Singlet-triplet splitting and Mollow triplet dynamics in impurity-doped core/shell quantum dots

Abstract

While previous studies have explored singlet–triplet splitting in quantum dots with various geometries and material compositions for high-fidelity qubit design, core/shell QDs—particularly CdSe/CdS— have not been extensively investigated for this purpose. Given their advantageous optical properties, this study aims to investigate the electronic and optical behavior of a two-electron system confined in a CdSe/CdS core/shell QD modeled with a Woods–Saxon potential, incorporating a monovalent off-center impurity. We analyze how the singlet–triplet energy splitting depends on the core-to-shell size ratio and explore the system's response to external electric fields.

Using the effective mass approximation and perturbation theory, we show that for a core radius of 0.5 nm, increasing the shell thickness from 0.5 nm to 2 nm reduces the singlet–triplet splitting from 0.245 eV to 0.085 eV (a 65.3 % decrease). For a 2 nm core, the reduction is smaller—28.4 %—indicating weaker confinement effects. The dynamics under an external electric field reveal Rabi oscillations with tunable frequency and amplitude; increasing the field from 0.5 to 2 kV/cm shortens the oscillation period from ∼0.10 ps to ∼0.05 ps. At field strengths above 1.5 kV/cm, the triplet state becomes more probable than the singlet state during certain time intervals. We also analyze the Mollow triplet evolution and find that sideband separation grows proportionally with the Rabi frequency. These results highlight the tunability of singlet–triplet transitions in CdSe/CdS QDs, offering promising avenues for optical control in solid-state quantum information applications.