Gadolinium Gallium Garnet Crystals Research Articles

Probability ofYb3+4f−4ftransitions in gadolinium gallium garnet crystals at high hydrostatic pressures

We show that the probability of $f\text{\ensuremath{-}}f$ radiative transitions $(^{2}F_{7∕2}\ensuremath{\leftrightarrow}^{2}F_{5∕2})$ of ${\mathrm{Yb}}^{3+}$ ions in gadolinium gallium garnet (GGG) crystals is sensitive to hydrostatic pressure application. The changes of the transition energy with pressure are very small, almost negligible. However, we observed much more pronounced variations of the transition probability, which are ascribed to a coupling between the $4{f}^{13}$ with the $4{f}^{12}5{d}^{1}$ electronic configurations and variations of the coupling coefficient with pressure. The theoretical model, explaining this behavior, assumes nonmonotonic changes of the odd-parity crystal-field terms associated with lattice distortions induced by pressure. The magnitude of these distortions correlates well with the magnitude of the distortion from the noncubic symmetry of the dodecahedral crystallographic sites in GGG host, monitored by the pressure dependence of the splitting of the $^{4}F_{3∕2}$ level of ${\mathrm{Nd}}^{3+}$ ions.

Laser Performance of Nd:GGG Operating at 938 nm

We report an efficient diode-pumped Nd-doped Gadolinium gallium garnet(GGG) continuous-wave (CW) laser operating at 938 nm. Laser action of1.6at.% Nd-doped GGG crystals with different lengths and temperaturesare also investigated. The maximum output power of 620 mW is obtainedat the incident pump power of 5.0 W with a slope efficiency of 15%.

Study of energy transfer in gadolinium—gallium garnet crystals doped with Yb3+ and Ho3+ ions

The luminescence kinetics of Yb3+ donors and Ho3+ acceptors is quantitatively studied in gallium—gadolinium garnet (GGG) crystals doped with Yb3+ and Ho3+ ions. It is shown that the sensitisation of transitions in Ho3+ ions occurs due to migration-accelerated (hopping) energy transfer. The microparameters of donor—donor energy transfer are determined at 300 and 77 K. The microparameters of donor—acceptor energy transfer are found at the same temperatures at the first stage of successive sensitisation (resulting in the population of the 5I6 state of Ho3+ ions) and at the second stage of successive sensitisation of the 5S2, 5F4→5I8 transition in Ho3+ ions. At the second stage of sensitisation, the values of the microparameter of reverse energy transfer are also determined. The possibility of obtaining lasing at sensitised transitions in Ho3+ ions in Yb3+:Ho3+:GGG crystals pumped into the absorption band of Yb3+ ions is discussed.

Influence of high pressure on the luminescence transitions ofMn4+-doped gadolinium gallium garnet

We present results of a high-pressure photoluminescence study ofMn- and Ca-doped gadolinium gallium garnet (GGG) crystals. Since theMn4+ ion has the same electronic structure of the open3d3 shellas Cr3+, its spectroscopic properties are expected to be similar to those ofthe Cr-doped GGG. We observed the luminescence R-lines of variousMn4+ centres in GGG. In the 0–118 kbar pressure range those R-lines shiftlinearly to lower energies. The pressure coefficients are of the order of2 cm−1 kbar−1, about three timeslarger than that for Cr3+ ions. This fact can be explained by different strength of coupling of isoelectronicCr3+ and Mn4+ ions with GGG host. The changes of the radiative decay times of theMn4+ centres as a function of pressure are fitted with a theoretical model.

Experimental determination of track cross-section in Gd 3Ga 5O 12 and comparison to the inelastic thermal spike model applied to several materials

Single crystals of gadolinium gallium garnet, Gd 3Ga 5O 12 have been irradiated with various swift heavy ions ( 32S, 52Cr, 63Cu, 86Kr, 128Te, 129Xe, 181Ta, 208Pb, 238U) in the electronic stopping power regime. The extent of the induced damage is extracted from channelling Rutherford backscattering experiments and the corresponding track radii are deduced. At low beam energy (around 1.5 MeV/u), the electronic stopping power threshold of damage creation is 7.2 ± 1.2 keV/nm while it is 9.3 ± 0.9 keV/nm for beam energy around 6 MeV/u. The inelastic Thermal Spike model (i-TS) is used in order to calculate the track radii versus (d E/d x) e using λ, the mean diffusion length of the energy deposited on the electrons, as the only fitting parameter model. The i-TS model was extended to some other amorphizable materials like YBa 2Cu 3O 7− δ , GeS and LiNbO 3. The results, combined with previous ones extracted from BaFe 12O 19, Y 3Fe 5O 12, Y 3Al 5O 12 and α-SiO 2 quartz data, showed that λ decreases when the band gap energy E g increases. By extrapolation and depending of the material, the damage threshold induced by electronic excitation can appear at beam energy as low as 10 −3 to 10 −1 MeV/u.

Thermal decay of stable colour centres in Gd3Ga5O12 crystals

This work is devoted to an analysis of thermal decay processes of the stable colour centres (CC) created in gadolinium gallium garnet (GGG) Gd3Ga5O12, crystals under irradiation by gamma-quanta (E=1.25 MeV, D=106 Gy). An analysis of the model of CC accumulation under irradiation as well as an isochronous heating model is presented. The decay activation energy (Δ E) for O− and F centres in GGG crystals is calculated in accordance with this model.

Luminescence of Bulk and Thin-Film Single Crystals of Gadolinium Gallium Garnet Excited by UV Radiation

A difference between the luminescence spectra of bulk and thin-film single crystals of Gd3Ga5O12 gadolinium gallium garnet excited by UV radiation from a deuterium lamp has been studied. The films were grown by liquid phase epitaxy from supercooled melts based on the PbO-B2O3 and Bi2O3-B2O3 solid solution systems.

Magneto-optical properties of yttrium iron garnet (YIG) thin films elaborated by radio frequency sputtering

Thin films of yttrium iron garnet (YIG) are grown by radio frequency magnetron non reactive sputtering system. Thin films are crystallised by heat-treatment to obtain magneto-optical properties. On quartz substrate, the network of cracks observed on the annealed samples can be explained by the difference between the thermal expansion coefficient of substrate and YIG. Physico-chemical analysis shown that the obtained material has a correct stoichiometry and is crystallised as FCC. The Faraday rotation of thin films is measured with a classical ellipsometric system based on transmission which allows us to obtained an accuracy of 0.01°. The variation of Faraday rotation is studied on the one hand versus radio frequency power applied to the cathode during the deposition and on the other hand versus the applied magnetic field. The results are compared with those obtained by vibrating sample magnetometer analysis in perpendicular configuration. A maximum Faraday rotation is observed to be 1900°/cm at the wavelength of 594nm for a YIG thin film formed on quartz substrate and annealed at 740°C. The values of the Faraday rotation coefficients obtained in the study versus the wavelength are comparable to those of the literature for the bulk material. In order to eliminate the stress due to the heat-treatment, we made some films on single crystals of gadolinium gallium garnet (111) substrates for which thermal expansion coefficient is near than the YIG one. The material crystallises with no crackles and the Faraday effect is equivalent.

Magnetic properties of pulsed laser ablated YIG thin films on different substrates

Thin films of yttrium iron garnet (YIG) were deposited on single crystals of gadolinium gallium garnet (GGG) (1 1 1) and Si (1 0 0) substrates by pulsed laser deposition (PLD). The films deposited at substrate temperature ( T s ) of 750°C on GGG substrates show textured YIG phase. However, films deposited on Si substrates at T s of 600–750°C, show YFeO 3 phase along with one YIG (4 0 0) peak. For a T s of 850°C only one peak of very low intensity is observed which is close to (8 2 0) of YIG and (2 3 1) of YFeO 3 .

Powerful visible (530�??770 nm) luminescence in Yb,Ho:GGG with IR diode pumping

Powerful visible luminescence in a Gadolinium Gallium Garnet (GGG) crystal, co-activated with Yb3+ (~15 at.%) and Ho3+ (~0.1 at.%) ions, is investigated under CW laser diode pumping (lambda = 938 and 976 nm). The main visible emission band is observed in the green with its peak at lambda ~540 nm) and measured to be about 10% with respect to Yb3+ IR luminescence (lambda ~1000 nm). Red (lambda ~650 nm) and near-IR (lambda ~755 nm) emission bands are also observed but are weaker (about 3-5%). Analysis of the crystal absorption and luminescence spectra allows one to conclude that Yb3+ - Ho3+ stepwise up-conversion is the mechanism explaining the phenomenon. Ho3+ ions embedded in the crystal in small concentration are shown to form an effective reservoir for energy transferred from the excited Yb3+ subsystem and to be an efficient source of the visible emission.

Crystal growth and morphology of substituted gadolinium gallium garnet

Substituted gadolinium gallium garnet (SGGG) crystals with calcium, magnesium and zirconium as substituting ions have been used as substrates for bismuth-substituted iron garnet epitaxial films. Single crystals have been grown from the melt composition Gd 2.68Ca 0.32Ga 4.04Mg 0.32Zr 0.64O 12 using the Czochralski pulling technique. Spiral growth is the most common problem in this system. With proper adjustment of the growth environment, colorless and optically transparent single crystals of SGGG up to 2 in diameter and 125 mm in length were produced. We will discuss the effect of different growth parameters on the crystal morphology, and propose an optimum growth condition for SGGG crystals.

Optical Spectra of Gd 3 Ga 5 O 12 :Mn Crystals

The absorption and luminescence spectra of gadolinium gallium garnet crystals activated with Mn during high-temperature solution growth are analyzed in relation to the valence state of Mn. The results indicate that, in contrast to melt growth, the flux growth process stabilizes manganese in the valence state 3+.

A high-resolution thermometer for the range 0.75–1.0 K

We report on a new high-resolution thermometer (HRT) for use near the tricritical point in 3He– 4He mixtures. It is based on an existing HRT design that uses a DC-SQUID to detect the magnetization of a paramagnetic sensing element. We present the first test results of an HRT using a gadolinium gallium garnet (GGG) crystal. Sensitivity over the temperature range 0.4–3.5 K for applied fields from 0.41 to 20 G is presented.

Resonant magnetic properties of gadolinium–gallium garnet single crystals

The results of experimental investigations of resonant magnetic properties of gadolinium–gallium garnet (GGG) single crystals at temperatures 4.2–300 K in the frequency range 1.6–9.3 GHz are considered. It is found that magnetic losses in GGG are determined by the initial splitting of energy levels for gadolinium ions in the garnet crystal lattice and by the dipole broadening. The width and shape of the electron paramagnetic resonance (EPR) line in the GGG crystal, whose asymmetry is manifested most strongly at low frequencies, can be explained by the influence of these factors. Magnetic losses in GGG increase with frequency and upon cooling. It is found that the EPR linewidth increases considerably with decreasing temperature due to the presence of rapidly relaxing impurities.

Out-of-plane swelling of gadolinium gallium garnet induced by swift heavy ions

Single crystals of gadolinium gallium garnet (Gd3Ga5O12) have been irradiated with various ions (Cr 10.6 MeV/u, Cu 0.8 MeV/u, Kr 9 MeV/u, and Pb 4 MeV/u) in the electronic stopping power regime. The irradiated areas of the crystals exhibited a pronounced volume expansion. Using a profilometer, the out-of-plane swelling was measured by scanning over the border line between an irradiated and virgin area of the sample surface. The step height varied between 25 and 160 nm depending on the fluence, the electronic stopping power and the total range of the ions. In the high fluence regime, the swelling effect approaches saturation. In order to compare the results obtained for different ion species, the initial swelling per ion was normalised by the length of the damage track. Such an analysis makes evident that swelling occurs only above a critical energy loss of 7±2 keV/nm. The results of Gd3Ga5O12 will be compared with data obtained earlier in SiO2 and LiNbO3.

磁気冷凍の研究動向

Recent progress in magnetic refrigeration has been reviewed. Adiabatic demagnetization refrigerators (ADR) are under development at NASA for large deployable reflectors and X-ray spectrometers. The refrigerators provide temperatures of 2K at 0.027W for 165s and 0.1K at 0.01W for 13h using gadolinium gallium garnet and paramagnetic salt crystals. The Gas-Gap heat switch is used for the isothermal portion of a Carnot cycle. Active magnetic regenerator (AMR) cycles have the advantage of the Carnot cycle for larger temperature span and refrigeration capacity, in particular, above 20K. A test apparatus of the AMR demonstrated that the refrigeration capacity of 1.5-3.5W was obtainable at the temperature spans of 13-25K with a Carnot efficiency of 42%. Practical applications of magnetic refrigeration are also described.

Energy transfer in GGG:Yb 3+,Ho 3+ crystals

The energy transfer between Yb 3+ and Ho 3+ ions and the energy migration on Yb 3+ ions as a function of temperature were studied in gadolinium gallium garnet crystals doped with Yb 3+ (5%) and Ho 3+ (3%). It was found that the commonly accepted energy transfer theories do not provide a good description of the system studied. The application of several theories gives contradictory results.

A study of dislocations generated by inclusions in gadolinium gallium garnet crystals by birefringence topography

By means of birefringence topography, the stress fields around and the dislocations generated from iridium inclusions of various shapes in gadolinium gallium garnet (GGG) have been characterized. The Burgers vectors of dislocations have been identified as 1 2 a〈111〉, a〈100〉 and a〈110〉. Some of the dislocations with a〈110〉 Burgers vector are decomposed into two perfect dislocations with 1 2 a〈111〉 Burgers vector. It has been demonstrated that the inclusions acted as effective sources to generate dislocations in GGG crystal during growth and cooling processes.

Energy transfer from tetrahedral Fe3+ sites to Tm3+ in garnets.

A very efficient energy transfer from ${\mathrm{Fe}}^{3+}$ (d tetrahedral sites) to ${\mathrm{Tm}}^{3+}$ (c dodecahedral sites) in yttrium aluminum garnet and gadolinium gallium garnet crystals was observed. The site-selective spectroscopy shows that in case of near-neighbor Fe(d)-Tm pairs the transfer efficiency is 100%. For the other pairs it is less efficient and increases with temperature due to a better superposition of ${\mathrm{Fe}}^{3+}$ emission and $^{3}$${\mathit{H}}_{4}$ ${\mathrm{Tm}}^{3+}$ absorption bands.

Anisotropic effects in the third-harmonic-generation process in cubic crystals

The third-harmonic-generation process is governed by the fourth-rank tensor χ(3), which reflects the crystal symmetry. We use the fundamental output of a Nd:YAG laser as excitation energy to show the anisotropic character of the nonlinear response of a cubic crystal for different samples. A clear dependence of the third-harmonic-generation efficiency on the crystal orientation has been observed in NaF, LiF, Ge, Si, and gadolinium gallium garnet crystals, the last of which has the lowest value of anisotropy among the crystals considered here. The strengths of the anisotropic nonlinear response in Si and Ge are of the same order of magnitude.