Temperature-dependent mechanical losses of Eu3+:Y2SiO5 for spectral hole burning laser stabilization

We investigate the mechanical loss characteristics of Eu3+:Y2SiO5—a promising candidate for ultra-low-noise frequency stabilization through the spectral hole burning technique. Three different mechanical oscillators with varying surface-to-volume ratios and crystal orientations are evaluated. In this context, we perform mechanical ringdown and spectral measurements spanning temperatures from room temperature down to 15 K. By doing so, we measure a maximum mechanical quality factor of Q = 3676, corresponding to a loss angle of ϕ = 2.72 × 10−4. For a spectral hole burning laser stabilization experiment at 300 mK, we can estimate the Allan deviation of the fractional frequency instability due to Brownian thermal noise to be below σδν/ν0=2.4×10−18, a value lower than the estimated thermal-noise limit of any current cavity-referenced ultra-stable laser experiment.