Exciton Spin Relaxation Research Articles

Time-resolved spin-polarization spectroscopy in GaAs/AlGaAs quantum wells

We present a study of spin relaxation of excitons in a set of nominally undoped GaAs/AlGaAs quantum well (QW) structures by using time-resolved photoluminescence techniques. The initial polarization and the spin relaxation time have been measured varying the excitation energy between the heavy- and light-hole exciton of each well, at a temperature of 4 K. Typical spin relaxation times in the 50–120 ps range are found when increasing the well width from 25 to 90 Å; however no dependence of the spin relaxation time on the excitation wavelength has been observed.

Subpicosecond spin relaxation dynamics of excitons and free carriers in GaAs quantum wells

We have obtained a coherent understanding of spin relaxation processes of electrons, holes, and excitons in quantum walls by investigating subpicosecond dynamics of luminescence polarization. We show that the spin behavior for electrons and holes in quasi-two-dimensional systems is distinct from that in bulk semiconductors and that many-body effects and formation process play an important role in exciton spin relaxation.

Spin relaxation and thermalization of excitons in GaAs quantum wells

We present results on the picosecond dynamics of spin relaxation and thermalization of excitons in GaAs quantum wells. The spin relaxation time constant is ≊50 ps for resonant excitation and less than 10 ps for nonresonant excitation, considerably shorter than those for free electrons in the bulk. Photoexcited cold, nonthermal excitons thermalize in ≊50 ps at 10 K and less than 10 ps at 35 K. Exciton-acoustic phonon energy exchange rates are determined from these measurements.

Direct observation of picosecond spin relaxation of excitons in GaAs/AlGaAs quantum wells using spin-dependent optical nonlinearity

We have directly observed spin relaxation of excitons in the picosecond region using time-resolved polarization absorption measurements. With the help of spin-dependent optical nonlinearity of excitonic absorption, we obtained a fast decay of spin-up carriers and a fast accumulation of spin-down carriers with a spin relaxation time of 32 ps for a GaAs/Al0.51Ga0.49As multiple quantum well at room temperature with a time resolution of 1 ps.

Spin relaxation of triplet excitons in molecular crystals

The primary electronic spin relaxation mechanism for triplet excitons in many molecular crystals arises from hopping transport between two orientationally inequivalent sites and is the source of both EPR linewidth and level equilibration. A generalized stochastic theory of resonance linewidth due to Blume which was previously applied to this mechanism and shown in its random-phase approximation (RPA) to yield the exciton EPR linewidth spectrum is here shown to give a complete formulation of both linewidth and equilibration rate. The method employs the averaged time-development superoperator of Blume's theory in the RPA in order to construct an equation of motion for the appropriately averaged spin-density matrix from which these two can be extracted. We show associations between this work and related studies by Kubo and Suna. Comparison is made between rates calculated for anthracene and values of ${T}_{1}$ deduced by Haarer and Wolf from a Bloch analysis of their EPR saturation measurements.

The effect of spin relaxation on triplet-triplet annihilation and delayed fluorescence in molecular crystals

It is generally necessary to include the effects of the magnetic field dependence of triplet exciton spin relaxation, and especially its anisotropy, in order to explain the magnetic field modulation of delayed fluorescence in molecular crystals. It is then possible to account for several observations made in recent years on naphthalene, pyrene and 1-4-dibromonaphthalene.