Submillisecond electron spin relaxation in InP quantum dots

Light-induced orientation of electron spins in the negatively charged InP quantum dots is found to persist longer than $100\mu \mathrm{s}$. We have proved experimentally that the long-lived orientation is due to slow relaxation of the electron spins rather than to the dynamic nuclear polarization effects.

Figure 1

(a) PL kinetics under strong ${\sigma }^{+}$ excitation in copolarization $\left({\sigma }^{+}{\sigma }^{+}\right)$ and cross-polarization $\left({\sigma }^{+}{\sigma }^{-}\right)$. $P_{\mathrm{ex}}=90\mathrm{W}∕{\mathrm{cm}}^2$, $U_{\mathrm{bias}}=-0.1\mathrm{V}$, $T=5\mathrm{K}$. The inset shows PL spectra in the copolarization and cross-polarization under excitation at $E_{\mathrm{exc}}=1771\mathrm{meV}$. (b) Kinetics of the degree of circular polarization. Left inset shows the dependence of the NCP amplitude on the pump power density. Right inset shows the dependence of the NCP amplitude on the bias voltage at weak excitation $P_{\mathrm{ex}}\sim 1\mathrm{W}∕{\mathrm{cm}}^2$.

Figure 2

Kinetics of the degree of circular polarization for the copolarized and cross-polarized pump and probe pulses. Time positions of the pulses are indicated by arrows. The scheme illustrates the model of the NCP formation.

Figure 3

(a) The time delay dependence of $A_{\mathrm{NCP}}$ for circularly polarized probe with copolarized (circles), cross-polarized (triangles), and linearly (squares) polarized pumps. $T=2\mathrm{K}$. (b) The delay dependence of $\Delta A_{\mathrm{NCP}}=A_{\mathrm{NCP}}\left(\text{cross}\right)-A_{\mathrm{NCP}}\left(\mathrm{co}\right)$ (diamonds) and multiexponential fit (solid line). Left inset: temperature dependence of $\Delta A_{\mathrm{NCP}}$ at different delays ${\tau }_d$ (symbols); solid curves are the fits by the function $f\left(T\right)=f_0{(1+a\beta )}^{-1}$, where $\beta =\exp (-\Delta E∕kT)$. Activation energy $\Delta E=4\mathrm{meV}$ for all the curves (Ref. 20). Right inset shows magnetic field dependence of $A_{\mathrm{NCP}}$ under cross-polarized excitation for the $11\text{−}\mathrm{ns}$ delay.