Exciton Spin Relaxation Research Articles

Single-spin optical read-out in CdTe/ZnTe quantum dot studied by photon correlation spectroscopy

Received 9 February 2008; revised manuscript received 5 May 2008; published 5 June 2008Spin dynamics of a single electron and an exciton confined in CdTe/ZnTe quantum dot is investigated bypolarization-resolved correlation spectroscopy. Spin memory effects extending over at least a few tens ofnanoseconds have been directly observed in magnetic field and described quantitatively in terms of a simplerate equation model. We demonstrate an effective 68% all-optical read-out of the single carrier spin statethrough probing the degree of circular polarization of exciton emission after the capture of an oppositelycharged carrier. The perturbation introduced by the pulsed optical excitation serving to study the spin dynamicshas been found to be the main source of the polarization loss in the read-out process. In the limit of low laserpower, the read-out efficiency extrapolates to a value close to 100%. The measurements allowed us as well todetermine neutral exciton spin relaxation time ranging from 3.4 0.1 ns at

Excitonic spin relaxation in GaN

AbstractBy performing non‐degenerate pump‐probe experiments, we study the relaxation dynamics of spin‐polarized A and B excitons in wurtzite epitaxial GaN. By analyzing the differential reflection of the two circularly polarized probe components, we extract characteristic times of the different spin relaxation processes. We show that the spin relaxation of the exciton as a whole is negligible in regard to the spin relaxation of the individual carriers. We determined Te = 15 ps for the electron in the conduction band, Thh = 5 ps, and Tlh = 1.5 ps for the heavy hole and for the light hole, respectively. The quite long HH relaxation time can be related to the band structure in which the degeneracy between different spin valence bands is lifted. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Experimental investigation of excitonic spin relaxation dynamics in GaN

By performing nondegenerate pump-probe experiments, we study the relaxation dynamics of spin-polarized A and B excitons in wurtzite epitaxial GaN. By analyzing the differential reflectivity spectra $(\ensuremath{\Delta}R∕R)$ of the two circularly polarized probe components, we are able to identify each spin relaxation channel (electron, light- and heavy-hole spin flips) separately and to extract characteristic times of the different spin relaxation processes. In addition, spectral oscillatory features are observed for negative delays. They show a rapid rise determined by the fast dephasing time ${T}_{2}$ of the excitonic transitions. We show that the high density of dislocations increases the spin relaxation of electrons and holes through the Elliot--Yafet mechanism and makes the exciton dephasing time very short. The measured heavy-hole relaxation time, which is not extremely short compared to the electron relaxation time, can be related to the band structure, in which the degeneracy between different spin valence bands is partially lifted.

Localized and free exciton spin relaxation dynamics in GaInNAs∕GaAs quantum well

We have investigated the exciton spin relaxation in a GaInNAs∕GaAs quantum well. The recombination from free and localized excitons is resolved on the basis of an analysis of the photoluminescence characteristics. The free exciton spin relaxation time is measured to be 192ps at 10K, while the localized exciton spin relaxation time is one order of magnitude longer than that of the free exciton. The dependence of the free exciton spin relaxation time on the temperature above 50K suggests that both the D’yakonov–Perel’ and the Elliot–Yafet effects dominate the spin relaxation process. The temperature independence below 50K is considered to be due to the spin exchange interaction. The ultralong spin relaxation time of the localized excitons is explained to be due to the influence of nonradiative deep centers.

Radiative and nonradiative recombination kinetics of indirect bound excitons studied bytime-resolved level anticrossing experiments

Time-resolved investigation of the level anticrossing effect in exciton emission is first usedto determine separately the radiative and nonradiative indirect bound exciton lifetimes in asemiconductor crystal using GaSe as an example. A theoretical treatment of theexperimental level anticrossing signal is carried out for different instances of theindirect bound exciton lifetime. The indirect bound exciton radiative lifetimeτr = 4.8 µs is found to be about five times shorter than the nonradiative lifetimeτ0 = 19 µs and about two orders of magnitude longer than the radiative lifetimeof direct bound excitons in the same compound. Within the two-levelscheme, the value of the indirect bound exciton spin relaxation timeT1 is assessedto be 100 µs.

FINE STRUCTURE OF EXCITED EXCITONIC STATES IN QUANTUM DISKS

We report on a theoretical study of the fine structure of excited excitonic levels in semiconductor quantum disks. A particular attention is paid to the effect of electron–hole long-range exchange interaction. We demonstrate that even in the axisymmetric quantum disks, the exciton P-shell is split into three sublevels. The analytical results are obtained in the limiting cases of strong and weak confinement. A possibility of exciton spin relaxation due to the resonant LO-phonon-assisted coupling between the P and S shells is discussed.

Dynamics of exciton-spin injection, transfer, and relaxation in self-assembled quantum dots of CdSe coupled with a diluted magnetic semiconductor layer ofZn0.80Mn0.20Se

We study the dynamics of exciton-spin injection, transfer, and relaxation in self-assembled quantum dots (QDs) of CdSe coupled with a diluted magnetic semiconductor (DMS) layer of ${\mathrm{Zn}}_{0.80}{\mathrm{Mn}}_{0.20}\mathrm{Se}$, where spin-polarized excitons can be injected from the DMS into the QDs. The degree of circular polarization $P$ of excitonic photoluminescence (PL) at $5\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ in the coupled QDs exhibits a rapid increase with increasing delay time, up to $+0.3$ at $25\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ after the pulse excitation of the DMS by a linearly polarized light. This development of a positive $P$ value directly reflects the spin-injection dynamics from the DMS, since the intrinsic polarization of the QD excitons due to Zeeman splitting is $P\ensuremath{\sim}\ensuremath{-}0.1$ when only the QDs are selectively excited. The $P$ value gradually decays with time after reaching its maximum, as a result of the exciton-spin relaxation with a time constant of $800\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ in the QDs. Time-resolved circularly polarized PL spectra immediately after the pulse excitation directly show the exciton-energy dependence of the spin-injection dynamics in the QD ensemble, where two-dimensional-like QDs with higher exciton energies show higher receptivity to the spin-polarized excitons than three-dimensional-like dots with lower exciton energies. A rate equation analysis reveals all time constants responsible for the spin-injection dynamics. We deduce a time constant of $10\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ for the spin injection. The spin-injection efficiency of 0.94 is also obtained, which corresponds to the ratio between the number of the spin-polarized excitons responsible for the rise of the positive $P$ value in the QD emission and the total number of the excitons injected from the DMS. Moreover, we observe that interdot exciton transfer significantly affects the $P$ value within the QD emission band after the fast spin injection, in addition to the spin relaxation within the QDs.

Electron- and hole-spin relaxation within excitons in GaAs quantum wells by non-degenerate pump-and-probe measurements

We report on an experimental investigation of electron-, hole-, and exciton-spin relaxation times in intrinsic GaAs quantum wells. We present non-degenerate pump-and-probe measurements followed by a detailed analysis of the differential transmission spectra in order to identify different spin relaxation channels. Dynamics of spin relaxation is generally considered to be multiexponential and therefore a dynamical model taking into account four exciton states is used in order to determine the various relaxation times. In contrast to other experimental work, we also consider transitions to different biexciton states. This allows us to identify spin relaxation processes of electrons, holes, and excitons and to assign them fitted time constants.

Electron spin polarization through interactions between excitons, trions, and the two-dimensional electron gas

We use a two-color transient Kerr rotation technique to study the spin dynamics in an $n$-doped $\mathrm{Cd}\mathrm{Te}∕{\mathrm{Cd}}_{0.85}{\mathrm{Mg}}_{0.15}\mathrm{Te}$ quantum well. The dynamics displays the interplay between excitons, trions, and the two-dimensional electron gas. The spin relaxation of individual species is resolved by spectral selection. The spin dynamics are quantitatively described by rate equations involving the spin populations of excitons, trions, and the electron gas. Under resonant excitation of excitons, spin polarization of the electron gas is generated through trion formation, with the spin coherence partially lost through exciton spin relaxation. A maximum hole spin-flip time is observed around the trion resonance, with a rapid decrease for increasing excitation energy.

Two-phonon assisted exciton spin relaxation due to exchange interaction in spherical quantum dots

Spin relaxation in undoped quasispherical CdS quantum dots at zero magnetic fields is investigated using time- and polarization-resolved transient absorption measurements. Unlike in previous studies of these systems, the measured signals were corrected for spin-insensitive contributions to the exciton bleaching dynamics, allowing us to determine the pure spin-related exciton dynamics. To explain the observed room-temperature spin-relaxation time of several nanoseconds, we propose a mechanism based on intralevel exciton transitions with the emission of one LO phonon, the absorption of another LO phonon, and an electron spin flip, which is driven by the electron-hole exchange interaction. The transition rates, calculated in the present work for different sizes of quantum dots and temperatures, are in fair agreement with our experimental results.

Excitonic spin relaxation in GaN

By performing non-degenerate pump-probe experiments, we study the relaxation dynamics of spin-polarized A and B excitons in wurtzite epitaxial GaN. We show that the spin relaxation of the exciton as a whole is negligible with regard to the spin relaxation of the individual carriers. We determined ps for the electron in the conduction band, ps, and ps for the heavy hole (HH) and for the light hole (LH), respectively. The quite long HH relaxation time can be related to the band structure in which the degeneracy between different spin-valence bands is lifted.

Exciton spin relaxation in GaN observed by spin grating experiment

The authors studied the exciton spin relaxation of bulk GaN by creating spin polarization gratings using degenerate four-wave mixing spectroscopy. The spectrally resolved analysis achieved with this technique facilitated the direct evaluation of spin polarizations in the individual excitons (A and B excitons). The spin polarizations for each exciton decay very quickly (τs∼1ps) at low temperatures. Moreover the τs is faster than the dephasing time T2 throughout the measured temperature range, suggesting the existence of fast intrinsic spin relaxation processes, which can be attributed to a large exchange constant characterized in GaN.

Magneto‐optical spectroscopy of spin injection and spin relaxation in ZnMnSe/ZnCdSe and GaMnN/InGaN spin light‐emitting structures

AbstractIn this paper we review our recent results from in‐depth investigations of physical mechanisms which govern efficiency of several processes important for future spintronic devises, such as spin alignment within diluted magnetic semiconductors (DMS), spin injection from DMS to non‐magnetic spin detectors (SDs) and also spin depolarization within SD. Spin‐injection structures based on II–VIs (e.g. ZnMnSe/Zn(Cd)Se) and III–Vs (e.g. GaMnN/Ga(In)N) were studied as model cases. Exciton spin relaxation within ZnMnSe DMS, important for spin alignment, was found to critically depend on Zeeman splitting of the exciton states and is largely facilitated by involvement of longitudinal optical (LO) phonons. Optical spin injection in ZnMnSe/Zn(Cd)Se was shown to be governed by (i) commonly believed tunneling of individual carriers or excitons and (ii) energy transfer via localized excitons and spatially separated localized electron–hole pairs (LEHP) located within DMS. Unexpectedly, the latter mechanism is in fact found to dominate spin injections. We shall also show that spin depolarization in the studied structures is essentially determined by ef‐ ficient spin relaxation within non‐magnetic spin detectors, which is an important factor limiting efficiency of spin detection. Detailed physical mechanisms leading to efficient spin depolarization will be discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Interwell exciton relaxation in semimagnetic asymmetric double quantum wells

The possibility of magnetic field control of the spectral and polarization characteristics of exciton recombination is examined in Cd(Mg, Mn) Te-based asymmetric double quantum wells. At low fields, the exciton transition in a semimagnetic well is higher in energy than that in a nonmagnetic well and the interwell exciton relaxation is fast. In contrast, when the energy order of the exciton transitions reverses at high fields, unexpectedly slow relaxation of σ− polarized excitons from the nonmagnetic well to the σ+-polarized ground state in the semimagnetic well is observed. Strong dependence of the total circular polarization degree on the heavy-light hole splitting Δhh-lh in the nonmagnetic well is found and attributed to the spin dependent interwell tunneling controlled by exciton spin relaxation. Such a slowing down of the relaxation allows separation of oppositely spin-polarized excitons in adjacent wells.

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.

Internal and external polarization memory loss in single semiconductor quantum dots

By using nonlinear resonant optical control and polarized photoluminescence, the authors studied the intensity dependence of exciton spin relaxation and photoluminescence polarization in single self-assembled InGaAs quantum dots. Data from dots with different dipole moments reveal two distinctive channels for polarization memory loss: (i) an external pathway due to carrier escape and capture to and from the wetting layer that is responsible for memory loss increasing with intensity and (ii) an internal loss channel due to intrinsic spin relaxation. The values obtained rule out a universal freezing of exciton spin relaxation in single self-assembled quantum dots related to the wetting layer.

Exciton‐spin relaxation in weakly confining quantum dots due to spin–orbit interaction

AbstractIn weakly confining quantum structures such as interfacial islands or quantum disks the exciton‐spin relaxation is governed by two independent electron and hole spin flip processes between the optically active and dark states. A microscopic theory for these transitions is presented which is based on second order spin–orbit and carrier–phonon interaction processes. We found that the sequential relaxation between bright and dark states leads to much faster exciton‐spin relaxation than for strongly confining (“small”) quantum dots where the dominant process stems from electron–hole exchange interaction plus hole deformation potential coupling. In addition, the fast exciton spin relaxation implies that the (exciton‐bound) electron spin flip time is also much shorter than for a single electron. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Nanocrystal Shape and the Mechanism of Exciton Spin Relaxation

The rate of exciton spin relaxation (flips) between the bright exciton states (F = +/-1) of CdSe nanocrystals is reported as a function of shape, for dots and nanorods. The spin relaxation is measured using an ultrafast transient grating method with a crossed linearly polarization sequence. It is found that the spin relaxation rate depends on the radius, not length, of the nanocrystals. That observation is explained by deriving an expression for the electronic coupling matrix element that mixes the bright exciton states.

Spin dynamics in a diluted magnetic semiconductor quantum well studied by pump-probe absorption spectroscopy: Magnetic-field-induced suppression of electron-spin relaxation

Exciton spin dynamics is studied in a diluted magnetic semiconductor quantum well of Cd0.95Mn0.05Te by pump-probe absorption spectroscopy under magnetic fields. The time dependences of the saturated absorbance for the higher- and lower-energy spin states of heavy-hole (hh) excitons clarify the following exciton-spin relaxation process in magnetic fields: ultrafast hh-spin relaxation with the formation of dark excitons and subsequent electron-spin relaxation. The electron-spin relaxation due to the s-d exchange mechanism involving Mn spins is suppressed in a high magnetic field by field-induced pinning of the Mn spins.

Transient pump–probe measurements for polarized excitons in strained GaN epitaxial layers

We have studied exciton spin relaxation in strained GaN epilayers on various sapphire substrates using spin-dependent transient reflectivity and three-pulse four-wave mixing measurements. Strikingly fast exciton spin relaxation (<1 ps) at 10 K was observed in the lowest exciton level (A-exciton) both in bulk and GaN layer on c-sapphire. In contrast, the transient signals in the GaN layer on a-sapphire show the break down of the spin polarization caused by the exchange interaction.