  The optical properties of a spherical topological insulator embedded
concentrically in a single-electron system consisting of a core-shell GaAs
quantum dot are analyzed, when the system is under a uniform external magnetic
field. The topological insulator's magnetoelectric response is computed in the
effective framework of $\theta$-electrodynamics, which allows analytical
calculations for the induced electric and magnetic potentials by Green's
Function method. The GaAs Hamiltonian is constructed in the effective-mass
approximation, and its corresponding Schr\"odinger equation is numerically
solved through the Lagrange-mesh method. We compute the total absorption
coefficients and refractive index changes given by the non-linear iterative
density matrix formalism up to third order. Our results show that the presence
of the magnetoelectric material causes new dipolar transitions otherwise not
allowed. Also, an enhancement of the photon absorption is found when the
incident light polarization is oriented parallel to the external magnetic
field, in comparison with perpendicular polarization. Moreover, we report an
appreciable blue shift in the optical functions when the values of the
$\theta$-parameter are increased. These results can be useful for indirect
experimental measures of the magnetoelectric polarizability which is
proportional to the QED fine-structure constant $\alpha$.
