YAG Single Crystals Research Articles

Deformation mechanism and force modelling of the grinding of YAG single crystals

YAG single crystals are the primary host materials for solid-state lasers at multi-kW scale and must be processed using ultra-precision grinding to achieve a satisfactory dimensional precision and surface integrity. However, the deformation mechanism of YAG crystals is not well understood, which has thus hindered the development of high efficiency grinding technology for the crystals. In this work, precision grinding of YAG single crystals was investigated. Ductile-like surfaces that are free of cracks and brittle-ductile surfaces that consist of fractured spots and ductile striations were found after grinding. The deformation mechanisms associated with the two types of surfaces were explored with the aid of transmission electron microscopy (TEM). The results indicated that the deformation involved in the formation of the ductile-like surface was mainly caused by the slippage of (0 0 1) crystal planes, along with the formation of dislocations and stacking faults and the distortion of atomic planes. The brittle-ductile surfaces were generated by the plastic deformation due to the formation of nanocrystals and nanovoids, combined with brittle fracture caused by the crack propagation initiated at intersections of slip lines. A theoretical model was developed to predict the grinding force in the ductile-like grinding process, which has taken the combined effect of strain rate, random distribution of abrasive radii and elastic-to-plastic transition depth into account for the first time. The key model parameters were obtained using a genetic algorithm trained using the experimental force data. The modelled force agrees well with the measured. This model enabled an in-depth understanding of the deformation mechanism of a crystal solid involved in ultraprecision grinding and the effect of strain rate on its material removal.

Investigation of laser-induced luminescence saturation in a single-crystal YAG:Ce phosphor: Towards unique architecture, high saturation threshold, and high-brightness laser-driven white lighting

A significant obstacle in the development of phosphor-converted white-laser diodes (pc-wLDs) is associated with the laser-induced luminescence saturation of phosphors. YAG:Ce is a canonical yellow-emitting phosphor. However, there has been few systematic research of measurement and understanding of the luminescence saturation in YAG:Ce yet. Here, we analyze the luminescence saturation of a single-crystal YAG:Ce with a unique phosphor geometry. A novel characterization method, which enables the optical signals to be fully collected and analyzed, is introduced. The effects of both laser power and laser power density on the luminescence properties and colorimetric properties of the YAG:Ce are studied systematically. The results show that a single-crystal YAG:Ce has a high saturation threshold (over 360 W/mm2) so that only a very slight saturation behavior can be observed at a high power density. Furthermore, a pc-wLDs light source based on the combination of a single laser diode and a YAG:Ce phosphor exhibits remarkable efficiency and stability. Under the excitation of a 3.38 W blue laser (∼100 W/mm2), the white light emission with the luminous flux of 465 lm, the luminous efficacy (LE) of 145 lm/W, and the correlated color temperature (CCT) of 4980 K can be achieved.

Interstitial Li+ Controls the UV Transmission and the Radiation Hardness in YAG

Optical absorption spectra measured in as‐grown and gamma‐ray irradiated un‐doped YAG, Li‐doped YAG, and Ca‐doped YAG single crystals are compared for characterization of effects introduced by impurities. The studies are conducted on single crystals grown by the vertical Bridgman method. Basing on the Li+ incorporation mechanism, clarification is done regarding the optimal amounts of Li+ for preparation of YAG:Li crystals with low concentration of anion vacancies and related defects giving rise to absorption in the UV range. Differences in behavior are recorded in variously doped crystals subjected to gamma‐ray irradiation; in comparison to un‐doped YAG, the induced absorption coefficient in Li‐doped YAG of optimal composition is lower more than six times. High transmission in the UV and high radiation tolerance are important for most applications of YAG, especially when operated in high radiation fields.

Influence of annealing atmosphere on the optical quality and pulsed laser performance of Ce, Nd:YAG single crystal

Hydrogen, nitrogen and air atmospheres were used for annealing of Ce, Nd:YAG rods cut from single crystal grown by Czochralski method in nitrogen atmosphere. The optical quality of the rods was studied in terms of the peak to valley deviation of wave front in number of fringes per inch, extinction ratio and lateral scattering loss coefficient. Furthermore, the pulse laser performance of the rods was compared together after annealing in different atmospheres. Better optical quality and laser performance is achieved in H2 annealed laser rod and results proved that annealing in hydrogen atmosphere is more desirable than nitrogen and air.

Prospective scintillation electron detectors for S(T)EM based on garnet film scintillators.

The performance of a scintillation electron detector for a scanning electron microscope and/or a scanning transmission electron microscope (S(T)EM) based on new epitaxial garnet film scintillators was explored. The LuGAGG:Ce and LuGAGG:Ce,Mg film scintillators with chemical formula (Ce0.01 Lu0.27 Gd0.74 )3-w Mgw (Ga2.48 Al2.46 )O12 were prepared and their cathodoluminescence (CL) and optical properties were studied and compared with the properties of current standard bulk single crystal YAG:Ce and YAP:Ce scintillators. More specifically, CL decay characteristics, CL emission spectra, CL intensities, optical absorption coefficients, and the refractive indices of the mentioned scintillators were measured. Furthermore, electron interaction volumes with absorbed energy distributions, photomultiplier (PMT) photocathode matchings, modulation transfer functions (MTF), and the photon transport efficiencies of scintillation detectors with the mentioned scintillators were calculated. A CL decay time for the LuGAGG:Ce,Mg film scintillator as low as 28 ns with an afterglow of only 0.02% at 1 μs after the e-beam excitation was observed. As determined from calculated MTFs, the scintillation detectors with the new film scintillators lose contrast transfer ability above 0.6 lp/pixel, while the currently commonly used YAG:Ce single crystal scintillators already do so above 0.1 lp/pixel. It was also calculated that the new studied film scintillators have an 8% higher photon transfer efficiency, even for a simple disk shape compared with the standard bulk single crystal YAG:Ce scintillator. The studied LuGAGG:Ce,Mg epitaxial garnet film scintillators were evaluated as prospective fast scintillators for electron detectors, not only in S(T)EM but also in other e-beam devices.

The Growth of Ce,Nd:YAG Single Crystal Under New Atmosphere Condition

Ce3+,Nd3+:YAG single crystal was grown by Czochralski method without using any reduction gas. The growth was done in the atmosphere of pure nitrogen. The limited-flow of nitrogen gas was used for evacuating the produced oxygen in order to change Ce4+ to Ce3+ ions. The X-ray diffraction, absorption and luminescence spectrums were obtained from the sample of the grown crystal. The obtained results were discussed and compared with previously published data. The lattice parameter a = 12.001 A was determined by the X-ray powder diffraction data. Distribution of the cores in a disc sample was studied. There was only one small central core in the sample.

Optical characteristics of Ce,Eu:YAG single crystal grown by Czochralski method

Eu-doped Ce:YAG single crystal (SC) was fabricated by the Czochralski method. The crystal structure, optical and photoelectric property of the constructed w-LED was investigated. The XRD and HRTEM results show that YAG crystal structure has little change when Eu3+, Ce3+ are doped. Absorption spectra and photoluminescence spectra show that the Ce,Eu:YAG single crystal can effectively absorb the 460 nm blue light to form a broadband emission center at 530 nm, decay curves of the crystal show that the energy transfer between Ce3+ and Eu3+ is highly suppressed. Compared with commercial Ce:YAG phosphor, Ce,Eu:YAG exhibits better thermal stability.

Origin and Future of Polycrystalline Ceramic Lasers

Since highly efficient laser oscillation by Nd:YAG ceramics has been reported, various types of transparent ceramics have been developed and also power scaling has been performed. Theoretical clarification on why polycrystalline ceramics having numerous structural defects (grain boundaries) could generate highly efficient laser oscillation is very important, and it will be a road map for the development of polycrystalline laser gain media for the future. The sintering aid was useful for reducing optical loss of ceramics but further improvements in performance were obtained by the development of a laser gain medium without using sintering aids and as a result a slope efficiency of 74% with an output power of 1.8 kW was obtained. This material had 5.5 kW output as a maximum and its power density exceeded 1 MW/cm3. Forming composite gain media by ceramic technology is very important for further power scaling and sophistication of laser functionality. In this report, advantages of bonding technology to form composite gain media are described. Also, by removing the grain boundary completely from the ceramic material, high quality single crystal can be synthesized. The slope efficiency of laser oscillation from 2.4% Nd:YAG polycrystal and single crystal showed high values of 57% and 63%, respectively.

Chemical Mechanical Polishing and Direct Bonding of YAG

Current limitations in both single crystal and polycrystalline (ceramic) solid state laser technologies for high power applications stem from thermal effects that cause degradation in both lasing efficiency and beam quality. YAG and Y2O3 have favorable material properties for producing these high power lasers. We demonstrate chemical mechanical polishing (CMP) processes for YAG and Y2O3 resulting in smooth surfaces, (< 1 nm RMS roughness), defect free, and subsequently suitable for direct bonding. The CMP process has been used in conjunction with surface activation to form single crystal YAG-YAG and polycrystalline Y2O3-Y2O3 bonded elements showing a proof of concept for the fabrication of composite laser elements. YAG single crystals were successfully polished with a 70-nm colloidal silica solution containing NaOH (pH 9.9). X-ray photoelectron spectroscopy measurements provided insights to the chemical impact of the NaOH. After NaOH exposure, YAG showed complete removal of Al from tetrahedral sites and a change in the Y3/2 and Y5/2 peak area ratios, indicating that the surface of YAG was sufficiently modified to allow the silica particles to abrade the byproduct surface layer. The final surface roughness after polishing was measured at 0.1 nm RMS roughness with no scratches deeper than 0.3 nm. YAG single crystals were also polished with a 70-nm Al2O3 slurry containing NaOCl (pH 11.4), however AFM measurements showed that the surface produced, 0.5 nm RMS roughness with no scratches deeper than 6.0 nm, was not as good as polishing with the colloidal silica slurry which was attributed to the effect of the chemical action of NaOH with the YAG surface. Polishing polycrystalline Y2O3 required a slurry with less aggressive chemical reactions, so the NaOCl/Al2O3 slurry was used to successfully polish the substrates to RMS roughness values of 0.5 nm and scratches no deeper than 1.0 nm. Triple axis diffraction rocking curves and double crystal x-ray diffraction imaging were used to demonstrate that the CMP process also removed subsurface damage that was present in the as-supplied material. The triple axis technique was also successfully employed for the first time to demonstrate that the polycrystalline Y2O3 subsurface damage was also significant reduced after the CMP process. After CMP and surface treatment, YAG-YAG and Y2O3-Y2O3 were bonded together and annealed at 1425 °C with no applied pressure. Cross section and plan view high resolution transmission electron images of the YAG-YAG bonded interface revealed a defect free bonding interface. Light transmission measurements showed that 90% of the interface area was bonded at room temperature contact and was further strengthened with annealing at 1425 °C for 72 hours. These conditions possess a much lower thermal budget than those previously shown to be required to bond YAG without a CMP step. The CMP and surface treatment processes that have been developed are applicable towards the creation of solid state laser composite elements and enable further progress and development for high power laser applications.

Electronic excitation energy transfer processes in Er:YAG under variable pump duration

For the first time, the luminescence decay dependences of 4I13/2 and 4I11/2 energy levels on pulsed excitation duration are studied with and without pinhole technique in erbium-doped YAG single crystals. Luminescence decay investigations of 4I13/2 → 4I15/2 (1565 nm) and 4I11/2 → 4I15/2 (970 nm) transitions are carried out on 5 at.% and 40 at.% doped samples. Time-resolved measurements of luminescence decay have demonstrated non-exponential behavior depending on excitation duration as well as impurity concentration, thus, indicating the presence of various energy transfer processes. In order to eliminate the influence of radiation trapping effect on luminescence decay from 4I13/2 and 4I11/2 levels the pinhole technique is used. Qualitative comparisons of luminescence decay curves are made to estimate the influence of radiation trapping effect. The influence of non-radiative energy transfer processes on population redistribution of 4I13/2 and 4I11/2 energy levels is evaluated. A theoretical model for non-radiative transfer of electronic excitation energy in YAG:Er3+ crystals is proposed, in the frame of which the constants of rate equations are determined. The experimental and theoretical investigations are made for luminescence corresponding to both 4I13/2 → 4I15/2 and 4I11/2 → 4I15/2 transitions. The following non-radiative energy transfer processes are taken into account: 4I13/2 → 4I15/2 + 4I13/2 → 4I9/2, 4I11/2 → 4I15/2 + 4I11/2 → 4F7/2, 4S3/2(2H11/2)→4I9/2 + 4I15/2 → 4I13/2, 4I11/2 → 4I15/2 + 4I13/2 → 4F9/2.

Single fiber temperature probe configuration using anti-Stokes luminescence from Cr:GdAlO3

Single-photon excitation of anti-Stokes-shifted emission from a thermographic phosphor allows operation of a luminescence decay-based single fiber temperature probe with negligible interference from background fiber-generated Raman scattering. While single fiber probe configurations for luminescence-based fiber optic thermometers offer advantages of simple design, compactness, and superior emission light collection efficiency, their effective use has been limited by interference from Raman scattering in the fiber probe and excitation delivery fiber that produces distortion of the luminescence decay that follows the excitation pulse. The near elimination of interference by background fiber-generated Raman scattering was demonstrated by incorporating a Cr-doped GdAlO3 (Cr:GdAlO3) thermographic phosphor as the sensing element at the end of a single fiber luminescence decay-based thermometer and detecting anti-Stokes-shifted luminescence centered at 542 or 593 nm produced by 695 nm excitation. Measurements were performed using both silica (up to 1150 °C) and single-crystal YAG (up to 1200 °C) fiber-based thermometers. Selection of emission detection centered at 542 nm greatly benefited the YAG fiber probe measurements by practically eliminating detection of otherwise significant luminescence from Cr3+ impurities in the YAG fiber. For both the silica and YAG fiber probes, the relative benefit of adopting single-photon excitation of anti-Stokes-shifted luminescence was evaluated by comparison with results obtained by conventional 532 nm excitation of Stokes-shifted luminescence.

Red-emitting carbon dots phosphors: a promising red color convertor toward warm white light emitting diodes

Various luminescent carbon materials are artificially synthesized and as admirable fluorescent materials because of the increased research of carbon nanodots (CDs). Herein, a novel solvent-dependent red CDs (rCDs) is introduced through solvothermal method. The as-prepared rCDs remain benzene structure and possess abundant surface function groups that endow them well solubility in various solvents. Furthermore, multicolor luminescence are observed when rCDs are dissolved in different solvents, and the emission wavelength of these materials can be well-tuned from 475 to 624 nm, which presents intensity solvent-dependent properties. Red luminescence carbon phosphors are successfully synthesized by mix rCDs with silica powder. Finally, warm white light emitting diodes (WLEDs) are constructed using rCDs powder with Ce3+: YAG single crystal and blue GaN chips. The results demonstrate that the rCDs can act as red component to modify the correlated color temperature (CCT) and the color rendering index (CRI) of WLEDs .

Pressureless glass crystallization of transparent yttrium aluminum garnet-based nanoceramics

Transparent crystalline yttrium aluminum garnet (YAG; Y3Al5O12) is a dominant host material used in phosphors, scintillators, and solid state lasers. However, YAG single crystals and transparent ceramics face several technological limitations including complex, time-consuming, and costly synthetic approaches. Here we report facile elaboration of transparent YAG-based ceramics by pressureless nano-crystallization of Y2O3–Al2O3 bulk glasses. The resulting ceramics present a nanostructuration composed of YAG nanocrystals (77 wt%) separated by small Al2O3 crystalline domains (23 wt%). The hardness of these YAG-Al2O3 nanoceramics is 10% higher than that of YAG single crystals. When doped by Ce3+, the YAG-Al2O3 ceramics show a 87.5% quantum efficiency. The combination of these mechanical and optical properties, coupled with their simple, economical, and innovative preparation method, could drive the development of technologically relevant materials with potential applications in wide optical fields such as scintillators, lenses, gem stones, and phosphor converters in high-power white-light LED and laser diode.

Composite Laser Ceramics by Advanced Bonding Technology.

Composites obtained by bonding materials with the same crystal structure and different chemical compositions can create new functions that do not exist in conventional concepts. We have succeeded in bonding polycrystalline YAG and Nd:YAG ceramics without any interstices at the bonding interface, and the bonding state of this composite was at the atomic level, similar to the grain boundary structure in ceramics. The mechanical strength of the bonded composite reached 278 MPa, which was not less than the strength of each host material (269 and 255 MPa). Thermal conductivity of the composite was 12.3 W/mK (theoretical value) which is intermediate between the thermal conductivities of YAG and Nd:YAG (14.1 and 10.2 W/mK, respectively). Light scattering cannot be detected at the bonding interface of the ceramic composite by laser tomography. Since the scattering coefficients of the monolithic material and the composite material formed by bonding up to 15 layers of the same materials were both 0.10%/cm, there was no occurrence of light scattering due to the bonding. In addition, it was not detected that the optical distortion and non-uniformity of the refractive index variation were caused by the bonding. An excitation light source (LD = 808 nm) was collimated to 200 μm and irradiated into a commercial 1% Nd:YAG single crystal, but fracture damage occurred at a low damage threshold of 80 kW/cm2. On the other hand, the same test was conducted on the bonded interface of 1% Nd:YAG-YAG composite ceramics fabricated in this study, but it was not damaged until the excitation density reached 127 kW/cm2. 0.6% Nd:YAG-YAG composite ceramics showed high damage resistance (up to 223 kW/cm2). It was concluded that composites formed by bonding polycrystalline ceramics are ideal in terms of thermo-mechanical and optical properties.

Plasmonic nanoparticles embedded in single crystals synthesized by gold ion implantation for enhanced optical nonlinearity and efficient Q-switched lasing

We report on the synthesis of embedded gold (Au) nanoparticles (NPs) in Nd:YAG single crystals using ion implantation and subsequent thermal annealing. Both linear and nonlinear absorption of the Nd:YAG crystals have been enhanced significantly due to the embedded Au NPs, which is induced by the surface plasmon resonance (SPR) effect in the visible light wavelength band. Particularly, through a typical Z-scan system excited by a femtosecond laser at 515 nm within the SPR band, the nonlinear absorption coefficients of crystals with Au NPs have been observed to be nearly 5 orders of magnitude larger than that without Au NPs. This giant enhancement of nonlinear absorption properties is correlated with the saturable absorption (SA) effect, which is the basis of passive Q-switching or mode-locking for pulsed laser generation. In addition, the linear and nonlinear absorption enhancement could be tailored by varying the fluence of implanted Au+ ions, corresponding to the NP size and concentration modulation. Finally, the Nd:YAG wafer with embedded Au NPs has been applied as a saturable absorber in a Pr:LuLiF4 crystal laser cavity, and efficient pulsed laser generation at 639 nm has been realized, which presents superior performance to the MoS2 saturable absorber based system. This work opens an avenue to enhance and modulate the nonlinearities of dielectrics by embedding plasmonic Au NPs for efficient pulsed laser operation.

YAG/Nd:LuAG composite laser materials prepared by the ceramization of YAG single crystals

Composite YAG/Nd:LuAG laser materials were prepared by the ceramization of YAG single crystals from the original bonding interfaces of green bodies (optical contacted YAG crystals and Nd:LuAG ceramics). And the morphology, microstructural characteristics and optical properties of the composite materials were studied. The results indicated that Nd:LuAG ceramics and YAG crystals were completely combined without bonding interfaces. They were pore-free at the original contacting interfaces of the YAG/Nd:LuAG transparent materials and the in-line transmittance of the specimens reached 84% at 1064 nm. This suggests that ceramization of YAG single crystals is a promising way to fabricate composite YAG/Nd:LuAG transparent materials.

Spectra and Kinetics of YAG:0.5%Er3+ Radiation Under Pulsed Laser Thermal Excitation

The dependences of single-crystal YAG:0.5%Er3+ spectra in the range 200–1700 nm and the kinetics of the integrated emission on the excitation-energy density W of a single CO2-laser pulse of ~100 ns FWHM duration at λex = 10.6 μm were investigated experimentally. The spectra changed qualitatively when W exceeded the generation threshold Wt of laser plasma near the single-crystal surface. The YAG:0.5%Er3+ spectra at W Wt. Oscillograms of the integral emission in the range 600–1700 nm were characterized by a pulse-front duration of τf = 1.0–1.6 μs and a decay time τd ≈ 2.5 μs if W changed by one order of magnitude. The kinetics of the integrated emission intensity were explained by calculating the single-crystal temperature and the populations of Er3+ energy levels during conversion of thermal energy into SE during unsteady energy transfer by internal radiation and thermal conductivity.

Scandium doped Tm:YAG ceramics and single crystals: Coherent and high resolution spectroscopy

We report a detailed study about the effect of Scandium (Sc3+) doping on the 3H6-3H4 transition in Tm3+:Y3Al5O12 ceramics at cryogenic temperature. Inhomogeneous, homogeneous linewidths and Zeeman lifetime as a function of the Sc3+ codoping are presented and compared to values obtained in single crystals. We demonstrate that Sc3+ can be used to increase the inhomogeneous broadening without being detrimental to coherence properties and spectral hole persistence. This controlled disorder therefore provides larger bandwidth for signal processing applications such as spectral analyzers and quantum information processing. Moreover, we show that opaque ceramics prepared by simple solid-state reaction exhibit a long coherence lifetime and even narrower inhomogeneous linewidths than single crystals. Opaque ceramics are very attractive for material development because they offer ease and low cost of fabrication and enable the study of a larger range of composition compared to single crystals. However we show that the manufacturing and the initial stoichiometry of the ceramics have a significant impact on the inhomogeneous broadening.

Phase transformation under direct laser writing in a YAG single crystal

The transformation from Y3Al5O12 to perovskite YAlO3 crystal phase was observed during ultrafast laser writing in the bulk of a Y3Al5O12 single crystal. The control of the phase transformation was demonstrated by tuning parameters of laser writing. The phenomenon is interpreted in terms of structural changes in the overheated garnet melt under laser heating followed by rapid solidification, and at least 1-ms dwell time of overheated melt is required to start spontaneous crystallization. The appearance of the perovskite phase was confirmed by Raman spectroscopy, quantitative phase and polarization microscopies.

Preparation, characterization and optical properties of Dy-doped yttrium aluminum garnet

The Y[Formula: see text]Dy[Formula: see text]Al5O[Formula: see text] (Dy:YAG) ceramic rods were sintered at different temperatures from 1000[Formula: see text]C to 1450[Formula: see text]C by solid-state reaction method and the Dy:YAG single crystals were grown by the optical floating zone method. The XRD found the Y4Al2O9 (YAM) phase after sintering at 1000[Formula: see text]C and disappeared after sintering at 1450[Formula: see text]C. The YAG phase and the YAlO3 (YAP) phase were found in the ceramic sintered at [Formula: see text]1200[Formula: see text]C. As the sintering temperature increases, the amount of the YAG phase increases, while YAP phase decreases. There is only a single YAG phase in the Dy:YAG single crystal. The SEM images showed that the grain size in the Dy:YAG ceramic increases with the sintering temperature. There are several emission bands in the photoluminescence spectrum of the Dy:YAG single crystal located at 483 nm in the blue region, 580 nm in yellow region and 670 nm in red region. Dy:YAG is a good candidate crystal for white light emission.