Observation of extremely slow hole spin relaxation in self-assembled quantum dots

We report the measurement of extremely slow hole spin relaxation dynamics in small ensembles of self-assembled $\mathrm{InGaAs}$ quantum dots. Individual spin oriented holes are optically created in the lowest orbital state of each dot and read out after a defined storage time using spin memory devices. The resulting luminescence signal exhibits a pronounced polarization memory effect that vanishes for long storage times. The hole spin relaxation dynamics are measured as a function of external magnetic field and lattice temperature. We show that hole spin relaxation can occur over remarkably long time scales in strongly confined quantum dots (up to $\sim 270\mathrm{\mu }\mathrm{s}$), as predicted by recent theory. Our findings are supported by calculations that reproduce both the observed magnetic field and temperature dependencies. The results suggest that hole spin relaxation in strongly confined quantum dots is due to spin-orbit-mediated phonon scattering between Zeeman levels, in marked contrast to higher-dimensional nanostructures where it is limited by valence band mixing.

Figure 1

(Color online) Schematic band profile of the devices investigated in the spin generation (a) and readout (b) phases of the time-gated EL measurement. (c) The temporal sequence of optical and electrical control pulses during the measurement and detector gating.

Figure 2

(Color online) (a) Comparison of a nonresonantly excited PL spectrum (dashed line) with a hole storage EL spectrum ($\hslash {\omega }_{laser}\sim 1.252\mathrm{eV}$, $\mathrm{\Delta }t=300\mathrm{ns}$ at $f_{rep}\sim 200\mathrm{kHz}$). (b) Temporal evolution of the peak intensity of $R$ recorded with circularly polarized excitation and detection $\left[I_{n∕m}\right(\mathrm{\Delta }t\left)\right]$, where $n,m∊\{+,-\}$ denote the helicity of the excitation and detection, respectively. The dotted line shows the intensity $⟨I⟩$ averaged over the two polarizations. (c) Temporal evolution of the degree of circular polarization.

Figure 3

(Color online) (a) Temporal dependence of the excess circular polarization for $B$ ranging from 5 to $11\mathrm{T}$. (b) Comparison of the $B$-field dependence of $T_1^h$ (open symbols) and $T_1^e$ (filled symbols) for identical QD material and experimental conditions. The filled line shows the calculated hole spin relaxation time using the model of Ref. 26. (c) Comparison of measured (open symbols) and calculated (full lines) dependence of $T_1^h$ on lattice temperature.