Characterization of bubble core and cloudiness in Yb3+:Sr5(PO4)3F crystals using micro-Raman spectroscopy

Abstract

Ytterbium doped strontium fluoroapatite Yb3+:Sr5(PO4)3F (Yb:S-FAP) crystals have been used in high average power laser systems as gain medium. Growth induced defects associated with the crystal often affect their performance. In order to improve the crystal quality and its optical applications, it is imperative to understand the nature of these defects. In this study, we utilize micro-Raman spectroscopy to characterize two common growth-induced defects: bubble core and cloudiness. We find the bubble core consist of voids and microcrystals of Yb:S-FAP. These microcrystals have very different orientation from that of the pure crystal outside the bubble core. In contrast to a previous report, neither Sr3(PO4)2 nor Yb2O3 are observed in the bubble core regions. On the other hand, the cloudy regions are made up of the host materials blended with a structural deformation along with impurities which include CaCO3, YbPO4, SrHPO4, and Sr2P2O7 . The impurities are randomly distributed in the cloudy regions. This analysis is necessary for understanding and eliminating these growth defects in Yb:S-FAP crystals.