Controlling the Polarization Eigenstate of a Quantum Dot Exciton with Light

We demonstrate optical control of the polarization eigenstates of a neutral quantum dot exciton without any external fields. By varying the excitation power of a circularly polarized laser in microphotoluminescence experiments on individual InGaAs quantum dots we control the magnitude and direction of an effective internal magnetic field created via optical pumping of nuclear spins. The adjustable nuclear magnetic field allows us to tune the linear and circular polarization degree of the neutral exciton emission. The quantum dot can thus act as a tunable light polarization converter.

Figure 1

QD III. $E_{\mathrm{laser}}=1.44\mathrm{eV}$. (a) $X^0$ PL with $E_{X0}=1.340\mathrm{eV}$ recorded with interferometer for ${\pi }^Y({\sigma }^{+})$ laser polarization in lower (upper) panel. ${\pi }^Y({\pi }^X)$ detection shown as solid squares (triangles). (b) $X^{+}$ PL with $E_{X+}=1.346\mathrm{eV}$ recorded with interferometer for ${\pi }^Y({\sigma }^{+})$ laser polarization in lower (upper) panel. ${\sigma }^{+}({\sigma }^{-})$ detection shown as solid circles (triangles). QD I: The circular polarization $P_c$ (c), the linear polarization $P_l$ (d). Overhauser shift ${\delta }_n$ (e) shown as a function of laser power for $X^0$. Solid circles (hollow squares) represent ${\sigma }^{+}$ (${\pi }^X$) laser polarization.

Figure 2

QD I. $E_{\mathrm{laser}}=1.44\mathrm{eV}$. ${\delta }_1=-9\mu \mathrm{eV}$. (a) Circular polarization $P_c$ and (b) Linear polarization $P_l$ for $X^0$ as a function of $B_N$ for ${\sigma }^{+}$ (${\sigma }^{-}$) laser polarization shown as solid circles (hollow triangles) assuming $|g_e|=0.48$. Solid lines calculated with Eqs. (2, 3). Dashed vertical line: measured value of $|{\delta }_1|$.

Figure 3

Circular polarization $P_c$ and Overhauser shift ${\delta }_n$ as a function of $E_{\mathrm{laser}}$ for ${\sigma }^{+}$ (${\sigma }^{-}$) laser polarization shown as solid circles (hollow triangles) for (a),(b) $X^0$ of dot I, (c),(d) $X^{+}$ of dot I, (e),(f) $X^0$ of dot II and (g),(h) $X^{+}$ of dot II. PL detection energy dot I (II) $\simeq 1.358\mathrm{eV}$ ($\simeq 1.342\mathrm{eV}$).