High-resolution transient and permanent spectral hole burning in Ce3+:Y2SiO5 at liquid helium temperatures

We perform hole burning with a low-drift stabilized laser within the zero phonon line of the 4f - 5d transition in Ce3+:Y2SiO5 at 2 K. The narrowest spectral holes appear for small applied magnetic fields and are 6 +/- 4 MHz wide ( FWHM). This puts an upper bound on the homogeneous linewidth of the transition to 3 +/- 2 MHz, which is close to lifetime limited. The spin level relaxation time is measured to 72 +/- 21 ms with a magnetic field of 10 mT. A slow permanent hole burning mechanism is observed. If the excitation frequency is not changed the fluorescence intensity is reduced by more than 50% after a couple of minutes of continuous excitation. The spectral hole created by the permanent hole burning has a width in the tens of MHz range, which indicates that a trapping mechanism occurs via the 5d state.