We report the first observation of the nearly degenerate four-wave–mixing spectrum of ${\mathrm{Nd}}^{3+}$-doped β’’-Na-alumina. In a new application of continuous-wave high-resolution nearly degenerate four-wave mixing, a linewidth analysis of ∼1-kHz-wide optical resonances in heavily doped samples has been used to explain concentration-dependent, nonradiative decay of Nd ions which is obscured in conventional fluorescence-decay studies. Quantitative four-wave–mixing saturation studies also reveal a transition between a ‘‘closed’’ and an ‘‘open’’ quantum system due to pair interactions at a Nd density of ∼6×${10}^{20}$ ${\mathrm{cm}}^{\mathrm{-}3}$.

• Received 8 December 1986

DOI:https://doi.org/10.1103/PhysRevLett.59.597