Probing the Spin State of a Single Magnetic Ion in an Individual Quantum Dot

The magnetic state of a single magnetic ion (${\mathrm{M}\mathrm{n}}^{2+}$) embedded in an individual quantum dot is optically probed using microspectroscopy. The fine structure of a confined exciton in the exchange field of a single ${\mathrm{M}\mathrm{n}}^{2+}$ ion ($S=5/2$) is analyzed in detail. The exciton-${\mathrm{M}\mathrm{n}}^{2+}$ exchange interaction shifts the energy of the exciton depending on the ${\mathrm{M}\mathrm{n}}^{2+}$ spin component and six emission lines are observed at zero magnetic field. Magneto-optic measurements reveal that the emission intensities in both circular polarizations are controlled by the ${\mathrm{M}\mathrm{n}}^{2+}$ spin distribution imposed by the exchange interaction with the exciton, the magnetic field, and an effective manganese temperature which depends on both the lattice temperature and the density of photocreated carriers. Under magnetic field, the electron-Mn interaction induces a mixing of the bright and dark exciton states.

Figure 1

Low temperature (T=5 K) PL spectra obtained at $B=0\mathrm{T}$ and $B=11\mathrm{T}$ for an individual isotropic $\mathrm{C}\mathrm{d}\mathrm{T}\mathrm{e}/\mathrm{Z}\mathrm{n}\mathrm{T}\mathrm{e}$ QD (a) and a Mn-doped QD (c). (b) Scheme of the bright states energy levels of the Mn-exciton coupled system at zero magnetic field. The exciton-${\mathrm{M}\mathrm{n}}^{2+}$ exchange interaction shifts the energy of the exciton depending on the ${\mathrm{M}\mathrm{n}}^{2+}$ spin projection.

Figure 2

(a) Magnetic field dependence of the emission of a Mn-doped QD showing the anticrossing of bright and dark states around 6 T. (b) Optical transitions in $\sigma -$ polarization obtained from the diagonalization of the spin Hamiltonian and Zeeman Hamiltonian (the diamagnetic component is not included) in the subspace of the heavy-hole exciton and ${\mathrm{M}\mathrm{n}}^{2+}$ spin projections. The thickness of the lines is proportional to the oscillator strength of the transition. (c) Detail of the circularly polarized emission spectra around the anticrossing. Seven emission lines are resolved at $B=6\mathrm{T}$ in $\sigma -$ polarization.

Figure 3

(a) Magnetic field dependence of the calculated energy levels of the Mn-exciton coupled system. The dark states are presented in gray. All the bright states associated with the $J_z=-1$ exciton are degenerate for an applied magnetic field of about 1.5 T. (b) Emission rate (emission intensity of each line normalized by the total integrated intensity) of the six PL lines observed in $\sigma +$ and $\sigma -$ polarization as a function of magnetic field for a fixed lattice temperature and a fixed excitation intensity. The dotted lines correspond to the Boltzmann distribution describing the thermal equilibrium of the population of the initial state (a) with an effective temperature $T_{\mathrm{M}\mathrm{n}}=12\mathrm{K}$. For simplicity the anticrossings with the dark states are neglected in this calculation.

Figure 4

(a) Normalized PL (PL spectra divided by the total integrated intensity) in $\sigma +$ polarization versus excitation intensity for a fixed temperature (5 K) and magnetic field (7 T). (b) Excitation intensity dependence of the zero magnetic field emission of a single Mn-doped QD for a fixed lattice temperature $T=5\mathrm{K}$. (c), (d): extracted emission rates of each PL line as a function of the excitation intensity at 7 T  (c) and 0 T (d). The inset plots the extracted Mn effective temperature.