Optical spin orientation of an individual ${\mathrm{Mn}}^{2+}$ ion in a CdSe/ZnSe quantum dot

We demonstrate optical spin orientation of an individual ${\mathrm{Mn}}^{2+}$ ion embedded in a CdSe/ZnSe quantum dot. It is achieved through the injection of spin-polarized excitons to the dot under circularly polarized below-the-barrier optical excitation at energy sufficiently close to the dot emission energy. The efficiencies of both the exciton spin-transfer and optical pumping of the ${\mathrm{Mn}}^{2+}$ spin are studied by means of polarization-resolved single-dot spectroscopy performed at magnetic field in a Faraday configuration. The ion spin orientation efficiency is found to increase significantly with the magnetic field, while the spin-polarization degree of injected excitons remains constant and yields about 40%.

Figure 1

(a) A PL spectrum of a CdSe/ZnSe QD containing a single ${\mathrm{Mn}}^{2+}$ ion. (b) Simplified diagram of neutral exciton and biexciton energy levels in Mn-doped QD at zero magnetic field. The states are displayed as a function of their energy and ${\mathrm{Mn}}^{2+}$ ion spin projection on the QD growth axis. Color of the neutral exciton levels denotes the $X$ spin projection on the growth axis as well as the circular polarization of corresponding $X$ optical transitions (red – ${\sigma }^{+}$, blue – ${\sigma }^{-}$). For clarity, only the transitions related to 3/2 spin projection of the ${\mathrm{Mn}}^{2+}$ ion are shown.

Figure 2

(a)-(i) PL spectra of the neutral exciton confined in Mn-doped QD under excitation at 488 nm with indicated circular or linear polarizations. The spectra were detected in ${\sigma }^{+}$ (dashed red lines) and ${\sigma }^{-}$ (solid blue lines) circular polarizations and measured at magnetic field (a)–(c) $B=0$, (d)–(f) $B=4$ T, and (g)–(i) $B=8$ T. (j) Magnetic field dependence of the ratio of integrated $X$ PL intensities detected in ${\sigma }^{-}$ and ${\sigma }^{+}$ circular polarizations under linearly polarized excitation. (k) Circular polarization degree (absolute value) of the integrated $X$ PL under excitation with ${\sigma }^{+}$ (squares) and ${\sigma }^{-}$ (circles) polarization as a function of the magnetic field. The average values of these two degrees are marked with triangles.

Figure 3

(a)–(f) The zero-field PL spectra of biexciton confined in Mn-doped QD. The spectra were excited with indicated circular or linear polarizations and detected in ${\sigma }^{-}$ [(a)–(c) blue] and ${\sigma }^{+}$ [(d)–(f) red] polarizations. The ion spin projections corresponding to the emission lines of extreme energies are indicated. Solid lines representing the directions of change of $2X$ intensities are drawn to guide the eye. (g) Magnetic field dependence of the mean spin of the ${\mathrm{Mn}}^{2+}$ ion determined in ${\sigma }^{+}$ polarization of detection under linearly or circularly polarized excitation (as indicated). (h) The efficiency of optically induced ${\mathrm{Mn}}^{2+}$ spin orientation measured as a function of the magnetic field. Circles (squares) represent the efficiencies determined in ${\sigma }^{-}$ (${\sigma }^{+}$) polarization of detection. The excitation power was kept well below the saturation power.