Ultraslow optical dephasing in ${\mathrm{Eu}}^{3+}$:${\mathrm{Y}}_2$${\mathrm{SiO}}_5$

We report the measurement of what we believe to be the narrowest known optical resonance in a solid. This was made on the ${}_{}{}^7{}_{}{}^{}$ ${\mathit{F}}_0$ ${\to }^5$ ${\mathit{D}}_0$ transition of ${\mathrm{Eu}}^{3+}$:${\mathrm{Y}}_2$ ${\mathrm{SiO}}_5$ at 1.4 K using photon echoes with sufficiently low laser power to almost eliminate contributions from instantaneous diffusion. The optical resonance widths measured for ${\mathrm{Eu}}^{3+}$ ions in the two crystallographic sites of ${\mathrm{Y}}_2$ ${\mathrm{SiO}}_5$ and in a magnetic field of 100 G are 122 Hz (site 1) and 167 Hz (site 2). These widths are dominated by population decay, but up to 20 Hz is attributed to ${}_{}{}^{89}{}_{}{}^{}\mathrm{Y}$ spin fluctuations which have been reduced by the applied magnetic field.