Exciton Spin-flip Research Articles

Intramolecular Sensitization Assisting High-Efficiency TADF Conjugated Polymers with Accelerating Exciton Spin Flip for Solution-Processed Electroluminescent Devices

Intramolecular Sensitization Assisting High-Efficiency TADF Conjugated Polymers with Accelerating Exciton Spin Flip for Solution-Processed Electroluminescent Devices

Theory of acoustic-phonon involved exciton spin flip in perovskite semiconductors

We present a theory of the acoustic phonon assisted spin-flip Raman scattering (SFRS), or resonant photoluminescence with the spin flip of a photoexcited exciton localized in a bulk cubic-phase perovskite semiconductor. We consider the spin-flip transitions between the ground-state exciton spin sublevels in external magnetic field B and discuss the variation of their probability rate and polarization selection rules with the increase of B. The transitions are treated as two-quantum processes with the virtual to and fro transfer of the electron in the electron-hole pair between the bottom and first excited conduction bands. The transfer occurs due to both the electron-hole exchange interaction and the electron-phonon interaction. The theoretical results allow one to distinguish the phonon assisted Raman scattering from (a) the resonant Raman scattering with the combined spin flip of the localized resident electron and hole and (b) the biexciton-mediated SFRS analyzed previously.

Optical control of a Mn spin embedded in a quantum dot

We demonstrate a high degree of an optical spin preparation of a single Mn atom embedded in a CdTe/ZnTe quantum dot (QD). Due to the strong exchange interaction of the manganese atom with an exciton injected into the QD the spin orientation can be achieved by quasi-resonant or fully-resonant optical creation of the polarized electron-hole pairs. A measured spin memory of the isolated Mn atom, in most of the cases, is in the microsecond range, and depends on the built-in strain in the quantum dot. During the resonant optical pumping process exciton spin-flip can occur without a change of the Mn spin providing a way to directly read the dynamics of the pumped spin state. The manganese spin orientation is achieved in a few tens of ns.

Two-Photon Coherent Spin Flip and Polarization Rotation of Excitons in Quantum Dots

It is shown that the spin orientation and polarization of an exciton confined in a QD can be controlled via two-photon transitions, by virtually coupling the single-exciton states to the detuned ground and biexciton states. Furthermore, phonon-induced dephasing leading to loss of fidelity in the two-photon optical control schemes is studied. Exciton spin flip can be performed with errors as low as 10−3.

High-resolution spin-flip Raman scattering in CdTe quantum wells atH3etemperature

The high-resolution spin-flip Raman-scattering experiments were carried out on high-quality CdTe quantum wells below 0.5 K in high magnetic fields up to 14 T. An obvious exciton spin-flip Raman-scattering signal was observed in each quantum well under the Voigt configuration. The exciton exchange splitting in each quantum well was precisely determined.

Gigahertz bandwidth electrical control over a dark exciton-based memory bit in a single quantum dot

An optical write-store-read process is demonstrated in a single InGaAs quantum dot within a charge-tunable device. A single dark exciton is created by nongeminate optical excitation allowing a dark exciton-based memory bit to be stored for over ∼1 μs. Read-out is performed with a gigahertz bandwidth electrical pulse, forcing an electron spin-flip followed by recombination as a bright neutral exciton, or by charging with an additional electron followed by a recombination as a negative trion. These processes have been used to determine accurately the dark exciton spin-flip lifetime as it varies with static electric field.

Relaxation in the Exciton Fine Structure of Semiconductor Nanocrystals

A spectroscopic study is reported of relaxation in the exciton fine structure of CdSe nanorods measured using ultrafast cross-polarized heterodyned third-order transient grating (CPH-3TG) spectroscopy. The CPH-3TG spectroscopy probes the dynamics of population transfer between states accompanying exciton spin flip (radiationless transitions in the exciton fine structure associated with changes in the sign of the total angular momentum). Analysis of the data enabled elucidation of pathways for exciton fine structure relaxation (EFSR) involving all of the exciton fine structure states. The mechanism and origin of the EFSR dynamics in CdSe nanorods are discussed in the exciton state picture with an analogy to molecular radiationless transition processes such as internal conversion and intersystem crossing. Key conclusions are that the fast transitions are between states with total angular momentum of ±1 and ∓2 and that the rate of this transition follows an inverse diameter to the fourth power size dependence, which originates from the Dresselhaus spin−orbit coupling effect.

Mechanism and Origin of Exciton Spin Relaxation in CdSe Nanorods

The dynamics of exciton spin relaxation in CdSe nanorods of various sizes and shapes are measured by an ultrafast transient polarization grating technique. The measurement of the third-order transient grating (3-TG) signal utilizing linear cross-polarized pump pulses enables us to monitor the history of spin relaxation among the bright exciton states with a total angular momentum of F = +/-1. From the measured exciton spin relaxation dynamics, it is found that the effective mechanism of exciton spin relaxation is sensitive to the size of the nanorod. Most of the measured cross-polarized 3-TG signals show single-exponential spin relaxation dynamics, while biexponential spin relaxation dynamics are observed in the nanorod of the largest diameter. This analysis suggests that a direct exciton spin flip process between the bright exciton states with F = +/-1 is the dominant spin relaxation mechanism in small nanocrystals, and an indirect spin flip via the dark states with F = +/-2 contributes as the size of the nanocrystal increases. This idea is examined by simulations of 3-TG signals with a kinetic model for exciton spin relaxation considering the states in the exciton fine structure. Also, it is revealed that the rate of exciton spin relaxation has a strong correlation with the diameter, d, of the nanorod, scaled by the power law of 1/d4, rather than other shape parameters such as length, volume, or aspect ratio.

Exciton spin–flip processes in single quantum dots

We study the relaxation of the exciton spin (longitudinal spin–relaxation time T1) in single asymmetrical quantum dots (QD) at low temperatures. The main relaxation mechanism is due to the exciton spin–acoustic phonon coupling via the strain–dependent exchange interaction; piezoelectric coupling is less efficient. For zero magnetic field, relaxation within the radiative exciton–doublet for typical QDs is very slow compared to the exciton lifetime. Relaxation to nonradiative states becomes important for QDs with large singlet–triplet splitting of a few meVs. The calculated relaxation times strongly decrease in high magnetic fields. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Spin Relaxation of Negatively Charged Excitons in CdTe Quantum Wells

We investigate the spin relaxation of negatively charged excitons in modulation-doped CdTe quantum wells by time-resolved photoluminescence. The charged exciton spin flip process is found to be very sensitive to the excitation energy but not to a change of electron concentration. We exclude the Bir-Aronov-Pikus mechanism, which is based on the exchange interaction, as responsible for the X- spin flip.

Excitonic resonant spin-flip Raman scattering in Cd 1-xMn xTe multilayers

We report the observation of resonant spin-flip Raman scattering due to localized heavy-hole excitons in quasi-bulk Cd 1-xMn xTe multilayers. The exciton- and electron spin-flip energies were compared with excitonic Zeeman splittings obtained from magnetoreflectivity measurements on the same sample. A good agreement was observed between the electron spin splitting values obtained from both experiments. The exciton spin-flip energy was found to be slightly reduced with respect to the Zeeman splitting value. Possible mechanisms of this reduction are discussed.

Buried single CdTe/CdMnTe quantum dots realized by focused ion beam lithography

Buried single CdTe/CdMnTe quantum dots are realized by implantation-induced intermixing using a focused 100 keV Ga+ ion beam. For an implantation dose of 5×1013 cm−2 and an annealing temperature of 390 °C, a lateral potential depth of about 65 meV is obtained. By means of photoluminescence spectroscopy, the formation of zero-dimensional multiexcitons in single quantum dots is investigated, yielding a biexciton binding energy of about 3.5 meV. In addition, the occurrence of an excited biexciton transition in the photoluminescence spectrum gives clear evidence of a suppressed exciton spin-flip process in quantum dots.

Magnetic-field-induced resonance in four-wave mixing in GaAs.

The frequency-domain four-wave-mixing response of excitons in GaAs in a magnetic field is theoretically and experimentally studied. For linearly cross-polarized excitation beams, the application of a magnetic field induces a resonance that leads to the observation of a narrow dip in the primary resonance. The primary resonance has a width given by the exciton spin-flip rate while the narrow dip has a width determined by the recombination rate. The results reflect the dynamics of two observables defined in terms of the appropriate Fourier components of the net population and the net population transfer between the two oppositely spin-oriented excitonic states, similar to the normal decay modes of the system. The relative amplitude of these observables is determined by the relative polarization of the incident fields and the strength of the applied magnetic field.