Optical Orientation Of Excitons Research Articles

Optical Alignment and Optical Orientation of Excitons in CdSe/CdS Colloidal Nanoplatelets.

Optical alignment and optical orientation of excitons are studied experimentally on an ensemble of core/shell CdSe/CdS colloidal nanoplatelets. Linear and circular polarization of photoluminescence during resonant excitation of excitons is measured at cryogenic temperatures and with magnetic fields applied in the Faraday geometry. The developed theory addresses the optical alignment and optical orientation of excitons in colloidal nanocrystals, taking into account both bright and dark exciton states in the presence of strong electron-hole exchange interaction and the random in-plane orientation of nanoplatelets within the ensemble. Our theoretical analysis of the obtained experimental data allows us to evaluate the exciton fine structure parameters, the g-factors, and the spin lifetimes of the bright and dark excitons. The optical alignment effect enables the identification of the exciton and trion contributions to the emission spectrum, even in the absence of their clear separation in the spectra.

Optical Orientation of Excitons in a Longitudinal Magnetic Field in Indirect-Band-Gap (In,Al)As/AlAs Quantum Dots with Type-I Band Alignment.

Exciton recombination and spin dynamics in (In,Al)As/AlAs quantum dots (QDs) with indirect band gap and type-I band alignment were studied. The negligible (less than 0.2 μeV) value of the anisotropic exchange interaction in these QDs prevents the mixing of the excitonic basis states and makes the formation of spin-polarized bright excitons possible under quasi-resonant, circularly polarized excitation. The recombination and spin dynamics of excitons are controlled by the hyperfine interaction between the electron and nuclear spins. A QD blockade by dark excitons was observed in the magnetic field, that eliminates the impact of nuclear spin fluctuations. A kinetic model which accounts for the population dynamics of the bright and dark exciton states as well as for the spin dynamics was developed to quantitatively describe the experimental data.

Optical orientation of excitons and carriers in quantum dots

The optical orientation (optical pumping) of excitons and carriers in semiconductor quantum dots (QDs) is discussed. A brief review of main experimental studies on optical pumping is given. We discuss different manifestations of size quantization on spin phenomena— enhancement of the electron–hole exchange interaction, hyperfine interaction between electron and nuclear spins, etc. The peculiarities of spin polarization in doped QDs are considered. It is demonstrated that the Hanle effect is a powerful technique for the study of spin dynamics.

Optical Orientation of Trions in Charge-Tunable InAs/GaAs Quantum Dots

We report on optical orientation of excitons and trions (singly charged exciton) in individual charge-tunable self-assembled InAs/GaAs quantum dots. When the number of electrons varies from 0 to 2, the trion photoluminescence under quasi-resonant excitation gets progressively polarized from zero to≈ 100%. We discuss this behavior as the efficient quenching of exciton spin quantum beats in anisotropic quantum dots due to the trion formation. This result indicates a long hole-spin relaxation time larger than the radiative lifetime, confirmed by time-resolved photoluminescence measurements carried out on a quantum dots ensemble.

Observation of exchange interaction effects under optical orientation of excitons in AlGaAs

Exciton luminescence in AlGaAs layers is studied under interband excitation by circularly polarized light. Curves of luminescence depolarization in a transverse magnetic field (Hanle effect) exhibit peaks arranged symmetrically about a point H = 0. It is shown that this effect is attributable to crossings of fine-structure levels in the magnetic field. The exchange splitting of bulk exciton levels and also recombination and spin-relaxation times are determined from a comparison between theoretical and experimental dependences.

Coherent control in semiconductor nanostructures

This paper reviews recent experimental results on temporal coherent control of electronic excitations in semiconductor nanostructures. We demonstrate that the optical orientation of excitons can be coherently controlled and directly observed in a resonant time-resolved Secondary Emission experiment. Optical and quantum interferences are clearly distinguished in this coherent control experiment which relies on the linear optical response of the crystal.

Coherent Manipulation of the Exciton Orientation in Quantum-Wells

We demonstrate that the optical orientation of excitons can be coherently controlled and directly observed in a resonant time-resolved photoluminescence experiment. The optical dephasing time of excitons, their longitudinal and transverse spin relaxation times, and their radiative lifetime, are measured with strictly the same experimental conditions. This technique, which relies on the linear response of the crystal, is used to investigate the spin-dependent exciton–exciton interactions.

Determination of interface preference by observation of linear-to-circular polarization conversion under optical orientation of excitons in type-II GaAs/AlAs superlattices

A comprehensive investigation of exciton optical orientation and alignment in type-II GaAs/AlAs(001) superlattices in longitudinal and transverse magnetic fields is presented. We observe the previously predicted type-II orientation-to-alignment conversion induced by a longitudinal field due to the anisotropic exchange splitting of the localized-exciton radiative doublet. A theory of polarized exciton photoluminescence under polarized excitation is developed, that takes into account the fine structure of the excitonic quartet, the exciton spin relaxation, and the difference in the lifetimes of electric-dipole-active and inactive states. By comparing experimental and theoretical curves, the main parameters characterizing the level splitting, the recombination rates, and the spin relaxation of localized excitons are deduced. Field-induced polarization conversion provides an effective method to measure the interface preference, i.e., the difference between the fractions of excitons localized on AlAs/GaAs and GaAs/AlAs interfaces. We have also observed the field-induced transformation from [100] to [110] linear polarization, which is direct evidence of exciton cascade kinetics.

Hanle effect in an asymmetric CdTe/CdMnTe double quantum well

The observation of optical orientation of excitons and the Hanle effect in an asymmetric CdTe/CdMnTe double quantum well is reported. The characteristics of the magnetic depolarization of radiation from CdTe/CdMnTe quantum wells are discussed.

Optical orientation of excitons in quantum wells with semimagnetic barriers

Optical orientation of excitons in quantum wells CdTe CdMnTe has been observed in a steady-state regime. The luminescence polarization arising due to optical orientation of exciton angular momenta under optical pumping has been studied as a function of excitation quantum energy and transverse magnetic field. The magnetic depolarization of exciton radiation has been qualitatively understood by separate decay of electron and hole angular momenta taking into account the magnetic field induced mixing of involved states. The observation of luminescence linear polarization confirms this interpretation.

Optical orientation of excitons in (Cd,Mn)Se and (Cd,Mn)Te

This paper reports 2 K optical pumping studies of luminescence by localized excitons in Cd 1−xMn xSe and Cd 1−xMn xTe. The luminescence is weakly polarized when the crystals are excited with circularly polarized light at or above the free exciton energy, implying free carrier spin relaxation times in the picosecond range. The signal abruptly increases when the pump laser frequency is tuned below the free exciton. Peak polarizations of 50 and 15% are observed in (Cd,Mn)Se and (Cd,Mn)Te. With low energy pumping, excitons can only be created in regions of magnetic fluctuation whose large internal fields make spin flip of the hole energetically unfavorable; the effect is enhanced by BMP formation. Exciton spins “trapped” in this way have relaxation times comparable to the exciton lifetime.

Optical orientation of excitons in CdS

The optical orientation by circularly polarized light of free and bound excitons in CdS is reported and discussed. The observed degrees of polarization are generally large and vary significantly with the exciting energy (a "negative" polarization is sometimes obtained). These results, together with the effects of a magnetic field, provide a lot of information about the levels excited, the luminescent levels (lifetimes and disorientation times), and the transfer of polarization from the first to the second. The optical orientation by linearly polarized light is also observed for the first time in solids. It is shown why these unusual effects can be seen on free excitons and not on other levels. The additional information that they provide leads to a better comprehension of the mechanisms of thermalization of the excitons, and of the symmetry properties of $K\ensuremath{\ne}0$ exciton states.