glass laser focused on a solid target. The X-ray energy range for image acquisition was in the water-window spectral range for single shot contact microscopy of very thin biological samples (single cells) and around 1 keV for multishots microradiography. The main aim of this article is to highlight the possibility of using a LiF crystal as a detector for the biological imaging using soft X-ray radiation and to demonstrate its ability to visualize the microstructure within living cells.

Thermal conductivity of γ-irradiated LiF single crystals

The thermal conductivity of γ-irradiated lithium fluoride (LiF) single crystals has been studied using the method of stationary longitudinal heat flux in a temperature range of 50–300 K. An increase in the irradiation dose to 2 × 109 rad is accompanied by a monotonic decrease in the thermal conductivity by 10% at 300 K and by a factor of 10 at 50 K. This increase in the irradiation dose also leads to an increase in the microhardness of LiF crystals from 140 to 222.5 kgf/mm2.

Luminescence excitation spectra and characteristics of luminescence centers with overlapping absorption bands

For an ensemble of different types of luminescence centers with overlapping absorption bands, with no restrictions on the optical densities, we have obtained relations describing the luminescence excitation spectra for each type of center. We consider transformations of the relations in some limiting cases. We suggest a procedure for using the equations obtained to determine the characteristics of the luminescence centers. Some of these procedures have been experimentally implemented in study of intrinsic radiation color centers in lithium fluoride crystals. We have determined the ratios of the luminescence quantum yields for F2 and F3+ color centers, and we have observed that a major role is played by nonradiative transitions in deactivation of the first excited singlet state of F3+ centers.

Velocity correction and refractive index changes for [100] lithium fluoride optical windows under shock compression, recompression, and unloading

Plate impact experiments were conducted to produce two and three step shock wave loadings in [100] ultrapure, lithium fluoride (LiF) crystals to examine the role of loading history on optical window response in laser interferometry measurements. Peak compressive stresses ranged between 5.0 and 17.5 GPa, and the window response was characterized by measuring the difference between the apparent and actual velocities of reflecting surfaces by using a velocity interferometer. In some experimental configurations, this velocity correction was obtained independently from the projectile velocity. Our results show that the velocity correction in [100] lithium fluoride windows can be described in all cases by a single linear relation, Δu=(0.2739±0.0016)u. Because this correction is independent of the loading history, it is applicable to arbitrary loading, which includes ramp-wave or shockless compression. By using the velocity correction and the measured particle and shock velocities, we have also determined the density dependence of the refractive index for [100] lithium fluoride at 532 nm to be n=(1.2769±0.0024)+(0.0443±0.000 82)ρ.

Li colloids formation study induced by reactor neutrons in LiF single crystals

Single crystal of lithium fluoride was irradiated with neutrons at about 40°C. The irradiation was performed with reactor neutrons at fast (E>1MeV) and thermal neutrons fluence of 1.8×1018cm-2 and 3.3×1018cm-2, respectively. The resulting damage was characterised by optical measurements, scanning electron microscopy and X-ray diffraction analysis. The obtained results show large Li colloids of about 0.513μm that are formed after intense reactor neutrons irradiation. Photoluminescence spectra obtained after annealing under argon atmosphere show the F centers aggregation process with increasing annealing temperature. Li colloids intensity increases with increasing annealing temperature and shifts towards higher wavelength (500nm).

Spatial structures based on color centers created by electrons in lithium fluoride crystals: Absorption and luminescence characteristics

We have shown that in spatial structures based on color centers created by electrons in a lithium fluoride crystal, the distances between centers reach 1.6 nm and 3.6 nm for F1 and F2 centers respectively. This suggests considerable potential opportunities for using electron technology to form structures in the crystals with spatial resolution of such an order of magnitude. We measured the decrease in fluorine content on the irradiated surface of the crystal. We found the concentrations of F1, F2, F3+, F3(R2), and F4(N1) centers. We established that the specific characteristics of color center formation by electrons leads to an increase in the efficiency of creation of F3 and F4 centers. We determined the decrease in the average luminescence lifetimes of F2 and F3+ centers as a result of concentration quenching. We observed distortion of the luminescence contour for F2 centers as a result of absorption of its short-wavelength portion by other centers and emission of radiation by the latter in its long-wavelength portion.

Fabrication of Optoelectronic Devices in Lithium Fluoride Crystals by Interfering Femtosecond Laser Pulses

Fine-pitched microgratings either on or beneath surface of bulk lithium fluoride (LiF) are holographically fabricated by interfering with the second harmonic (400 nm) of a mode-locked Ti:sapphire oscillator–amplifier laser. The laser-active F2 and F3+ color centers in LiF are excellent candidates for producing visible laser action from the green-to-red spectral range when excited with a single wavelength. Here a green distributed feedback (DFB) laser action with a narrower oscillating linewidth is demonstrated by utilizing simultaneous formation of the F3+ color centers and waveguide with the microgratings encoded by interference of 400 nm femtosecond laser pulses. In addition, the possibility of a dual-beam DFB laser based on these color centers in LiF is discussed.

Effect of the surface energy state on the dynamics of dislocations in ionic crystals

The effect of the surface state and boundary physicochemical conditions on the starting stresses and dynamics of near-surface dislocations in lithium fluoride and sodium chloride crystals has been investigated by the method of prick dislocation rosettes arising in alkali halide single crystals upon microindentation. It has been shown that crystals with ionic bonds are characterized by a surface energy (potential) barrier that hinders the movement of near-surface dislocations and influences their starting stresses in crystal lattices.

Permanent bleaching of F2 color centers in lithium fluoride under UV light illumination

AbstractPermanent bleaching of F and F2 color centers in gamma irradiated lithium fluoride crystals has been obtained under low power, CW laser illumination at 244 nm. Optical absorption and photoluminescence measurements reveal a selective change in the concentrations of these kinds of electronic defects. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Advanced optical characterization of active micro-strips induced on Lithium Fluoride crystals by a monochromatic soft X-ray beam

Broad-band visible-emitting micro-strips were induced in Lithium Fluoride crystals by direct writing with a monochromatic soft X-ray beam. They were successfully characterized via fluorescence imaging, photoluminescence and optical transmittance. The irradiation gave rise to stable primary and aggregate color centers within a thin surface layer of the crystals. Their volume concentrations were evaluated together with the induced modification of the complex refractive index in the irradiated volume.

Confocal laser scanning microscopy of active color centers based strips induced on LiF by low energy electron beams

A confocal laser scanning microscope has been used in a non-conventional approach to reconstruct the depth distribution of stable, optically active color centers induced at the surface of a lithium fluoride crystal by a low energy electron beam. The formation of F 2 and F 3 + aggregate electronic defects was limited to the penetration depth of the electrons in the material, as obtained from Monte Carlo simulations. This novel measurement technique could be easily used for spatial characterization of luminescent optical waveguides in transparent dielectrics.

Color center conversion by femtosecond pulse laser irradiation in LiF:F_2 crystals

We report the observation of intense spontaneous emission of green light from LiF:F(2):F(3) (+) centers in active channel waveguides generated in lithium fluoride crystals by near-infrared femtosecond laser radiation. While irradiating the crystal at room temperature with 405 nm light from a laser diode, yellow and green emission was seen by the naked eye. Stripe waveguides were fabricated by translating the crystal along the irradiated laser pulse, and their guiding properties and fluorescence spectra at 540 nm demonstrated. This single-step process inducing a waveguide structure offers a good prospect for the development of a waveguide laser in bulk LiF crystals.

Broad band light-emitting nanostructured substrates by ion beam irradiation of lithium fluoride crystals

We present experimental results on simultaneous surface nanostructuring and optical activation of lithium fluoride crystals by 800 eV off-normal Ar + sputtering. Our data demonstrate that the formation of periodic nanostructures is accompanied by the efficient production of stable electronic defects, optically active in the green and red parts of the visible spectra, thus providing the possibility to conceive and fabricate advanced insulating substrates.

Five Simultaneously Q-Switch Mode-Locked Passive Laser Modulators

Five types of passive Q-switched as well as simultaneously Q-switch mode-locked modulators: plastic dye sheets (Kodak 9850 cellulose acetate dye sheets), lithium fluoride crystals containing F2 − color centers (LiF:F2 −), chromium-doped yttrium–aluminum–garnet crystals (Cr4+:YAG), ionic color filter glass (Schott RG1000 color filter glass), and the single crystal semiconductor wafers (GaAs, Fe-doped InP, Zn-doped InP, S-doped InP, etc.) used for modulation of the Nd:hosted(Nd:YAG, Nd:YVO4, and Nd:LSB) lasers were investigated in detail in our research. We also investigated applications of the Q-switch mode-locked pulse train for the development of a higher resolution solid-state laser range finder.

Radiation-induced color centers in the near-surface layer of lithium fluoride crystals

For radiation-induced intrinsic color centers, we show that the concentrations of identical centers and the concentration ratios of different centers are quite different in the near-surface layer and within the interior volume of a lithium fluoride crystal. We have established that these differences also depend on the sign of the difference between the temperature at which the crystal was irradiated with γ photons and the vacancy mobility temperature. We provide an interpretation for the results obtained, based on the structural features of the near-surface layer and the concentration ratio of vacancies and electrons in the layer, serving as the starting components for color center formation. We found that the concentrations of centers change over the course of a few days by tens of percent in the layer “emerging” from the interior onto the surface as a result of cleavage of the crystal. We measured the luminescence lifetimes of F3 (R2) and F4 centers: 6.6 nsec and 11.7 nsec.

Electrolytic coloration of oxygen-doped lithium fluoride crystals

Oxygen-doped lithium fluoride crystals were colored electrolytically by using a pointed cathode and a flat anode at various temperatures and electric field strengths, which mainly benefit appropriate coloration temperatures and electric field strengths as well as anode structure of used electrolysis apparatus. Characteristic spectral bands of O2−-Va+ dipoles and O2−-Va+ dipole complexes and magnesium-related F, M, magnesium-perturbed F2+, oxygen-perturbed F2+, F3− color centers were observed in absorption spectra of the colored crystals. Current-time curves for the electrolytic colorations and their relationships with the electrolytic coloration processes were given. Color-center formation in the electrolytic coloration was explained.

Color-center waveguides in low-energy electron-bombarded lithium fluoride

We employ a differential version of m-line spectroscopy through grating coupling in order to measure the refractive index of low-energy electron irradiated lithium fluoride crystal channel waveguides for broadband emission. Using photoresist films and a holographic setup for grating fabrication, we perform an accurate characterization of bidimensional structures and a direct quantitative comparison between treated and blank materials.

Surface nanostructuring and optical activation of lithium fluoride crystals by ion beam irradiation

We present results on simultaneous nanostructuring and optical activation of lithium fluoride crystals by 800eV off-normal Ar+ sputtering at different ion doses. The samples were studied by atomic force microscopy and optical spectroscopy. After ion irradiation smoothening of the initial random roughness is achieved and well-defined self-organized ripple structures appear, having a mean periodicity of 30nm and a mean height of 3nm. The simultaneous optical activation of the irradiated samples is due to the stable formation of electronic defects with intense photoluminescence in the visible spectral range.