Rubyene: a promising platform for quantum technologies and optical nanosensing

In this paper, we have investigated the optical properties of a newly synthesized material consisting of a few-nm-thin flakes of ruby ( $\mathrm{Al}_2\mathrm{O}_3\mathrm{:Cr}^{3+}$ ), referred to as rubyene . Both theoretical and experimental approaches focus on the photoluminescence of $\mathrm{Cr}^{3+}$ , particularly exploring the effects of the refractive index of the surrounding medium on the radiative and nonradiative lifetimes of the photoexcited chromium ions, and the consequences of the diverse shapes of the thin rubyene flakes. Notably, we demonstrate how such susceptibility of the radiative rates to the refractive index allows for insight into the orientation of the crystallographic axes relative to the surface of the flakes. Although the effect of the environment on the nonradiative relaxation rates is an order of magnitude smaller than its effect on the radiative rates, the achieved sensitivity of our measurements was sufficient to observe both. The presented theoretical analysis of nonradiative transitions has broader implications for Förster-type energy transfer, especially in scenarios where the refractive index of the medium is non-uniform. Our results demonstrate good agreement between the theory and the experiment and establish the potential of rubyene in optical nanosensing applications, highlighting its sensitivity to environmental refractive index changes.