Gadolinium Scandium Gallium Garnet Research Articles

Efficient pulsed laser operation of Yb-doped Gd3Sc2Ga3O12 mixed crystal

We demonstrate the laser performance of a gadolinium scandium gallium garnet (GSGG) single crystal with 10 at.% Yb3 + -doping concentration. In the case of continuous-wave operation, the laser wavelength was blueshifted in the range from 1067.1 to 1027.2 nm with increasing the transmission of the output coupler from 0.5% to 30%. The maximum output power produced was 3.2 W with 3% output transmission. By employing a Cr4 + : YAG crystal as the saturable absorber, a stable Q-switched laser beam with 21-ns pulse duration and 38-μJ single-pulse energy was achieved at a 20-kHz repetition rate. This laser crystal should be a promising candidate for nanosecond pulse generation especially in harsh environments, such as outer space, due to its wide absorption and emission spectral bandwidths and strong radiation resistance.

Growth and spectroscopic investigations of the 1.5 at.% Er:GSGG laser crystal

The 1.5 at.% Er3+ doped gadolinium scandium gallium garnet (GSGG) laser crystal with high optical quality was successfully grown by the Czochralski method. The structural parameters were obtained by x-ray Rietveld refinement. A high crystalline quality of Er:GSGG was determined by x-ray rocking curve. The spectroscopic parameters of Er3+ ion were calculated and analyzed using the Judd–Ofelt theory. Furthermore, the stimulated-emission cross-section spectra were investigated for the 4I13/2 → 4I15/2 transitions at 1.5–1.6 µm, which indicates the great potential of Er:GSGG for multi-wavelength emission at 1.5–1.6 µm. The larger emission cross-section and long fluorescence lifetimes around 0.55 and 0.67 µm mean that the 1.5 at.% Er:GSGG crystal is beneficial for green and red laser generation. Meanwhile, the intensity of the emission spectra was compared between the 1.5 at.% and 2.0 at.% Er-doped GSGG. The roles of cross-upconversion, cross-relaxation and the excited-state absorption process were discussed in the 1.5 at.% Er:GSGG crystal for generating visible and 1.5–1.6 µm lasers.

Evaluation of growth, thermal and spectroscopic properties of Yb^3+-doped GSGG crystals for use in ultrashort pulsed and tunable lasers

A series of high quality Yb3+-doped gadolinium scandium gallium garnet (Yb:GSGG) single crystals with different Yb3+ ion concentration (7.5, 10, 15 and 30 at.%) has been grown by means of optical floating zone (OFZ). Crystal structure was analyzed with X-ray powder diffraction (XRPD), showing that the Yb:GSGG crystal possesses a cubic structure with space group Ia3d, and the lattice constants decrease linearly as the Yb3+ concentration increases. Effective elemental segregation coefficients and chemical composition were investigated by X-ray fluorescence (XRF), suggesting that some Yb3+ ions occupy the octahedral site when the Yb3+ ion concentration is higher than 10 at.%. The thermal properties of Yb:GSGG crystals were systematically studied by measuring the specific heat, thermal expansion and thermal diffusion coefficients. Thermal conductivity was then calculated, and it exhibits a tendency to decrease with increasing Yb3+ ion concentration. Spectroscopic characterization of Yb:GSGG crystals was performed at room temperature (RT), and the stimulated emission cross section was calculated using the reciprocity method (RM) and the Fuchtbauer−Ladenburg (F−L) formula, respectively. In comparison with the radiative lifetime of the 2F5/2→2F7/2 transition, the effect of radiation trapping on the fluorescence lifetime is discussed. These research results indicate that Yb:GSGG can be regarded as a promising candidate for use in an ultrashort pulsed and tunable laser.

Improved laser efficiency by direct diode laser pumping of the radiation-resistant Nd:gadolinium–scandium–gallium garnet

Spectroscopic investigation of the radiation-resistant Nd:GSGG crystals and ceramics reported in this paper evidences that the absorption line at 883 nm corresponding to the unusual situation of quasi-degenerate absorption transitions 4I9/2(2) →4F3/2(1) and 4I9/2(3) →4F3/2(2) can be used for efficient direct diode laser pumping of this material, with stable absorption over an extended temperature range. It is inferred that the reduction of the quantum defect at this wavelength of pump compared to the traditional 807 nm pumping could improve the laser parameters and reduce drastically the heat generation, leading to a considerable extension of the power scalability. This possibility is demonstrated for the first time in the case of the 1061 nm laser emission in a continuous-wave and in the repetitive active acousto-optic and passive Q-switched laser emission. In all regimes this manifests in the reduction of the laser threshold, increase of slope efficiency and extension of the average power range. Additionally, in the active Q-switching this enables increased pulse energy and reduced pulse duration. The direct pumping could revitalize the utilization of Nd:GSGG for construction of lasers with storage of population inversion or working in an ionizing radiation environment.

Thermal, spectroscopic and laser properties of Nd3+ in gadolinium scandium gallium garnet crystal produced by optical floating zone method

A neodymium-doped gadolinium scandium gallium garnet (Nd:GSGG) single crystal with dimensions of Φ 5×20mm2 has been grown by means of optical floating zone (OFZ). X-ray powder diffraction (XRPD) result shows that the as-grown Nd:GSGG crystal possesses a cubic structure with space group Ia3d and a cell parameter of a=1.2561nm. Effective elemental segregation coefficients of the Nd:GSGG as-grown crystal were calculated by using X-ray fluorescence (XRF). The thermal properties of the Nd:GSGG crystal were systematically studied by measuring the specific heat, thermal expansion and thermal diffusion coefficient, and the thermal conductivity of this crystal was calculated. The absorption and luminescence spectra of Nd:GSGG were measured at room temperature (RT). By using the Judd–Ofelt (J–O) theory, the theoretical radiative lifetime was calculated and compared with the experimental result. Continuous wave (CW) laser performance was achieved with the Nd:GSGG at the wavelength of 1062nm when it was pumped by a laser diode (LD). A maximum output power of 0.792W at 1062nm was obtained with a slope efficiency of 11.89% under a pump power of 7.36W, and an optical–optical conversion efficiency of 11.72%.

Optical pressure and temperature sensor based on the luminescence properties of Nd^3+ ion in a gadolinium scandium gallium garnet crystal

Hypersensitivity to pressure and temperature is observed in the near-infrared emission lines of the Nd(3+) ion in a Cr(3+),Nd(3+):Gd(3)Sc(2)Ga(3)O(12) crystal, associated to the R(1,2)((4)F(3/2))→Z(5)((4)I(9/2)) and R(1,2)((4)F(3/2))→Z(1)((4)I(9/2)) transitions. The former emissions show large linear pressure coefficients of -11.3 cm(-1)/GPa and -8.8 cm(-1)/GPa, while the latter show high thermal sensitivity in the low temperature range. Thus this garnet crystal can be considered a potential optical pressure and/or temperature sensor in high pressure and temperature experiments up to 12 GPa and below room temperature, used in diamond anvil cells and excited with different UV and visible commercial laser due to the multiple Cr(3+) and Nd(3+) absorption bands.

Temperature dependence of the 1.06-microm stimulated emission cross section of neodymium in YAG and in GSGG.

A linear temperature dependence between -70 degrees C and +70 degrees C is reported for the peak stimulated emission cross section of Nd3+ ions in both yttrium aluminum garnet (YAG) and gadolinium scandium gallium garnet (GSGG).

High-power single-frequency tunable solid-state lasers

Experimental results are presented on the achievement of single-frequency tunable lasing in ruby, Nd-glass, and Nd:YAG lasers with electrooptic Q switching of the cavity by the injection of an external signal. An optimization of the parameters is carried out for lasers on neodymium ions in yttrium aluminum garnet, lanthanum beryllate, chromium-doped gadolinium scandium gallium garnet, and lanthanum hexaaluminate with passive Q switching of the cavity by means of lithium fluoride shutters containing F 2 − color centers. High-power single-frequency generation of giant pulses is achieved, with the output wavelength tunable over the half-width of the gain lines of the active media.

High-pressure and high-temperature studies on oxide garnets.

We report high-pressure and high-temperature studies on a series of oxide garnets of chemical composition ${A}_{3}{B}_{2}{C}_{3}{\mathrm{O}}_{12}$. The members of this family investigated are gadolinium scandium gallium garnet (GSGG), gadolinium gallium garnet (GGG), and yttrium aluminum garnet (YAG). The GSGG and GGG are doped with both neodymium and chromium while the YAG is doped only with neodymium. Photoluminescence, synchrotron x-ray-diffraction, and laser heating studies were carried out in a diamond-anvil cell. Variety of optical sensors (ruby, Sm-doped YAG) and x-ray pressure marker (copper) were employed for pressure measurement. Pressure-induced amorphization was observed in GSGG at 58\ifmmode\pm\else\textpm\fi{}3 GPa and GGG at 84\ifmmode\pm\else\textpm\fi{}4 GPa by x-ray-diffraction studies. The photoluminescence studies show only gradual broadening of emission bands through the amorphization transition. On increasing pressure beyond amorphization, very broad and featureless emission bands were observed in the fluorescence spectra at 77\ifmmode\pm\else\textpm\fi{}2 GPa for GSGG and at 88\ifmmode\pm\else\textpm\fi{}2 GPa for GGG. Laser heating of the pressure-induced amorphous phase in GSGG caused recrystallization to the stable cubic phase. High-pressure x-ray study on YAG shows that it retains cubic phase up to 101\ifmmode\pm\else\textpm\fi{}4 GPa. A pressure-volume relation for each member of the oxide garnet at ambient temperatures is presented, structural transformation mechanisms, and application of oxide garnets as pressure sensors are also discussed.

Gain in pulsed x-ray irradiated Cr,Nd:GdScGa-garnet

The transmission properties of gadolinium scandium gallium garnet doped with chromium and neodymium have been studied during and after a 20 ns high-energy x-ray pulse. Gain is observed in the transmission which indicates a small signal gain of 0.175/cm. About 5% of the absorbed dose is converted to inversion, which is comparable to flash-lamp-pumped inversion. The small signal gain scales with dose.

Laser-related properties of chromium- and neodymium-doped gadolinium scandium aluminum garnet (Cr:Nd:GSAG)

The spectroscopic, lasing, and thermal properties of chromium- and neodymium-doped gadolinium scandium aluminum garnet (Cr:Nd:GSAG) are reported. The Judd–Ofelt parameters, radiative lifetime, and branching ratios were determined, and an effective emission cross-section value of 0.92 × 10−19 cm2 was derived at 1.06 μm from our spectroscopic measurements. The temporal behavior of the Cr3+ fluorescence intensity agreed with the Inokuti–Hirayama model for the chromium–neodymium energy-transfer mechanism, which invokes a static dipole-dipole interaction and no migrational energy transfer. Under identical flash-lamp-pumping conditions the measured pumping efficiency of Cr:Nd:GSAG for threshold lasing is nearly equal to the pumping efficiency of codoped gadolinium scandium gallium garnet (Cr:Nd:GSGG). From the threshold-lasing data, an effective emission cross section of 0.7 × 10−19 cm2 for Cr:Nd:GSAG was independently determined, as well as a value of 1.3 × 10−19 cm 2 for Cr:Nd:GSGG. The normalized heating parameter χ, defined as the heat deposited per unit stored energy, experimentally ranges from 2.1 to 2.7, depending on the effective emission cross-section value. Our calculated value of χ is 1.9 ± 0.3. Based on our heating-efficiency measurements, calculated Cr:Nd:GSAG thermal lensing values are ~2.1–2.6 D/kW, which indicates a thermally induced focal length that is weaker than that of Nd:YAG by approximately a factor of 2 at the same output power. Finally, the benefits and trade-offs of laser-diode pumping are discussed.

Optical spectra, energy levels, and emission intensity calculations of trivalent thulium ions in gadolinium scandium gallium garnet

Absorption spectra of trivalent thulium ions in gadolinium scandium gallium garnet are reported between 1.9 and 0.25 μm at 4 K. Laser-induced fluorescence was observed at 4 K from the 1D2, 1G4, and 3H4 (4 and 300 K) manifolds. Site-selective excitation experiments reveal large fractions of thulium ions in both regular D2 sites and alternate sites. A crystal-field splitting calculation was carried out in which a parameterized Hamiltonian (including Coulombic, spin-orbit, and crystal-field terms in D2 symmetry) was diagonalized for all multiplet manifolds of the Tm3+ (4f12) configuration. The rms deviation between 56 experimental and calculated Stark levels was 10 cm−1. Calculations were carried out to predict branching ratios for emission from the 3H4 manifold to the 3H5, 3F4, and 3H6 manifolds, as well as line-to-line transition probabilities for transitions between the 3H4 manifold and the 3H5 manifold.

Thermal effects on cavity stability of chromium- and neodymium-doped gadolinium scandium gallium garnet laser under solar-simulator pumping

A chromium- and neodymium-codoped gadolinium scandium gallium garnet (Cr:Nd:GSGG) crystal was tested for pulsed- and continuous-wave (cw) laser operations with a flashlamp and solar simulator as pumping sources. The crystal has been considered as a good candidate for a solar-pumped laser because of its broad absorption bands over the solar spectrum. However, it was observed experimentally and theoretically that its cw laser operation is difficult at high pump powers because too many thermal lensing effects are induced. Only at low solar-simulator beam intensities of up to 1500 solar constants (203 W/cm2 ) was cw laser operation lasting longer than 10 min achieved. For higher pump beam intensities of up to 2500 solar constants (338 W/cm2 ), a quasi-cw-laser operation was obtained with continuously chopped pumping with a duty cycle of 0.5 and a repetition rate of 13 Hz. The experimental result was compared with the calculated stability condition of the laser resonator for various pump powers and showed that the thermal focusing of the Cr:Nd:GSGG is 15–8 times greater than that of the neodymium-doped yttrium aluminum garnet (Nd:YAG) laser for the pump beam intensities from 1000 to 3000 solar constants.

Stable pulsed picosecond GSGG:Nd3+laser with a resonator based on Sagnac interferometer

An investigation was made of the characteristics of a laser with a new active medium in the form of neodymium-activated gadolinium scandium gallium garnet (GSGG:Nd3+), containing chromium and emitting ultrashort pulses in the case of passive mode locking. Statistical dependences of the duration, energy, and reproducibility of the output pulses on the position of a cell with a saturable absorber (dye 3274 in ethanol) in an interferometer and on the initial transmission of the cell were determined. A new resonator configuration with an asymmetric position of the active element inside an interferometer ring was used: it ensured simple emission of the lowest transverse mode without additional spatial selection. A comparison of the main parameters of the ultrashort pulses emitted by YAG and GSGG lasers demonstrated characteristic advantages of the new active medium in generation of picosecond pulses.

Diode array side-pumped neodymium-doped gadolinium scandium gallium garnet rod and slab lasers

Gadolinium scandium gallium garnet (GSGG) was studied as a high average power laser host material utilizing diode array side pumping. Maximum optical slope efficiencies of 41.5% for Nd:GSGG in the rod geometry and 32.5% for a Nd:Cr:GSGG zig-zag slab were obtained at 1.06 μm. Nd:GSGG and Nd:YAG have similar fluorescence lifetimes while Nd:GSGG has better energy storage capabilities due to its smaller stimulated-emission cross section.

Fluorescence quantum efficiency and optical heating efficiency in laser crystals and glasses by laser calorimetry

A photocaloric technique is described for determining the fluorescence quantum efficiencies and optical heating efficiencies of optically active ions in laser materials. Optical absorption within the sample results in a temperature increase until the heat produced by the absorbed power is balanced by heat leakage to the surroundings. The fluorescence quantum efficiency and optical heating efficiency are determined from a measure of the absorbed power, the steady-state temperature, and the time constant associated with sample cooling following laser excitation. An alternative analysis utilizing only the absorbed power and the steady-state temperature as a function of excitation frequency is also shown to yield quantum efficiencies consistent with the first method. Theory and experiment are demonstrated by measuring the fluorescence quantum efficiency and optical heating efficiency for trivalent chromium in gadolinium scandium gallium garnet. Measurements are also reported for several neodymium-doped phosphate laser glasses.

Absorption spectra and energy levels of Gd3+, Nd3+, and Cr3+ in the garnet Gd3Sc2Ga3O12.

Absorption spectra measured between 0.3 and 6.7 \ensuremath{\mu}m are reported for ${\mathrm{Gd}}^{3+}$, ${\mathrm{Nd}}^{3+}$, and ${\mathrm{Cr}}^{3+}$ in single-crystal gadolinium scandium gallium garnet, ${\mathrm{Gd}}_{3}$${\mathrm{Sc}}_{2}$${\mathrm{Ga}}_{3}$${\mathrm{O}}_{12}$, at various temperatures. Energy levels split by the crystal field are identified between 1500 and 35 000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for ${\mathrm{Gd}}^{3+}$(4${f}^{7}$), ${\mathrm{Nd}}^{3+}$(4${f}^{3}$), and ${\mathrm{Cr}}^{3+}$(3${d}^{3}$). The rare-earth ions occupy sites of ${D}_{2}$ point-group symmetry, and chromium ions are found primarily in sites of ${C}_{3i}$ point-group symmetry. A Hamiltonian consisting of Coulombic, spin-orbit, and crystal-field terms is diagonalized to obtain theoretical energy levels for each symmetry. Empirical crystal-field parameters ${B}_{\mathrm{km}}$ were determined by fitting calculated energy levels to observed levels through a variation of the ${B}_{\mathrm{km}}$; these were compared with a lattice-sum calculation. The rms deviations between calculated and observed levels are 1.7 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for ${\mathrm{Nd}}^{3+}$ (45 levels), 4.0 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for ${\mathrm{Gd}}^{3+}$ (14 levels), and 87.7 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for ${\mathrm{Cr}}^{3+}$ (35 levels).

Spectral, luminescence, and lasing characteristics of gadolinium scandium aluminum garnet doped with chromium and neodymium

An investigation was made of the spectral, luminescence, and lasing characteristics of gadolinium scandium aluminum garnet (GSAG) doped with chromium and neodymium. It was found that at the working activator concentrations, GSAG crystals are not inferior to gadolinium scandium gallium garnet crystals in terms of the efficiency of excitation energy transfer from the chromium to the neodymium ions. The maximum differential efficiency of 2.4% was obtained under free-running conditions.

Effects of impurities and atmosphere on the growth of Cr-doped gadolinium scandium gallium garnet. II

A model is introduced that details the complexing between divalent and tetravalent species in garnet melts. This explains the phenomenon of interface instability in Czochralski crystal growth of gallium-containing garnets. Chromium may participate in this process when it is oxidized to its 4+ valence state. This model explains the effects of uncompensated divalent ions and growth atmosphere on absorption in the crystal.

Effects of impurities and atmosphere on the growth of Cr-doped gadolinium scandium gallium garnet. I

The Czochralski growth technique was used to prepare crystals of Cr-doped gadolinium scandium gallium garnet. The concentrations of divalent and tetravalent dopants and the oxygen partial pressure in the growth atmosphere were varied to examine their effects on interface stability and growth morphology and on optical absorption at 1 μm in the crystal.