Spin inertia and polarization recovery in quantum dots: Role of pumping strength and resonant spin amplification

Abstract

Spin inertia measurements are a novel experimental tool to study long-time spin relaxation processes in semiconductor nanostructures. We develop a theory of the spin inertia effect for resident electrons and holes localized in quantum dots. We consider the spin orientation by short optical pulses with arbitrary pulse area and detuning from the trion resonance. The interaction with an external longitudinal magnetic field and the hyperfine interaction with the nuclear spin bath is considered both in the ground and excited (trion) states of the quantum dots. We analyze how the spin inertia signal depends on the magnetic field (polarization recovery), on the modulation frequency of the helicity of the pump pulses as well as on their power and detuning. In particular, we elaborate how approaching the saturation limit of the spin polarization influences the measurements. The quantitative description of spin inertia measurements will enable the determination of the parameters of spin dynamics such as the spin relaxation times in the ground and excited states and the parameters of the hyperfine interaction. Finally, we predict a novel longitudinal spin mode locking effect, which manifests itself as oscillations of the spin polarization as function of the longitudinal magnetic field.

Resonant spin amplification in Faraday geometry

“…The appearance of a modulation in the Faraday ellipticity at certain values of the longitudinal magnetic field in the case of large pump powers is most striking. This is the result of RSA in Faraday geometry enabled by nuclear fluctuation fields [25]. As demonstrated in Fig.…”

“…For larger pump powers, the spin polarization increases and the qualitative dependence on the magnetic field is very similar. But strong pulses suppress the in-plane electron spin components, effectively accelerating the spin relaxation, which in turn leads to a broadening of the zero field minimum in the PRC [25,30]. Clearly, this is the case in Fig.…”

“…In this Letter, however, we demonstrate that RSA can emerge in this geometry for an ensemble of n-doped (In,Ga)As/GaAs quantum dots (QDs), which allows us to measure the longitudinal g factor of the charge carriers with high accuracy. The effect is enabled by the hyperfine interaction between the resident electron spins and the fluctuating nuclear spin environment [25].…”

“…, where τ * s is the effective spin lifetime at the corresponding pump power. The extrapolation of τ * s to zero power allows for extracting the intrinsic spin relaxation τ s of the electrons [15,25]. The inset in Fig.…”

“…This effect is referred to as polarization recovery. For the n-doped QD sample studied in this work, the polarization recovery curves (PRCs) have the typical V-like shape (symmetric around zero field) [16,17,25,30,31]. Figure 2 shows the PRCs for a wide range of pump powers.…”