Er_Al:Al₂O₃ for Telecom-Band Photonics: Electronic Structure and Optical Properties

Er-doped Al₂O₃ is a promising, CMOS-compatible host for telecom-band integrated photonics. We combine ab initio calculations with a symmetry-resolved analysis to elucidate substitutional Er on the Al site (Er_Al) in α-Al₂O₃. First-principles relaxation confirms the structural stability of Er_Al. Using the local trigonal crystal-field symmetry, we classify the Er-derived impurity levels by irreducible representations and derive polarization-resolved electric-dipole selection rules, explicitly identifying the symmetry-allowed 4f–5d hybridization channels. Kubo–Greenwood absorption spectra computed from Kohn–Sham states quantitatively corroborate these symmetry predictions. We further connect the calculated intra-4f line strengths to Judd–Ofelt theory, clarifying the role of 4f–5d admixture in enabling optical activity. Notably, we predict a characteristic absorption near 1.47 µm (telecom band), directly relevant for on-chip amplification and emission. To our knowledge, a symmetry-resolved first-principles treatment of Er:Al₂O₃ with an explicit Judd–Ofelt interpretation has not been reported, providing a transferable framework for tailoring rare-earth dopants in wide-band-gap oxides for integrated photonics. Our theoretical spectrum is in good agreement with available experimental data.