CaF2 Single Crystals Research Articles

Influence of water on the machinability of calcium fluoride crystal

Calcium fluoride (CaF2) optical components are produced by ultra-precision machining, but the often-neglected ambient water may potentially affect its processing. This work provides a systematic study on the role of water in the machinability of single-crystal CaF2. Micro-scratching experiments revealed poor machinability and reduced deformability under water adsorption. Atomic scratching simulations reproduced the mechanical deformation and identified the enhancement in dislocation nucleation with water, which led to the reduction in deformability due to dense cross-linking of dislocations. Atomic scale cutting simulations also identified easier formation of subsurface cracks under water adsorption. With the aid of mechanical schematic models based on the simplification of the scratching process, this work revealed that while water enhances the plastic flow of the material during machining, the accumulation of dislocations creates stress conditions sufficient to induce material fracture, which is a unique influence of water adsorption on the reduction in machinability of CaF2.

Structures and Properties of High-Concentration Doped Th:CaF2 Single Crystals for Solid-State Nuclear Clock Materials.

Thorium-doped vacuum ultraviolet (VUV) transparent crystals is a promising candidate for establishing a solid-state nuclear clock. Here, we report the research results on high-concentration doping of 232Th:CaF2 single crystals. The structures, defects, and VUV transmittance performances of highly doped Th:CaF2 crystals are investigated by theoretical and experimental methods. The defect configurations formed by Th and the charge compensation mechanism (Ca vacancy or interstitial F atoms) located at its first nearest neighbor position are mainly considered and studied. The preferred defect configuration is identified according to the doping concentration dependence of structural changes caused by the defects and the formation energies of the defects at different Ca or F chemical potentials. The cultivated Th:CaF2 crystals maintain considerable high VUV transmittance levels while accommodating high doping concentrations, showcasing an exceptional comprehensive performance. The transmittances of 1-mm-thick samples with doping concentrations of 1.91 × 1020 and 2.76 × 1020 cm-3 can reach ∼62% and 53% at 150 nm, respectively. The VUV transmittance exhibits a weak negative doping concentration dependence. The system factors that may cause distortion and additional deterioration of the VUV transmittance are discussed. Balancing and controlling the impacts of various factors will be of great significance for fully exploiting the advantages of Th:CaF2 and other Th-doped crystals for a solid-state nuclear optical clock.

Relative Strength of Polycation Adsorption on Oxide Surfaces.

Polyelectrolyte adsorption onto surfaces is widely employed in water treatment and mining. However, little is known of the relative interaction strengths between surfaces and polymer. This fundamental property is assumed to be dominated by electrostatics, i.e., attractive interactions between opposite charges, which are set by the overall ionic strength ("salt concentration") of the solution, and charge densities of the surface and the polymer. A common, counterintuitive finding is a range of salt concentrations over which the amount of adsorbed polyelectrolyte increases as electrostatic interactions are tempered by the addition of salt. After an adsorption maximum, higher salt concentrations then produce the expected gradual desorption of polyelectrolyte. In this work, the salt response of the adsorption of the same narrow molecular weight distribution polycation, poly(N-methyl-4-vinylpyridinium), PM4VP, to a variety of surfaces was explored. Oxide powders for adsorption included Al2O3, SiO2, Fe2O3, Fe3O4, TiO2, ZnO, and CuO. Planar surfaces included silicon wafers, mica, calcium carbonate, and CaF2 single crystals. The PM4VP was radiolabeled with 14C so that sensitive, submonolayer amounts could be detected. The position of the peak maximum, or the lack of a peak, in response to added salt was used to rank the electrostatic component of the interaction. The importance of charge regulation, a shift in the surface pKa in response to solution species, was highlighted as a mechanism for adsorption on the "wrong" side of the isoelectric point and also as a factor contributing to the difficulty of reaching the totally desorbed state even at the highest salt concentrations.

Investigation of visible emission and luminescence center evolution in Pr–La:CaF2 single crystals for laser application

The study presents Pr,La:CaF2 single crystals grown using the temperature gradient technique, showing distinct Pr3+–La3+ cluster types, with two types of centers showing cubic symmetry.

Comparative study of optical and dosimetric properties of Mn-doped CaF2

Mn-doped CaF2 has been used as a thermoluminescence detector. On the other hand, dosimetric properties of Mn-doped CaF2 transparent ceramics have not been thoroughly investigated. In this study, transparent ceramics (TC) of Mn-doped CaF2 were fabricated, and the optical and thermally stimulated luminescence (TSL) properties were investigated in comparison with Mn-doped CaF2 single crystal (SC) and opaque ceramics (Cer). The transmittance at 500 nm of the TC, SC, and Cer specimens was 58, 91, and 0%, respectively. All the specimens showed a PL peak at 490 nm with 340, 400, and 450 nm excitations, originating from d-d transitions of Mn2+ ions. The emission and excitation spectra changed with X-ray irradiation, and this phenomenon was attributed to optically stimulated luminescence. All the specimens mainly had a TSL glow peak at 300 °C. The TC specimen exhibited a higher TSL intensity than the SC and Cer specimens.

Effects of temperature on factors relevant to laser operation near 1.6 µm wavelength in resonantly (in-band) pumped CaF2:Er and SrF2:Er crystals: a comparative study

Single crystals of CaF2 and SrF2 containing Er3+ ions with concentrations between 0.01 and 0.6 at% respectively, were grown using the temperature gradient technique (TGT). Absorption spectra related to the 4I15/2 → 4I13/2 transition of Er3+ ions were measured at 5 K to investigate the properties of the crystal field splitting of the 4I13/2 multiplet, and at several temperatures in the region 80 K – 300 K to determine the effect of temperature on the spectral bands. The 4I13/2 → 4I15/2 luminescence spectra were recorded within the same temperature range as for absorption measurement. The analysis of absorption and luminescence spectra provides an in-depth knowledge on the effect of temperature on the factors that govern the efficiency of resonant optical pumping near 1.6 µm and the resulting lasing ability of Er3+ ions imbedded in CaF2 and SrF2 hosts.

Insight into neodymium clusters and spectroscopic properties in CaF2 single crystals

In this work, the relationship between local structures and the photoluminescence parameters of neodymium activated fluorite crystals has been preliminary investigated in CaF2 crystals. The first principles calculation results reveal that as the size of codoped rare earth ions become smaller, the corresponding stable clusters twists progressively from cubic sublattice to square antiprism. The regulated spectral line properties and the concentration dependent photoluminescence parameters, including the absorption, emission and decays, agree well with the calculated results. In detail, the cubic stable clusters of Nd3+,La3+:CaF2 and Nd3+,Gd3+:CaF2 crystals possess similar spectral characteristics, while the other category of Nd3+,Y3+:CaF2 and Nd3+,Lu3+:CaF2 resembles both in the square antiprism cluster structures and spectra. Besides, a crystal with ultra long lifetime near 600 µs, wide emission bandwidth and high thermal conductivity has been obtained, which is promising for generation of the LD-pumped ultrafast lasers. By connecting the enriched spectral properties with local structures, it is highly expected to design new laser materials through regulating the local structures of active ions in rare earth doped fluorite crystals.

Crystalline Orientation of CaF2 window determined by Neutron Transmission Imaging using a Delay Line Current-Biased Kinetic-Inductance Detector

The combination of a high speed two-dimensional neutron detector and an intensive pulsed neutron source provides not only neutron transmission imaging but also information on crystal structures, orientations and constituent elements by analyzing neutron transmission spectra. The delay-line current-biased kinetic-inductance detector (CB-KID) is a two-dimensional superconducting neutron detector with high spatial and temporal resolutions, and multi-hit tolerance. We demonstrated that the delay-line CB-KID with a 10B neutron conversion layer can be applied for neutron transmission measurements up to 100 keV. We observed two-different dip structures in the transmission spectra in a CaF2 single crystal, i.e., one is from the Bragg diffraction and the other is from nuclear resonance absorption. We consider that some duplicated structures in resonance absorption dips are originating from a double-bunch structure of proton pulses for producing pulsed neutrons in the accelerator.

Surface characterization of CaF2 crystals irradiated with MeV ions below charge state equilibrium

Single crystals of CaF2, cleaved along the (111) plane, were irradiated by 35Cl, 39K, 52Cr, 58Ni, 79Br and 197Au ions with energies from 2 to 48 MeV, charge states from 1+ to 11+ and ion fluences from 2 × 1010 to 1 × 1013 ions/cm2. The surface properties of the irradiated samples were studied using Atomic Force Microscopy in contact mode. Nanoscale surface structures were observed for some surfaces irradiated by Cl, K, Cr, Ni, Br and Au ions. Thresholds for nanostructure formation were found in terms of projectile ion velocity and electronic stopping power (Se). Clear surface modification was observed for projectiles with specific energies above approximately 0.04 MeV/u. The threshold in Se for the generation of distinct triangular structures was observed to be velocity-dependent. The average formation efficiency, referring to the ratio of the number of distinct nanostructures observed on the surface to the number of incident ions, as well as the average height of nanostructures increases with Se.

Investigation of the temperature effect on cutting of calcium fluoride single crystal using molecular dynamics simulation

Thermal assisted machining has been demonstrated to enhance the machinability of single-crystal CaF2 but the underlying mechanisms of the thermal effect on the mechanical behavior of the material remain unclear. Therefore, molecular dynamics was employed in this research to study the deformation process during machining from the perspective of dislocation activity. The results illustrate the enhanced plasticity with longer dislocation line lengths and the reduction in cutting forces at an elevated temperature. Micro-cutting experiments confirm the simulated results by the delay in ductile–brittle transition by a magnitude of 1.34.

∼2.9 μm continuous-wavelength laser operation of fiber-pumped Ho,Pr:CaF2 single crystals

Ho,Pr:CaF2 single crystals were grown by modified temperature gradient technique method (TGT). A systematic study on the fluorescence characteristics around 2.9 μm has been carried out on this kind of materials for the first time, to the best of our knowledge. After co-doping with Pr3+, the population inversion is much easier to realize and the ratio of populations between the upper and lower energy levels is highest among the reported materials, which shows the Ho,Pr co-doped CaF2 is a kind of promising materials. The continuous-wavelength (CW) laser operation has been achieved with the maximum output power of 154 mW on 1%Ho,0.1%Pr:CaF2 crystals. It's the first report about the CW laser output on Ho-doped CaF2 single crystals.

Proton irradiation-induced defect aggregation and metallic nanoparticles in CaF2 single-crystal

Proton irradiation induced-defects aggregation and metallic nanoparticles in single-crystal CaF2 was investigated by X ray diffraction (XRD), Raman spectroscopy and UV–Vis spectrophotometry. The structural reports reveal the degradation of material crystallinity after irradiation without amorphization, as a result of generated anion point defects (F- and H-center pairs). Particularly, Raman analysis reveals the colloids formation and even the increase of their size with increasing energy. While confirming this result, the optical measurements show an anticorrelation between the concentration of F-centers and Ca colloids with increasing proton fluence. The blue shift of the colloid band, observed at higher energies, indicates that larger colloids are formed at deeper penetration, resulting in the development of a compressive stress in material.

Effect of point defects on the STE luminescence of CaF2 single crystals

The study is devoted to the radiation defects created in the calcium fluoride single crystals with reactor irradiation to the fluence of 3.7 × 1017 thermal and 4.6 × 1017 fast (En > 1 MeV) neutrons per cm2, and their effect on the luminescence of the crystals. UV–VIS absorption spectroscopy revealed the formation of point defects, mainly F and F2 centers. From the comparison of the annealing behavior of the absorption and the ion-beam-induced luminescence (IBIL), we conclude that the color centers are responsible for a non-radiation decay of self-trapped excitons (STEs) and thus cause the luminescence quenching. In addition, the Raman analysis revealed a disorder of about 44% and a stress formation in the material after the neutron irradiation. Annealing at 100 °C reduces the disorder to 13% and eliminates the stress.

Effect of Yb[formula omitted] concentration on Er[formula omitted] doped CaF[formula omitted] single crystal for temperature sensor applications

Yb–Er co-doped CaF2 crystals with different Yb3+ ions concentrations are successfully grown by the temperature gradient technique (TGT) method. Up-conversion (UC) luminescence characteristics and optical temperature sensing behavior of all samples are thoroughly investigated in this work. Under the excitation of a 980 nm laser-diode, the up-conversion luminescence intensity firstly increases and then decreases. By adjusting the content of Yb3+ ions, multi-color visible light emissions of red, yellow and green could be easily obtained. The fluorescence intensity ratio from two thermally coupled 2H11/2 and 4S3/2 levels technology is used to evaluate the temperature sensing performance of Yb–Er co-doped CaF2 crystals. The concentration of Yb3+ ions shows a significant influence on the temperature sensing performance. The sensitivity reaches the maximum value of 2.24 × 10−3 K−1 at 548 K with a 5 at.% doping concentration of Yb3+ ions. The experiment also confirms that Yb–Er co-doped CaF2 crystal exhibit high stability as optical temperature sensors. In general, it is proved that Yb–Er​ co-doped CaF2 single crystals are a kind of promising materials used in the optical temperature sensor.

KrF excimer laser induced damage and its mechanism of CaF2 single crystal with (1 1 1), (1 1 0) and (1 0 0) planes

CaF2 crystal generally has a fluorite-type crystal structure, in which the cleavage plane is {111} and the slip system is {100} 〈110〉. The mechanical properties of different crystal planes are obviously different. In this investigation, 248 nm KrF excimer laser damage experiments were carried out on three different crystal planes (100), (110) and (111) of CaF2 crystals. The laser induced damage threshold was obtained, and the relationship between crystal plane and damage characteristics was analyzed. Our work highlights were that by analyzing the relationship among crystal plane, cleavage plane and slip plane, the formation mechanism of surface morphology of (111), (110) and (100) crystal planes were summarized.

A Square Pulse Thermoreflectance Technique for the Measurement of Thermal Properties

We report on a laser-based square pulse thermoreflectance (SPTR) technique for the measurement of thermal properties for a wide range of materials. SPTR adopts the pump-probe thermoreflectance principle to monitor the evolution of local temperature after square pulse excitation. The technique features a compact setup, high spatial resolution, and fast data collection. By comparing the acquired SPTR signals with a continuum heat transfer model, material thermal properties can be obtained. Taking advantage of various spot sizes and modulation frequencies, SPTR can measure both the thermal diffusivity and thermal conductivity of poorly to moderately conductive materials and the thermal conductivity of conductive materials with satisfactory accuracy, with potential to be applied to more conductive materials. The technique was validated on three materials: fused silica, single crystal CaF2 and single crystal nickel (with conductivities ranging from 1 W·m−1·K−1 to 100 W·m−1·K−1) with typical measurement errors of 5 % to 20 %. The leading sources of error have been identified by Monte Carlo simulations, and the primary limitations of SPTR are discussed. The compact, fiberized platform we describe here will allow instruments based on this methodology to be deployed in complex, multi-analytical environments for the type of high-throughput correlative analyses that are key to materials design and discovery.

Investigation of the deposition of calcium fluoride nanoparticles on the chips of CaF2 single crystals

The deposition of calcium fluoride nanoparticles on single crystal chips of calcium fluoride was studied. CaF2 nanoparticles were synthesized by co-precipitation from aqueous nitrate solutions using hydrofluoric acid as a fluorinating agent at a batch system. The prepared samples were examined by atomic force microscopy, scanning electron microscopy, transmission electron microscopy and optical transmission. There is an inhomogeneous coating of the substrate surface with submicron particles of about 100–150 nm in size, which are clusters of nanoparticles of 15-20 nm in size. The initial nanoparticles coherently grow on the surface of the crystal substrate. Heat treatment of the substrate-deposited layer composite at 600 °C leads to the coalescence of submicron particles and the formation of a porous layer of a complex structure.

Electronic ionisation-induced annealing of pre-existing defects in Al2O3 and CaF2 single crystals

ABSTRACT Electronic ionisation-induced annealing of pre-existing defects in CaF2 and Al2O3 single crystals has been investigated. Pre-existing defects (vacancies and interstitials) and defect aggregates were first induced in both crystals by fast neutrons irradiation and then irradiated by 90 MeV Xe ions. After irradiation, the samples were characterized using Doppler Broadened Emission Spectroscopy (DBS). The S parameter and average S values obtained for the neutrons irradiated samples are compared with results obtained for samples irradiated by neutrons + Xe ions. Results of these experiments revealed ionisation-induced pre-existing defect annealing in Al2O3 while generating more defects in CaF2. It is also found that the defect recovery rate in Al2O3 crystal depends on the Xe ions fluence that confirms the different mechanisms of defect formation in both materials. The electronic ionisation and excitation energy are transferred to the defects formation in the self-trapped exciton (STE) mechanism resulting in defect formation in CaF2. Whereas in the case of Al2O3, it is transferred to electron–phonon coupling leading to lattice heating, which causes the annealing of the pre-existing defects.

Theoretical approach on the critical depth of cut of single crystal MgF2 and application to a microcavity

Optical microcavity, which can localize light at a certain spot for a short period of time, has a wide range of applications, such as optical signal processing and optical frequency combs. Single crystal CaF2 has excellent optical properties and many research on the manufacturing process to create a microcavity has been done. However, due to its unstable thermal conditions, stable oscillation of Kerr comb could not be done. Single crystal MgF2, which also has excellent optical properties, has stable thermal conditions suitable to create a microcavity. Like CaF2, MgF2 microcavity can only be manufactured by ultra-precision cylindrical turning, yet its appropriate processing conditions is not known. In this study, a method to derive the critical depth of cut theoretically in ductile mode machining was proposed in consideration of specific cutting energy, mechanical properties and slip system of workpiece, and tool geometry. The critical depth of cut calculated by the proposed method roughly agreed with the experimental value on the MgF2 plane. Unlike CaF2, MgF2 has an optical anisotropy; therefore, in order to create a microcavity where the refractive angle is constant, the resonance part needs to be made on a cylinder with (001) plane as end face. Therefore, a microcavity was fabricated according to the critical depth of cut on a cylinder with (001) as end face derived from the proposed method, and the surface integrity was evaluated from the viewpoint of microcavity performance.

Characterization of surface and subsurface defects induced by abrasive machining of optical crystals using grazing incidence X-ray diffraction and molecular dynamics

IntroductionSurface and subsurface defects were easily induced during abrasive machining process of optical crystals due to their high brittleness. Accurate characterization of these defects is the prerequisite for obtaining optical components with high surface integrity. ObjectivesThis work aims to evaluate subsurface defects of CaF2 single crystals induced by abrasive machining, the mappings between micro cracks and diffraction pattern curves, and the influence of micro cracks on lattice structures. MethodsMolecular dynamics simulation, grazing incidence X-ray diffraction experiments and cross-sectional TEM detection were used in this work. ResultsIn grazing incidence X-ray diffraction (GIXD) detection experiments on lapping and polished specimens, shifts phenomenon of the peak position under the non-theoretical grazing incidence angle indicated that the subsurface was damaged to a certain extent. The micro cracks of the subsurface were evaluated by the consistent characteristic of “peak drift” of the diffraction pattern curve in both experiments and simulations. In addition, cross-sectional TEM results showed that regular micro cracks were found on the subsurface, which agreed well with the simulation results. ConclusionThe subsurface defects of CaF2 single crystals induced by abrasive machining, and the influence of micro cracks on lattice structures can be evaluated by molecular dynamics simulation. The simulation results revealed the mappings between the micro cracks and diffraction pattern curve, which demonstrated that a phenomenon of “peak drift” occurred near the diffraction angle of a specific crystal plane. This work provided a novel technology for the nondestructive testing of defects of single crystal materials at nano- and micro- scales.