Spectroscopy and Coherence of an Excited-State Transition in Tm³⁺:YAlO₃ at Telecommunication Wavelength

We characterize the spectroscopic and coherence properties of the 1451.37 nm excited-state zero-phonon line (ZPL) between the ³F₄ and the ³H₄ manifolds of a thulium-doped yttrium aluminum perovskite $Tm³⁺$:$YAlO₃$ crystal at temperatures around 1.5 K. We measure the absorption spectrum between the ³H₆ - ³F₄ and ³F₄ - ³H₄ manifolds, the inhomogeneous broadening of the ³F₄ - ³H₄ (excited-state) ZPL, and the lifetimes of the higher-lying and lower-lying excited states. We also investigate spectral hole-burning spectra with varying magnetic fields, which provides insight into hyperfine and superhyperfine interactions. Using again spectral holes but also optical free induction decays (FIDs), we assess optical coherence times, finding a maximum of 1.1 μs when a 2T magnetic field is applied. 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.