Detailed site-selective spectroscopy has been performed as a function of temperature on the 7 F 0 ↔ 5 D 0 transition of Eu 3ϩ :Y 2 SiO 5 for Eu 3ϩ concentrations of 0.02%, 0.1%, 0.5%, and 1%. Time-domain optical dephasing, spectral hole lifetimes, anisotropic absorption coefficients, inhomogeneous linewidths, and fluorescence lifetimes for Eu 3ϩ ions at both crystallographic sites were measured. The temperature dependence of the optical dephasing, transition energy, and linewidth of the 7 F 0 → 5 D 0 absorption was measured and interpreted in terms of Raman scattering of phonons. Photon echo measurements of optical dephasing gave T 2 values as long as 2.6 ms, approaching the limit set by the fluorescence decay time. Spectral hole lifetimes were measured for temperatures from 2 K to 18 K, with observed lifetimes varying from 1 s at 18 K to an estimated value of greater than 20 days at 2 K. Anisotropic absorption coefficients were measured, and an increase in Eu 3ϩ concentration from 0.02% to 7% produced an increase in the inhomogeneous linewidth ⌫ inh from 0.5 GHz to ϳ150 GHz, indicating that Eu 3ϩ doping induces significant strain in the crystal. New determinations of many energy levels of 7 F J multiplets have been made for Jϭ0 to 6.

The influence of the anisotropic Zeeman effect on optical decoherence was studied for the 1.54 m telecom transition in Er 3+ :Y 2 SiO 5 using photon echo spectroscopy as a function of applied magnetic field orientation and strength. The decoherence strongly correlates with the Zeeman energy splittings described by the groundand excited-state g factor variations for all inequivalent Er 3+ sites, with the observed decoherence times arising from the combined effects of the magnetic dipole-dipole coupling strength and the ground-and excited-state spin-flip rates, along with the natural lifetime of the upper level. The decoherence time was maximized along a preferred magnetic field orientation that minimized the effects of spectral diffusion and that enabled the measurement of an exceptionally narrow optical resonance in a solid-demonstrating a homogeneous linewidth as narrow as 73 Hz.