Spectroscopy and Coherence of an Excited-State Transition in Tm${}^{3+}$:YAlO${}_3$ at Telecommunication Wavelength

We characterize spectroscopic and coherence properties of the 1451.37 nm excited-state zero-phonon line (ZPL) between the ${}^3{F}_4$ and the ${}^3{H}_4$ manifolds of a thulium-doped yttrium aluminum perovskite (Tm${}^{3+}$:YAlO${}_3$) crystal at temperatures around 1.5 K. We measure the absorption spectrum between the ${}^3{H}_6$ - ${}^3{F}_4$ and ${}^3{F}_4$ - ${}^3{H}_4$ manifolds, the inhomogeneous broadening of the ${}^3{F}_4$ - ${}^3{H}_4$ (excited-state) ZPL, and the lifetimes of the higher-lying and lower-lying excited states. We also investigate level shifts caused by the quadratic Zeeman interaction as well as spectral hole-burning spectra with varying magnetic fields, providing insights into hyperfine interactions. Using again spectral holes but also optical free induction decays (FIDs), we assess optical coherence times, finding a maximum of $4.75\pm 0.07\mu s$ at B=2T and low ion concentration in the ${}^3{F}_4$ level. Our results – the first to demonstrate coherence of an excited-state transition in a rare-earth crystal – suggest the possibility of exploiting such transitions for quantum technology.