Dynamics of Coherent and Incoherent Spin Polarizations in Ensembles of Quantum Dots

Abstract

The temperature dependence of spin coherence in InGaAs quantum dots is obtained from quantum beats observed in polarization-resolved pump-probe experiments. Within the same sample we clearly distinguish between coherent spin dynamics leading to quantum beats and incoherent long-lived spin-memory effects. Analysis of the coherent data using a theoretical model reveals approximately 10 times greater stability of the spin coherence at high temperature compared to that found previously for exciton states in four-wave-mixing experiments by Borri et al. [Phys. Rev. Lett. 87, 157401 (2001)]]. The data on incoherent polarization reveal a new form of spin memory based on charged quantum dots.

Optical anisotropy of cubic crystals, from bulk to quantum dots

“…The circular and linear polarizations thus beat in antiphase, decaying with spin coherence time t s to zero. This has been partly verified previously in quantum beat experiments [37,38] where precession of the linear (circular) polarization component excited with linearly (circularly) polarized light at Larmor frequency O was observed.…”

Fast spin relaxation in InGaN/GaN multiple quantum wells

“…Although an exact trend is not obvious from the data, the spin decoherence rate is observed to increase immediately as the temperature is raised above 5 K. This happens because after the system is prepared in a defined spin state by the exciting pump pulse, a fast loss of amplitude occurs due to fluctuations in the precessional frequency via phonon and impurity scattering. This is unlike the case for InGaAs quantum dots [9] where the data shows no change for temperatures below 50 K because of the dominance of inhomogeneous effects. The apparent plateau in our data above 60 K is most likely due to the fundamental time resolution of the experiment.…”

“…Figure 2(b) plots the degree of (or net) circular polarisation defined in . Such behaviour has been previously observed in a variety of quantum dots systems from In(Ga)As [8,9] to InP [10]. In the case of InP dots, the quantum beats understandably occur in the presence of a magnetic field.…”

“…However in the In(Ga)As system, quantum beats are present for neutral dots with no external fields. These have been identified as arising from fine structure splittings of the ground state exciton [8,9]. Following these works, we suggest that the spin quantum beat behaviour we observe is likely to arise from a fine structure splitting of the exciton states.…”

Spin injection in lateral InAs quantum dot structures by optical orientation spectroscopy

“…Therefore, the observed PL polarization must arise from the QDs with strongly reduced exchange interaction so that the circularly polarized ground states are recovered. The probable candidates are charged dots 21,22 , which may exist within the QD ensemble due to unintentional doping during the MBE growth. In the charged dots, the injected electron-hole pair forms, together with the resident carrier, a trion in which the exchange interaction is cancelled 23 .…”