Comparison of optical spectra of Nd³⁺ in NdAl₃(BO₃)₄ (NAB), Nd:GdAl₃(BO₃)₄ (NGAB) and Nd:Gd_0.2Y_0.8Al₃(BO₃)₄ (NGYAB) crystals

A comparative study of the optical absorption and luminescence of crystals of NdAl₃(BO₃)₄ (NAB), Nd:GdAl₃(BO₃)₄ (NGAB) and Nd:Gd_0.2Y_0.8Al₃(BO₃)₄ (NGYAB) is presented. The Judd-Ofelt theory is applied to the fitting of experimental absorption line strengths to obtain the intensity parameters Ω_t (t = 2,4,6) with root mean square errors of 11.8%, 3.8% and 8.6% for NAB, NGAB and NGYAB respectively. The intensity parameters are used to calculate the radiative transition rates, branching ratios and radiative lifetimes of Nd³⁺ ions undergoing transitions from ⁴F_3/2 to ⁴I_J manifolds. The relevant spectroscopic parameters for laser applications (quantum efficiency and emission cross section) are also estimated. The results are compared among the three samples and other borate crystals. It is concluded that NAB is a laser crystal with high gain and thus a promising candidate for microchip laser application, while NGAB (or NGYAB) can be used as an alternative to the celebrated nonlinear laser crystal NYAB.