Effect of carrier redistribution on spin polarization in n-doped InGaAs quantum dot ensemble

Semiconductor quantum dots (QDs) are found to have relatively long spin depolarization times, which makes them promising candidates for practical realization of qubits for spintronics based computation. For doped QDs the presence of the excess carriers in the dots modifies the electron-hole interaction leading to unusual spin polarization properties like negative spin polarization. From our experiments we found that the sign and magnitude of the spin polarization depended on the excitation energy and the QD transition involved. Therefore, to understand the spin dynamics in doped QDs and the role of excited states, we performed photoluminescence quenching measurements using circularly polarized interband excitation and temporally synchronized terahertz pulses to induce intersublevel transitions.

The sample studied was an ensemble of n-doped InGaAs/GaAs QDs with an average doping of one electron per dot. The s-p intersublevel energy was tuned to about 20 meV (5 THz) by thermal annealing. A circularly polarized Ti:sapphire laser was used for interband excitation, where the wavelength was adjusted to enable excitation in the barrier, the wetting layer (WL) or the QDs. Terahertz pulses from a free-electron laser (FEL), synchronized to the Ti:sapphire laser, were used to excite intersublevel transitions (~ 20 meV, i.e. 5 THz) in the QDs. A streak-camera coupled to a CCD enabled time and wavelength resolved PL detection.

Time-resolved photoluminescence measurements at ~10 K, for cross-circular (s+s+), i.e. excitation s+ and detection s+, and cross-circular (s+s-) polarizations showed weak negative spin polarization for the QD ground state transition (Figure 1(a)). When excited by long wavelengths, the PL spectrum showed multiple peaks due to selective excitation of the QD ensemble. Some of these features were associated with phonon mediated transitions and exhibited strong positive circular polarization anisotropy (Figure 1(b)). The effect of the FEL pulses on the spin polarization was found to depend on the excitation wavelength. While the strong positive spin polarization for excitation below the wetting layer was drastically reduced by the FEL pulse (as shown by the solid lines in Figure 1 (b)), the weak negative polarization was found to persist. This suggests different mechanisms involved in the generation of spin polarization for different excitation energies. We will discuss the origins of these spin polarizations associated with different transitions in the QD ensemble and their dynamical behaviour. By varying the time-delay of the FEL pulses with respect to the Ti:sapphire laser pulses for interband excitation in the barrier or the WL, we confirmed the existence of trapped carriers. Trapped carriers have been reported to have considerable effect on the PL response of QDs. The FEL pulses induced two competitive phenomena that affect the spin polarization, i.e. (i) diffusion of trapped carriers resulting in increased PL and (ii) intersublevel transitions resulting in a quenching of the PL signal. To summarize, this work investigates the influence of the intradot transitions and interdot diffusion of the trapped carriers on the spin polarization of the QD ensemble.