Charge Transitions in Rare Earth-Vacancy- Defect Complexes in MgO for Optical Memories

The realization of high-density and low power addressable memories is a major goal to accommodate the ever-increasing demands of computational resources. In principle, the use of rare earth (REs) atoms provides a way to increase memory density by exploiting the high degree of wavelength multiplexing within a diffraction limited memory voxel. In this work we theoretically explore a novel class of defect complexes of RE- native vacancies in oxides for creating optically addressable long-lived charged states suitable to build ultra-dense atomic memories. In particular, we address a critical question, namely the influence of the distance between the RE and the vacancy on (1) the charge transition levels of the complex, and (2) the near field non-radiative energy transfer (NRET) occurring via the Forster-Dexter mechanism. We focus on Erbium- V_O complexes in MgO and using Density Functional Theory and the Quantum Espresso Code we show that the presence of the Er substitutional sites results in significant (~3%) lattice distortions. In addition it results into charge redistribution affecting charge transition energies and dipoles of the states localized in proximity of the vacancy. Quantum analysis of the NRET process and the optical selection rules at the near field will be discussed, which provide key physical insights towards various processes relevant for classical and quantum optical memory applications.