Orientation of Nd3+ dipoles in yttrium aluminum garnet: Experiment and model

We report an experimental study of the 1.064 μm transition dipoles in neodymium-doped yttrium aluminum garnet (Nd:YAG) by measuring the coupling constant between two orthogonal modes of a laser cavity for different cuts of the YAG gain crystal. We propose a theoretical model in which the transition dipoles, slightly elliptic, are oriented along the crystallographic axes. Our experimental measurements show a very good quantitative agreement with this model and predict a dipole ellipticity between 2% and 3%. This work provides an experimental evidence for the simple description in which transition dipoles and crystallographic axes are collinear in Nd:YAG (with an accuracy better than 1°), a point that has been discussed for years.