High resolution spectroscopic properties and emission decay of rare‐earth doped transparent laser ceramics

Abstract

The paper discusses the use of high-resolution spectroscopy and emission decay for investigation of the variety, nature, structure and distribution of the centers formed by the rare earth ions in transparent polycrystalline ceramics of cubic oxide laser materials (garnets and sesquioxides). It is shown that for ceramics with grains of the order of tens of microns the spectroscopic and population dynamics properties of the doping ions can be described consistently based on the crystal structure of the host material and assuming random distribution even at concentrations much larger than that available for the single crystals. It is inferred that from spectroscopic point of view these ceramics can substitute the crystals as laser materials and their enhanced compositional versatility enables tailoring of new laser materials.1 Introduction Transparent ceramics activated with laser active rare earth (RE) ions are currently investigated as possible substitutes for the single crystal laser materials. These ceramics consist of tightly-packed, randomly oriented crystalline grains and can be useful in case of cubic laser materials such as the garnets or the cubic sesquioxides Ln 2 O 3 (Ln = Y, Lu, Sc). The high melting temperatures (1900-2000 °C for garnets and over 2400 °C for sesquioxides) makes the crystal growth difficult and costly; moreover, the size of the crystals is limited, while in many cases the segregation coefficient and the maximum doping concentration are quite low [1]. The fabrication of the transparent ceramics can be grouped in two basic techniques according to the synthesis method of the compound: solid-state synthesis [2] and wet-chemistry [3]. The grain sizes are determined by the thermal regime (temperature, duration) of the final sintering stage: high transparency is obtained for crystallites of several µm to a few tens of µm. Besides obvious technological advantages and lover cost, the transparent ceramics offer major functional advantages, such as enhanced compositional versatility, higher doping concentrations and the possibility to control the homogeneity of doping. Nevertheless, important problems, such as the variety, structure and spectroscopic properties (energy levels, transition probabilities) of the structural centers of the doping ions, the distribution of the RE 3+ at the available sites and in the ceramic grains as well as the effects of granular structure on the ion-ion or electron-phonon interactions remain of concern and the data from literature are still contradictory. This paper discusses the use of high resolution spectroscopy and emission decay for solving these problems in case of RE 3+ -doped garnet (YAG) or cubic sesquioxide (Y 2 O 3 , Sc 2 O 3 ) transparent ceramics produced by the method of solid-state synthesis.