We characterize spectroscopic and coherence properties of the 1451.37 nm excited-state zero-phonon line (ZPL) between the ³F₄ and the ³F₄ manifolds of a thulium-doped yttrium aluminum perovskite (Tm³⁺:YAlO₃) crystal at temperatures around 1.5 K. We measure the absorption spectrum between 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 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 spectral holes again but also optical free induction decays, we assess optical coherence times, finding a maximum of 4.75± 0.07 s at B=2 T and low ion concentration in the ³F₄ 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.

The creation of well-understood structures using spectral hole burning is an important task in the use of technologies based on rare-earth ion-doped crystals. We apply a series of different techniques to model and improve the frequency dependent population change in the atomic level structure of thulium yttrium gallium garnet (Tm:YGG). In particular we demonstrate that, at zero applied magnetic field, numerical solutions to frequency-dependent three-level rate equations show good agreement with spectral hole-burning results. This allows us to predict spectral structures given a specific hole-burning sequence, the underpinning spectroscopic material properties, and the relevant laser parameters. This enables us to largely eliminate power-dependent hole broadening through the use of adiabatic hole-burning pulses. Although this system of rate equations shows good agreement at zero field, the addition of a magnetic field results in unexpected spectral diffusion proportional to the induced Tm ion magnetic-dipole moment and average magnetic-field strength, which, through the quadratic Zeeman effect, dominates the optical spectrum over long timescales. Our results allow optimization of the preparation process for spectral structures in a large variety of rare-earth ion-doped materials for quantum memories and other applications.