Understanding the two-photon photoionization of a prototypical defect spin-qubit

Color centers in semiconductors implement qubit functionality via coupled spins, a spin-selective optical cycle, and optical read-out. Alternative read-out schemes based on spin-selective ionization and electrical detection of photo currents have been demonstrated for NV-center in diamond [1] as well as for silicon vacancies (V_Si) and di-vacancies (V_CV_Si) in 4H-SiC [2,3]. Experiments are interpreted as a two-photon process via photo-ionization from the first excited state into a dark charge state. A subsequent (photo-assisted) capture of a free electron (hole) restores the active qubit. While for the NV^--ionization leads to electron emission, the process for V_CV_Si involves holes instead. For V_Si, the dominant mechanism has not yet been determined by experiments. A key interest is to achieve optimal control of the two-photon ionization in a two-color scheme. This demands insight into the underlying mechanisms, dark states, and cross sections. Here we investigate this topic for the showcase V_Si^- combining density functional theory and our embedding method CI-CRPA capable of describing the spin multiplets. We explore the electronic structure of the dark charge states V_Si⁰ and V_Si^2-. Our analysis unravels that the two photon photo-ionization via V_Si^2- dominates by an order of magnitude over the competing process.

[1] E. Bourgeois et al., Nat. Commun. 6, 8577 (2015).

[2] M. Niethammer et al., Nat. Commun. 10, 5569 (2019).

[3] Ch. Anderson et al., Science Adv. 8, eabm5912 (2022); Science 366, 1225 (2019).