Rare-earth Doped YAG Research Articles

Hydrothermally cladded crystalline fibers for laser applications [Invited

Various rare earth doped single crystal YAG and sesquioxide fibers have been drawn using the laser heated pedestal growth (LHPG) method. Crystalline core/clad fibers have been successfully fabricated using a two-step hydrothermal growth method applied to core fibers. The development of a two-step process is essential to the growth of reasonable cladding layers. Various shapes of the cladding layer were observed, i.e., octagon, hexagon, square and intermediate shapes. EDX study shows a stoichiometry of YAG composition of the clad and there is no significant diffusion of the rare earth ion across the core/clad interface. No birefringence was observed under cross-polarized light indicating that a negligible stress between core/clad crystals was formed during hydrothermal process. Acid etching is an effective method to obtain faceted YAG fibers with a reduced diameter in a controlled manner. An etch rate of ∼0.15 µm/min was measured using 50:50 mixtures of phosphoric acid and sulfuric acid at elevated temperature. Scattering loss of 0.05 dB/cm and a net peak gain of 19 dB was measured from 10% Yb:YAG core/YAG clad fiber. The hydrothermal technique is a versatile epitaxial method to growth undoped YAG cladding of various thicknesses onto doped YAG core fibers and has a great potential to pave the way forward for improving laser performance.

Non-conventional Ce:YAG nanostructures via urea complexes

Ce:YAG nanostructures (Ce:YAG = Cerium in Yttrium Aluminium Garnet), easy to control and shape, have been prepared via templating approach using natural and synthetic materials (i.e. paper, cotton wool and glass wool) previously soaked with a gel-like metals precursor and then thermally treated to achieve the wished morphology. The final material, otherwise difficult to process, can be easily moulded, it is lightweight, portable and forms, at the nanoscale, homogeneous layers of interconnected but not agglomerated nanoparticles (15 ± 5 nm). Using the same synthetic route, called Urea-Glass-Route, but in absence of a template, extremely pure Ce:YAG nanoparticle (45 ± 5 nm) can be also prepared, highly crystalline and well-defined in size and shape. Both structural and optical properties of the final materials were investigated, showing high optical quality. The support allows the production of a multifunctional material with mouldable shape and potential lighting application for large structures combining the strength, chemical durability, fire resistance, and translucency of glass fibres. Last, but not least, the synthetic path also allows an easy scaling up of the process: the first, key step for practical application of nanosized rare-earth doped YAG on large scale.

Rapid, convenient and low-energy preparation of spherical rare earth doped YAG phosphors by a laser sintering method

Rare earth doped YAG phosphors are prepared rapidly and conveniently by using a laser sintering method with low energy consumption.

Co-precipitation Synthesis and Spectroscopic Studies of YAG and Yb:YAG Nanopowder for Opto-Electronic Applications

Rare-earth doped YAG nanopowders are widely used as luminescence material because of its technological importance in opto-electronic applications especially for solid state lasers. In this work, YAG and Yb:YAG were prepared by co-precipitation method. The thermal behavior was investigated by differential thermal analysis and thermo gravimetric analysis. The phase formation, elemental analysis, particle size distribution, optical absorbance, fluorescence behavior and morphology of YAG and Yb:YAG nanopowder were analyzed through powder X-ray diffraction technique, energy dispersive X-ray analysis, dynamic light scattering, UV–Vis, photoluminescence spectroscopy, high resolution scanning electron microscopy, high resolution transmission electron spectroscopy respectively. From the optical study, Yb:YAG gives optical absorption at 975 nm and predominant infrared emission at 1030 nm.

Color centers in YAG

Yttrium aluminum garnet (Y 3 Al 5 O 12 , YAG) is one of the most important optical materials with many existing and potential future applications in laser, illumination, and scintillation. X-rays, g-rays, and UV light can induce ionization and significant changes in the valency of impurities and defects in YAG single crystals which may lead to the formation of color centers. In fact, the use of YAG crystals in laser and scintillation devices involves significant formation of color centers and requires full understanding for their characteristics and effects on the material properties. In this work, the formation and characteristics of color centers in undoped and rare-earth doped YAG single crystals was investigated mainly through optical absorption spectroscopy. An increase in the absorption over a broad range of wavelengths was observed in the as-grown sample after UV irradiation. F-centers and iron impurities in the as-grown undoped crystals were found to be responsible for the formation of color centers. However, air- or oxygen-anneal seems to be effective in suppressing most color centers in the crystals.

Fabrication of Yb:YAG Transparent Ceramics Using Stearate Melting Derived Ultrafine Powders

Yb rare earth doped YAG ultrafine particles were synthesized by the stearate melting method using yttrium stearate, ytterbium stearate and aluminum tristearate as starting materials. The phase formation of Yb:YAG, the properties and the sintering activity of the powders were investigated by means of XRD, SEM, dilatometry and vacuum sintering. The results show that pure Yb:YAG nanopowders can be obtained by calcining the co-melted precursor at a relatively low temperature of 800 °C for 4 h. The powders calcined at 1000°C have better sintering activity than the powders calcined at other temperatures. For the Yb:YAG powders doping with 0.5% TEOS, the compact can be sintered to 99.2% of the theoretical density at 1600 °C and 99.7% at 1700 °C. The transparent Yb:YAG ceramics obtained by vacuum sintering at 1700 °C for 5 h exhibit a pore-free and uniform microstructure.

Novel transparent ceramics for solid-state lasers

AbstractRecent progress on rare-earth doped polycrystalline YAG transparent ceramics has made them an alternative novel solid-state laser gain material. In this paper we present results of our research on polycrystalline RE:YAG transparent ceramics. High optical quality YAG ceramics doped with various rare-earth (RE) ions such as ${\rm Nd}^{3+}$, ${\rm Yb}^{3+}$, ${\rm Er}^{3+}$, ${\rm Tm}^{3+}$, and ${\rm Ho}^{3+}$ have been successfully fabricated using the solid-state reactive sintering method. Highly efficient laser oscillations of the fabricated ceramics are demonstrated.

Revisiting the Hydrothermal growth of YAG

As new compositions and configurations of single crystals are specified for optical applications the crystal growth community must continue to optimize and explore new options for developing such materials. New crystal growth technology and a growing understanding of certain chemical and physical phenomena sometimes implore us to reconsider our perceptions even about the most studied systems. Thus, the hydrothermal growth of YAG was reexamined after nearly 40 years of inactivity and met with promising new results. While the hydrothermal growth of YAG at 500°C was previously met with slow growth rates or poor crystal quality we have found that growth at 600–630°C yields good quality single crystals at rates of about 0.5mm/side/week on (100) seeds. In the current study, high quality epitaxial layers of transition metal and rare earth doped YAG have been grown from 0.1 to 2.75mm in thickness. This has enabled several more advanced proof of concept designed growth experiments including the growth of co-doped Ca2+,Cr4+,Nd:YAG for self Q-switching, and the growth of Sm:YAG around Nd:YAG to potentially suppress amplified spontaneous emission. The interface quality of the substrate and the new growth was investigated and application of a very narrow thermal gradient at the early stages of growth has proven to greatly reduce optical scattering at the interface. The morphology of hydrothermally-grown YAG crystals is also discussed and several intermediate forms are occasionally observed.

Direct laser writing of near-IR step-index buried channel waveguides in rare earth doped YAG

A new (to our knowledge) ultrashort laser pulse irradiation regime that allows us to directly modify and increase the refractive index of rare earth doped YAG polycrystalline ceramics has been identified. Single-mode buried channel waveguides in both Ho:YAG and Er:YAG ceramics at the near-IR wavelengths of 1.55 μm and 1.95 μm are demonstrated by fabricating positive square step-index cores. Minimum propagation losses of 1.5 dB cm(-1) at a 1.51 μm wavelength have been preliminarily obtained. Confocal microluminescence mapping reveals that the increased refractive index regions retain the near-IR spectral properties of Er3+ ions in the YAG crystalline matrix.

Use of doped-YAG nanoparticles as down-converters for Photovoltaics

ABSTRACTIncreasing solar cell efficiency by using spectral conversion is addressed in this article. To that purpose rare-earth doped YAG nanoparticles exhibiting down-conversion and quantum cutting properties have been prepared. These nanoparticles have been synthesized with different concentrations of dopants in order to optimize the luminescence and the quantum cutting efficiency. Results on the incorporation of selected material into the encapsulating layer of c-Si based PV-modules are also presented. The effect of down-conversion has been demonstrated through the increase of photocurrent of encapsulated silicon solar cells.

Microwave-assisted polyol synthesis of sub-micrometer Y2O3 and Yb-Y2O3 particles for laser source application

Sub-micrometer powder (100–150nm diameter) of Yb-doped yttrium oxide was obtained, for the first time, by microwave-assisted polyol (diethylene glycol, DEG) method. This method is based on fast and homogeneous increase of temperature, due to the microwave heating, and on addition of the hydrolysing agent (water) at high temperature. This promotes a fast nucleation followed by a controlled growth of nuclei. Different procedures were used to process the as-synthesized powders. In some cases washing by ultrapure water was used to dissolve nitrate and DEG by-products, this treatment allowed the use of a lower calcination temperature (150–200°C less) to obtain the crystalline phase. Analysis of the calcined powder showed different levels of structures: from nanocrystal (10–15nm), to primary particles (100–150nm), to micrometer soft aggregates (2–4μm). The microwave-assisted polyol method resulted an easy way to dope yttria with the desired amount of Yb3+. This work was carried out in order to prepare particles to be used as rare-earth doped Y2O3 and YAG polycrystalline transparent ceramic for laser source applications.

Preparation of non-aggregation YAG-Ce phosphor particles by spray pyrolysis

Yttrium Aluminum Garnet(Y,AISOlz, YAG) materials have been widely studied in the application of fluorescent and solid state lasers(Schol1 and Trimmier, 1986; Robbins, 1982) YAG is a hard material which does not damage easily under conditions of high irradiance with an electron beam. Therefore, rare earth doped YAG materials are promising phosphor candidates in cathode-ray tubes (CRTs), field emission display (FED), scintillation, vacuum fluorescent displays (VFDs), and electroluminescent (EL). Recently, spray pyrolysis was successfully applied to the preparation of phosphor materials(Kang et al., 1997a, b). In this work, we prepared YAG:Ce green phosphors by spray pyrolysis using ultrasonic and FEAG process. In the system of YAG phosphors, the directly prepared particles in spray pyrolysis have amorphous phase, and activation of doping component was not occurred because of short residence time of particles inside hot wall reactor as several seconds. Therefore, anriealing process at high temperatures is required for the crystallization and activation of ceriuum-doped YAG particles. The peaks of cubic structure YAG appeared after annealing from 800 “C, and high crystallinity YAG was formed above 1000 “C. In all samples, small peak of CeO, was appeared, and intensity of peak was increased with increasing doping concentrations of dopant. Fig. 1 shows the SEM photographs of particles prepared by different processes. The overall solution concentration was 0.4M. The prepared particles have spherical morphology and submicron size in both processes. Additionally, the particles had nonaggregation and sphericity after annealing at high temperatures. The YAG:Ce particles prepared at low flow rate of carrier gas had more solid morphology and sphericity than those of high flow rate of carrier gas. The photoluminescence characteristics of YAG:Ce particles with spherical morphology were investigated. The all particles for the measurements of PL were prepared by ultrasonic spray pyrolysis at the conditions of 2 Nmin and 900 “C. The excitation spectra of YAG:Ce particles have maximum peak at wavelength of 465 nm. The emission spectra have a maximum intensity at 527 nm. The optimum brightness was obtained at doping concentration of lat.%. The effect of calcination temperature on the luminous intensities of particles at constant doping concentration( 1 at.%) was shown in Fig.2. The luminescence of particles was increased with increasing annealing temperatures because of good activation of Ce component into YAG matrix. In the XRD spectra, the peak intensity of CeO, was decreased with increasing annealing temperatures.

Possibility of Gamma-Induced Sensibilization Process in Rare-Earth Doped Yag Crystals

It is well established that color centers (CCs) produced by ionizing radiation or short-wave radiation of the pump lamp, strongly influence the optical characteristics of yttrium-aluminium garnet (YAG) single crystals. Many publica tions, including reviews and monographs [1-3], are devoted to this problem. Tle most thoroughly investigated aspect was the influence of various types of ionizing radiation (gamma quanta, electrons, neutrons) on performance of YAG crystals activated with Nd ions [2]. The CCs, arising in the crystals, absorb not only pump light but also laser light, significantly increasing laser energy losses [4]. Creating losses at the laser wavelength [5], the CCs reduce the slope of efficiency. In Ref. [6] the improvement of performance of Nd:YAG lasers was reported as an effect of small dose of gamma and electron irradiation. During optical investigations of Nd:YAG crystals, particular attention was paid to Cr3+ ions, because they not only sensibilize the laser medium but, at certain concentrations, they may also reduce the susceptibility of YAG crystals to ionizing radiation effects [7].