Optical Spin Orientation of a Single Manganese Atom in a Semiconductor Quantum Dot Using Quasiresonant Photoexcitation

An optical spin orientation is achieved for a Mn atom localized in a semiconductor quantum dot using quasiresonant excitation at zero magnetic field. Optically created spin-polarized carriers generate an energy splitting of the Mn spin and enable magnetic moment orientation controlled by the photon helicity and energy. The dynamics and the magnetic field dependence of the optical pumping mechanism show that the spin lifetime of an isolated Mn atom at zero magnetic field is controlled by a magnetic anisotropy induced by the built-in strain in the quantum dots.

Figure 1

(a) PL and PL excitation of a Mn-doped QD at $B=0\mathrm{T}$ and $T=5\mathrm{K}$. The PL is detected in circular polarization under an alternate ${\sigma }^{-}$ and ${\sigma }^{+}$ excitation at two different wavelengths into the same set of excited states: 1987.0 meV (black line) and 1987.4 meV [gray (red) line]. (b) PL transient under polarization switching at $B=0\mathrm{T}$. The PL is detected on the high energy line of $X$-Mn in ${\sigma }^{-}$ polarization (Mn spin $S_z=-5/2$). Transient (I) [(II)] was observed under resonant excitation at 1975 meV (1987 meV).

Figure 2

(a) PL transients at different values of the excitation power. Inset: power dependence of the inverse response time ${\tau }_r$, taken at the $1/e$ point of the spin-related transient. (b) PL transients recorded in ${\sigma }^{-}$ polarization on the high ($S_z=-5/2$) and low ($S_z=+5/2$) energy line of the $X$-Mn complex. (c) Simplified level diagram of a Mn-doped QD, as a function of Mn spin ($X$-Mn: bright exciton-Mn).

Figure 3

PL transients recorded under the optical polarization sequence displayed at the bottom of each plot. The prepared Mn spin is conserved during $t_{\mathrm{dark}}$: in (a) and (b), a transient is observed only for a different helicity of the pump and the probe. The injection of carriers with a nonresonant excitation (514 nm) forces the relaxation of the Mn spin (c). Inset: PL obtained under the resonant excitation condition used in the optical pumping measurements.

Figure 4

Mn spin transient as a function of a magnetic field applied in-plane (a) and out-of-plane (b). Inset: transverse field dependence of the transient amplitude $\Delta I/I$ (see Fig. 1). $B_{1/2}$ is the half width at half maximum.

Figure 5

(a) Magnetic field dependence of the fine structure of the Mn spin with out-of-plane (right) and in-plane (left) fields, calculated with $A=0.68\mu \mathrm{eV}$, $D_0=7\mu \mathrm{eV}$, and a crystal field parameter $a=0.32\mu \mathrm{eV}$. (b) Time evolution of ${\rho }_{+5/2}(t)$ calculated for different values of $D_0$ and $B_x$ with ${\rho }_{+5/2}(0)=1$. (c) Time average value of ${\rho }_{+5/2}(t)$ versus $B_x$ for different values of $D_0$ ($T_2=\infty$). The $B_{1/2}$ found experimentally is indicated with a dotted line.