Spin inertia of resident and photoexcited carriers in singly charged quantum dots

The spin dynamics in a broad range of systems can be studied using circularly polarized optical excitation with alternating helicity. The dependence of spin polarization on the frequency of helicity alternation, known as the spin inertia effect, is used here to study the spin dynamics in singly charged (In,Ga)As/GaAs quantum dots (QDs), providing insight into spin generation and accumulation processes. We demonstrate that the dependence of spin polarization in $n$- and $p$-type QDs on the external magnetic field has a characteristic $\mathsf{\text{V}}$- and $\mathsf{\text{M}}$-like shape, respectively. This difference is related to different microscopic mechanisms of the resident carriers' spin orientation. It allows us to determine the parameters of the spin dynamics both for the ground and excited states of singly charged QDs.

Figure 1

Spin polarization of carriers as a function of magnetic field at different modulation frequencies of pump helicity: (a) $n$-doped sample, laser pump/probe power is $4/0.8\mathrm{mW}$, $T=6\mathrm{K}$; (b) $p$-doped sample, pump/probe power is $1.5/0.5\mathrm{mW}$, $T=1.7\mathrm{K}$. Gray curves are calculations after Eq. (2) with parameters given in Table 1 [19]. Insets show examples of possible ground (GS) and excited state (ES) spin configurations: Thin green and thick black arrows represent the electron and hole spins, respectively. (c) and (d) show the frequency dependencies of carrier spin polarizations at fixed magnetic field for the $n$- and $p$-doped samples, respectively. The red line is a fit by Eq. (1). Insets are linear extrapolations of the power-dependent spin lifetime $T_s$ down to zero pump power giving the spin relaxation times in equilibrium $T_s^{\left(0\right)}$.

Figure 2

Temperature dependence of PRC for the $p$-doped sample. Laser pump/probe power is $2/0.5\mathrm{mW}$, $f_m=2\mathrm{kHz}$.

Figure 3

Decomposition of the PRC (dashed blue lines, shaded for better visibility, arb. units) into magnetic field dependence of spin polarization of resident and photoexcited carriers: (a) $n$-doped, (b) $p$-doped samples. Black curves give the spin lifetime of resident carriers and red curves the generation rate $Q$ determined by spin relaxation of the photoexcited carriers. The parameters of the calculation are the same as in Fig. 1.