Homogeneous linewidth behavior of narrow optical emitters at sub-kelvin temperatures

We explore the properties of ultranarrow spectral holes in ensembles of solid-state emitters in crystals over a range of sub-kelvin temperatures, as a new step toward leveraging their exceptional coherence properties more effectively. As an example, we consider the potential gain observed in their application in frequency stabilization schemes. We investigate how the parameters used to burn the spectral hole impact its shape, and how these factors determine the minimum achievable linewidth. In addition to the stability of the hole's center frequency, the linewidth and contrast play a crucial role in frequency locking. At sub-kelvin temperatures, the temperature-dependent T7 broadening from two-phonon Raman scattering is expected to be negligible, and the spectral hole's linewidth should therefore remain constant in this interval. We observe however a linear broadening with increasing temperature, highlighting the need for further investigation into the mechanisms governing the linewidth at ultra-low temperatures.