Undoped Yttrium Aluminum Garnet Research Articles

Importance of crystal length on the stability of a picosecond supercontinuum generated in undoped YAG

In this paper, we present the stability characteristics of a supercontinuum (SC) generated in a 60 mm, 100 mm, and 130 mm long undoped yttrium aluminum garnet (YAG) crystal pumped by a Yb thin-disk regenerative amplifier. The 1.2 ps pulses with a central wavelength of 1030 nm and repetition rate of 89 kHz are loosely focused (NA = 0.0031) into the crystal in order to prevent laser-induced damage and to increase the energy stability of the generated SC. Out of the tested crystal lengths, the optimal is experimentally determined to be 130 mm. The energy contained in the blueshifted side, which has a 530 nm spectral cutoff, is 71 nJ and exhibits average power stability of 0.4% RMS, which is 2.5× better than the stability of the pump beam.

Epitaxial Growth of Single Crystal YAG for Optical Devices

We report the latest progress on fabrication of rare earth doped single crystal yttrium aluminum garnet (YAG) core/undoped YAG cladded fibers. Rare-earth doped single crystal core fibers were grown with laser heated pedestal growth methods. In a second step, epitaxial methods were used to grow a single crystalline undoped YAG cladding onto the core fiber. Hydrothermal and liquid phase epitaxy methods utilize the core doped fiber as the seed. X-ray diffraction of cladding reveals an equilibrium (110) morphology. Energy-dispersive X-ray spectroscopy analysis shows there is minimal diffusion of rare-earth dopants into the cladding structure. The use of scandium doping is shown to substitute at the Al3+ site, thereby allowing an additional tunability of refractive index of core structure material besides conventional Y3+ site dopants. The use of these epitaxial growth methods enables material compatibility, tuning of refractive index, and conformal growth of cladding structures onto core fibers for optical devices.

Defect-Controlled Scintillation Process in Undoped Y3Al5O12Crystals

Optical absorption, luminescence spectra, scintillation decay curves, and scintillation light yield were measured in a series of undoped yttrium–aluminum garnet (YAG) crystals of different origins. This paper reveals a correlation between luminescent properties and scintillation efficiency of undoped YAG crystals. All the samples were separated into three groups, Type I, Type II, and Type III, based on their optical properties and scintillation efficiency. The intrinsic ultraviolet luminescence band peaking around 300 nm is found in the samples with high scintillation yield (Type I). Luminescence intensity of this band depends on the crystal purity and on the stoichiometry of the garnets. The UV emission may be related to e–h recombination in vicinity of vacancies. The garnets of Type II contain more impurities which quench the 300-nm emission. The samples of Type III do not exhibit any significant radioluminescence. Light yield of the finest undoped YAG samples in the series, Type I, is similar to the yield of Ce-doped YAG. This Type of crystals can potentially be used in scintillation detectors.

Low-loss 'crystalline-core/crystalline-clad' (C4) fibers for highly power scalable high efficiency fiber lasers.

We report the latest progress in fabrication and laser performance of the fully crystalline double-clad 'Yb:YAG-core/undoped-YAG-clad' fibers grown by the hybrid crystal growth method. The single-crystalline ytterbium (Yb) doped yttrium aluminum garnet (YAG) fiber cores were grown by the laser heated pedestal growth (LHPG) method, and the single-crystalline undoped YAG claddings were grown by the liquid phase epitaxy (LPE) technique, in which the single-crystalline Yb:YAG cores were used as the growth seeds. The key parameters of the hybrid-grown 'crystalline core/crystalline clad' (C4) fibers, including material composition, crystal structure, and fiber propagation loss, were characterized. The results confirmed that the grown C4 fibers, indeed, have both the single-crystalline fiber core and single-crystalline fiber clad. By utilizing a double-clad low-loss C4 fiber as a diode-cladding-pumped laser gain medium, we realized a fiber laser with the optical-to-optical conversion efficiency of 68.7% versus the incident pump power.

CW and passively Q-Switched operation of a Ho:YAG waveguide laser

We report the first demonstration of a Ho: YAG crystal fiber waveguide (CFW) laser operating at 2.09µm. The CFW structure was produced by adhesive free bonding of holmium doped yttrium aluminum garnet core to an undoped yttrium aluminum garnet cladding. The laser produced CW output powers greater than 500mW with slope efficiency of 17%. The same crystal when passively Q-switched with Cr: ZnSe, produced pulsed output with energies of 1μJ at a repetition frequency of 442kHz.

Cation diffusion at the interface of composite YAG/Re: LuAG (Re = Nd or Yb) transparent ceramics

Composite structures of YAG/1 at% Nd-doped LuAG/YAG, YAG/1 at% Nd-doped LuAG and YAG/5 at% Yb-doped LuAG transparent ceramics were successfully fabricated by non-aqueous tape-casting method and vacuum sintering technology. Cation (Y3+, Lu3+, Nd3+, Yb3+) diffusion behavior across the contact boundaries between ytterbium (Yb) or neodymium (Nd) doped lutetium aluminum garnet (LuAG) and un-doped yttrium aluminum garnet (YAG) was studied by the electron probe X-ray micro-analysis (EPMA). The diffusion concentration profile curves were well modeled by Harrison type B kinetics. Curves of concentration vs depth were well fitted by Fick’s second low and the log of the average concentration varies as 6/5 power of penetration depth in the diffusion range was demonstrated. Meanwhile, the bulk diffusion coefficient and grain boundary diffusion coefficient were calculated. For YAG/Nd:LuAG transparent ceramic, under an 808nm diode laser pumping, the 1064nm continuous wave (C.W.) laser output beam with good quality was achieved.

Structural properties and luminescence kinetics of white nanophosphor YAG:Dy

Dysprosium (Dy3+ ions) doped YAG nanopowders were prepared by the solution combustion synthesis (SCS) method and undoped yttrium aluminum garnet (YAG, Y3Al5O12) single crystal was grown by the Czochralski technique. The structure of the prepared materials has been confirmed and characterized using X-ray powder diffraction (XRD), scanning electron microscope (SEM), Raman and photoluminescence (PL) spectroscopy. PL spectra were acquired using Optical Parametric Oscillator (OPO) excitation tuned at 350nm. Several emission bands in Dy3+ emission spectrum were observed in the blue (470–500nm), yellow (560–600nm), red (660–685nm) and infra-red (750–780nm) regions, corresponding to 4F9/2–6H15/2, 4F9/2–6H13/2, 4F9/2–6H11/2, 4F9/2–(6H9/2+6H11/2) transitions in the 4f levels of Dy3+ ions, respectively. By using the Commission Internationale de l’Eclairage (CIE) chromaticity diagram of emission spectra it has been shown that this material can be used as a source of a white light. The result of lifetime analysis of the 4F9/2 level in dysprosium ion has been reported, as well.

Ion diffusion at the bonding interface of undoped YAG/Yb:YAG composite ceramics

Cation diffusion across a boundary between ytterbium (Yb)-doped and undoped yttrium aluminum garnet (YAG) ceramics was examined by electron microprobe analysis (EPMA). Polished Yb:YAG and undoped YAG ceramics were bonded by surface treatment with argon fast atom beam, and then heat-treated at 1400 or 1600°C for 50h or at 1400°C for 10h under vacuum. We obtained EPMA mapping images of the bonded samples that clearly showed the bulk and grain-boundary diffusion of Y and Yb ions. The number density profiles showed that the total diffusion distances of Yb and Y ions were almost equal and approximately 2 and 15μm at 1400 and 1600°C, respectively, and the dependence of diffusion distance on heating time was weak. The diffusion curves were well modeled by Harrison type B kinetics including bulk and grain-boundary diffusion. In addition, it was found that Si ions added to the samples as a sintering aid might be segregated at the grain boundary by heat treatment, and diffused only along grain boundaries.

Creep of polycrystalline yttrium aluminum garnet (YAG) at elevated temperature in air and in steam

Compressive creep of high-purity polycrystalline yttrium aluminum garnet (YAG, Y3Al5O12) was investigated at 1300°C and 50–200MPa in air and in steam. Compressive creep behavior of silica-doped polycrystalline YAG (Y3Al5O12–0.14wt% SiO2) was also studied. Creep specimens were microstructurally characterized by optical microscopy and TEM before and after creep. Steam slightly increased creep rates of material with grain size less than 1μm (the undoped YAG), but otherwise had little effect. The flow stress exponent was n≈1 for both SiO2-doped YAG and undoped YAG. Creep rates and microstructural observations are consistent with the Nabarro-Herring creep mechanism, with creep rate limited by lattice diffusion of yttrium cations (Y3+). Silica-doped YAG had a larger grain size of 2.41μm and lower creep rates than undoped YAG with 0.92μm grain size. However, creep rates normalized by grain size for Nabarro-Herring creep were higher in SiO2-doped YAG. Possible effects of SiO2 doping and steam on creep of YAG are discussed.

Energy levels of exciton traps in yttrium aluminum garnet single crystals

Electronic defects and exciton traps were studied in yttrium aluminum garnet (YAG) single crystals by wavelength and temperature resolved thermoluminescence (TL). Measurements were carried out from room temperature to 400 °C on a number of rare earth (RE) doped and undoped YAG crystals, and the trap parameters were determined. Although the TL emission spectrum is characteristic of the RE ion, the main trap levels are found to be characteristic of the undoped host crystal. Nevertheless, the thermal activation energies of the traps are slightly modified by doping. The trap kinetics are found to be of the first order in both undoped and Ce doped YAG single crystals indicating the absence of retrapping, which suggests that the traps and recombination centers exhibit a close spatial correlation. The effect of annealing on TL response suggests that some of the major traps are associated with oxygen vacancies.

Strong visible and near infrared luminescence in undoped YAG single crystals

Strong luminescence peaks were observed at 700 and 800 nm in undoped yttrium aluminum garnet (YAG) single crystals. They were attributed to low level of iron impurities as confirmed by Glow Discharge Mass Spectrometry analysis. The 800 nm was only excited by high energy band at 270 nm; the reason behind that was discussed. Photoluminescence measurements revealed a large number of luminescence peaks in all YAG crystals regardless of the growth conditions due to native defects and low-level impurities. These luminescence centers have significant effects on the optical properties of rare-earth doped YAG crystals and their performance in laser and scintillation applications. Excitons released in the lattice may be easily captured by iron ions instead of Ce3+ ions and the scintillation output is substantially decreased. Nevertheless, Undoped YAG crystals may have the potential to be developed into efficient scintillators.

Nd diffusion in YAG ceramics

Neodymium diffusion was observed in yttrium aluminum garnet (YAG) polycrystals with negligible change of microstructure during diffusion. Ceramic bi-layer samples were fabricated via a technique developed for ceramic laser amplifiers, from flame-spray pyrolized powders of un-doped YAG and 1% Nd:YAG. Diffusion studies were carried out on these samples at temperatures lower than the sintering temperature, such that average grain diameter remained approximately 0.8 μ m for all samples. Effective diffusivity was 117 ± 4 m 2 / s exp 623 ± 70 kJ/mol K RT for temperatures between 1500 and 1600 °C and with 1.2 ± 0.3 grain boundaries per micrometer.

ESR and X-ray diffraction measurements of Nd substituted yttrium aluminum garnet films

Yttrium aluminum garnet (YAG) layers, doped with some rare-earth ions can be used as thin film solid-state lasers. Thin films of Nd 3+-doped YAG have been grown on undoped YAG substrates by the isothermal liquid phase epitaxy (LPE) dipping technique. The layers have been obtained from a supercooled molten garnet-flux high temperature solution. Y 3− x Nd x Al 5O 12 films (2–30 μm) grown on the (111) plane of YAG substrate have been investigated as a function of neodymium concentration using electron spin resonance (ESR) and X-ray diffraction techniques. According to those measurements it can be concluded that the obtained thin films possess high quality.

Diode-pumped 1-W continuous-wave Er:YAG 3-?m laser

We have demonstrated 1.15 W of Gaussianlike (M(2) = 2) cw output at 2.94 mum from a diode laser end-pumped monolithic laser crystal composed of Er-doped yttrium aluminum garnet (YAG) bonded to undoped YAG. The laser was pumped with two polarization-coupled 2.5-W flared laser diodes that provided a 45-mum pump waist with a low N.A. (<0.04) . Output at 2.94 mum was generated with a 34% slope efficiency and a greater-than-unity quantum efficiency.

Refractive-index measurements of undoped yttrium aluminum garnet from 04 to 50 μm

We report measurements of the refractive index of undoped yttrium aluminum garnet from 0.4 to 5.0 mum and the calculation of Sellmeier coefficients based on our data. The data differ considerably from previously published reports. The effect of the new data on the design of optical devices such as intracavity etalons for lasers is discussed.

Optical absorption in undoped yttrium aluminum garnet

Room-temperature optical absorption over a large dynamic range is reported for single- crystal undoped yttrium aluminum garnet (Y3Al5O12 or YAG). Absorption results are presented for the energy ranges 1.8 to 3.0 and 4.5 to 6.5 eV. For this work, ultraviolet (UV) absorption measurements were performed on four different boules of Czochralski-grown undoped YAG. Absorption features were observed at 4.8 and 5.6 eV which can be attributed, at least in part, to trace impurity concentrations. Along with the UV measurements, calorimetric absorption results are presented at several laser wavelengths in the visible and near infrared (IR). The calorimetric results suggest a value of 1.5×10−3 cm−1 for the absorption coefficient of undoped YAG in the red and near IR. From the combination of UV and calorimetric measurements, an empirical estimate of the absorption coefficient from the visible to the UV is presented for undoped YAG. In addition to the absorption spectra, detailed chemical analyses and sample histories are given for the as-grown optical quality YAG utilized herein.