Pumping of Nuclear Spins by Optical Excitation of Spin-Forbidden Transitions in a Quantum Dot

We demonstrate that efficient optical pumping of nuclear spins in semiconductor quantum dots (QDs) can be achieved by resonant pumping of optically forbidden transitions. This process corresponds to one-to-one conversion of a photon absorbed by the dot into a polarized nuclear spin, and also has potential for initialization of hole spin in QDs. We find that by employing this spin-forbidden process, nuclear polarization of 65% can be achieved, markedly higher than from pumping the allowed transition, which saturates due to the low probability of electron-nuclear spin flip-flop.

Figure 1

(a) Pulse sequence used in the resonant nuclear spin pump-probe experiment. (b) Energy level diagram of a positively charged dot in magnetic field $B_z$. Electron and hole spin up (down) states are shown by $\uparrow (\downarrow )$ and $\Uparrow (\Downarrow )$, respectively. Long thick (thin) arrows show “allowed” (“forbidden”) optical transitions. Dotted arrow shows hole spin relaxation. (c) PL spectra of a positively charged dot measured with the probe pulse at $B_z=2.5\mathrm{T}$ for $B_N\approx 0$ (solid symbols) and at $B_N\approx -1.5\mathrm{T}$ (open symbols). Here the optics were optimized to maximize the PL signal, leading to differing sensitivity in detection of light with ${\sigma }^{+}$ and ${\sigma }^{-}$ polarizations and effectively unpolarized PL spectra.

Figure 2

Overhauser field $B_N$ on the dot (a),(c) and PL transitions energies $E_{\mathrm{PL}}$ (b),(d) as a function of the energy $E_l$ of the ${\sigma }^{+}$ polarized resonant laser with excitation power $P_{\mathrm{res}}=15\mu \mathrm{W}$ at $B_z=2.5\mathrm{T}$ (a),(b) and $B_z=0$ (c),(d). The full lines on panels (b) and (d) show the laser energy.

Figure 3

Results of calculations using Eqs. (1, 2) for $B_z=2.5\mathrm{T}$ (a),(b) and $B=0$ (c),(d): calculated nuclear field $B_N$ on the dot (a),(c) and PL transitions energies $E_{\mathrm{PL}}$ (b),(d) as functions of the energy $E_l$ of the ${\sigma }^{+}$ polarized laser [(crosses) circles show (meta) stable solutions]. Lines on panels (b)and (d) show the laser energy. Lines in (a) and (c) show nuclear spin pumping rate $|W_{\mathrm{pump}}|=|W_{\mathrm{SA}}+W_{\mathrm{SF}}|$ as a function of $E_l$ for a fixed $B_N=0$.