Exciton Spin Relaxation Time Research Articles

Transient circular dichroism and exciton spin dynamics in all-inorganic halide perovskites

All-inorganic metal halides perovskites (CsPbX3, X = Br or Cl) show strong excitonic and spin-orbital coupling effects, underpinning spin-selective excitonic transitions and therefore exhibiting great promise for spintronics and quantum-optics applications. Here we report spin-dependent optical nonlinearities in CsPbX3 single crystals by using ultrafast pump-probe spectroscopy. Many-body interactions between spin-polarized excitons act like a pseudo-magnetic field and thus lift the degeneracy of spin states resulting in a photoinduced circular dichroism. Such spontaneous spin splitting between “spin-up” and “spin-down” excitons can be several tens of milli-electron volts under intense excitations. The exciton spin relaxation time is ~20 picoseconds at very low pump fluence, the longest reported in the metal halides perovskites family at room temperature. The dominant spin-flip mechanism is attributed to the electron-hole exchange interactions. Our results provide essential understandings towards realizing practical spintronics applications of perovskite semiconductors.

Large anisotropic spin relaxation time of exciton bound to donor states in triple quantum wells

We have studied the spin dynamics of a dense two-dimensional electron gas confined in a GaAs/AlGaAs triple quantum well by using time-resolved Kerr rotation and resonant spin amplification. Strong anisotropy of the spin relaxation time up to a factor of 10 was found between the electron spins oriented in-plane and out-of-plane when the excitation energy is tuned to an exciton bound to neutral donor transition. We model this anisotropy using an internal magnetic field and the inhomogeneity of the electron g-factor. The data analysis allows us to determine the direction and magnitude of this internal field in the range of a few mT for our studied structure, which decreases with the sample temperature and optical power. The dependence of the anisotropic spin relaxation was directly measured as a function of several experimental parameters: excitation wavelength, sample temperature, pump-probe time delay, and pump power.

Dipolar excitons indirect in real and momentum space in a GaAs/AlAs heterostructure

For a Schottky-diode structure containing two narrow GaAs (3.5 nm) and AlAs (5 nm) heterolayers, the photoluminescence properties of long-living dipolar excitons, indirect in both real and momentum space, are studied in perpendicular magnetic fields in the Faraday configuration of measurements. With an external perpendicular electric field, the lifetimes of such excitons can be extended to ∼1 μs. Nevertheless the exciton spin subsystem remains nonequilibrium: the exciton spin-relaxation time is even longer. The degree of circular polarization of the photoluminescence attains 80% in a field of 6 T. With an electric field, it is possible to control the degree and sign of the circular polarization.

Exciton lifetime and spin dynamics in type-I In1−xAlxAs/Ga0.67Al0.33As quantum dots: Photoluminescence and pump-probe experiments

The exciton lifetime and spin relaxation have been studied in self-assembled In1−xAlxAs/Ga0.67Al0.33As quantum dots (QDs). Time-resolved photoluminescence and resonant pump-probe measurements were performed, at variable temperature and for different QD aluminium compositions. At low temperature, a long exciton-spin relaxation time has been measured, in agreement with the QD zero-dimensional confinement and the quenching of the relaxation mechanisms. The existence of a quasi-2D regime, in sample with a high QD density, has been observed. The importance of thermally-activated processes toward excited states is also evidenced, for QDs with different compositions and sizes.

Impact of artificial lateral quantum confinement on exciton-spin relaxation in a two-dimensional GaAs electronic system

We demonstrate the effect of artificial lateral quantum confinement on exciton-spin relaxation in a GaAs electronic system. GaAs nanodisks (NDs) were fabricated from a quantum well (QW) by top-down nanotechnology using neutral-beam etching aided by protein-engineered bio-nano-templates. The exciton-spin relaxation time was 1.4 ns due to ND formation, significantly extended compared to 0.44 ns for the original QW, which is attributed to weakening of the hole-state mixing in addition to freezing of the carrier momentum. The temperature dependence of the spin-relaxation time depends on the ND thickness, reflecting the degree of quantum confinement.

Slow exciton spin relaxation in single self-assembledIn1−xGaxAs/GaAsquantum dots

We calculate the acoustic phonon-assisted exciton spin relaxation via spin-orbit coupling in single self-assembled ${\mathrm{In}}_{1\ensuremath{-}x}{\mathrm{Ga}}_{x}\mathrm{As/GaAs}$ quantum dots using an atomistic empirical pseudopotential method. We show that the transition rate from bright to dark exciton states is zero under Hartree-Fock approximation. The exciton spin relaxation time obtained from sophisticated configuration interaction calculations is approximately 15--55 $\ensuremath{\mu}$s in pure InAs/GaAs QDs and even longer in alloy dots. These results are more than three orders of magnitude longer than previous theoretical and experimental results (a few ns), but agree with more recent experiments which suggest that excitons have long spin-relaxation times ($>$1 $\ensuremath{\mu}$s).

Longitudinal and transverse exciton-spin relaxation in a single InAsP quantum dot embedded inside a standing InP nanowire using photoluminescence spectroscopy

We have investigated the optical properties of a single InAsP quantum dot embedded in a standing InP nanowire. A regular array of nanowires was fabricated by epitaxial growth and electron-beam patterning. The elongation of transverse exciton spin relaxation time of the exciton state with decreasing excitation power was observed by first-order photon correlation measurements. This behavior is well explained by the motional narrowing mechanism induced by Gaussian fluctuations of environmental charges in the InP nanowire. The longitudinal exciton spin relaxation time was evaluated by the degree of the random polarization of emission originating from exciton state confined in a single nanowire quantum dots by using Mueller Calculus based on Stokes parameters representation.

Theory of excitons in cubic III-V semiconductor GaAs, InAs and GaN quantum dots: Fine structure and spin relaxation

Exciton fine structures in cubic III-V semiconductor GaAs, InAs and GaN quantum dots are investigated systematically and the exciton spin relaxation in GaN quantum dots is calculated by first setting up the effective exciton Hamiltonian. The electron-hole exchange interaction Hamiltonian, which consists of the long- and short-range parts, is derived within the effective-mass approximation by taking into account the conduction, heavy- and light-hole bands, and especially the split-off band. The scheme applied in this paper allows the description of excitons in both the strong- and weak-confinement regimes. The importance of treating the direct electron-hole Coulomb interaction unperturbatively is demonstrated. We show in our calculation that the light-hole and split-off bands are negligible when considering the exciton fine structure, even for GaN quantum dots, and the short-range exchange interaction is irrelevant when considering the optically active doublet splitting. We point out that the long-range exchange interaction, which is neglected in many previous works, contributes to the energy splitting between the bright and dark states, together with the short-range exchange interaction. Strong dependence of the optically active doublet splitting on the anisotropy of dot shape is reported. Large doublet splittings up to 600 $\ensuremath{\mu}$eV, and even up to several meV for small dot size with large anisotropy, are shown in GaN quantum dots. The spin relaxation between the lowest two optically active exciton states in GaN quantum dots is calculated, showing a strong dependence on the dot anisotropy. Long exciton spin relaxation time is reported in GaN quantum dots. These findings are in good agreement with the experimental results.

Optical alignment of the exciton in ZnO nanoparticles

The exciton spin dynamics of zinc oxide nanoparticles (NPs) of sizes ranging from 2.3 to 6.6 nm has been studied by time-resolved photoluminescence. Following a quasiresonant linearly polarized excitation, the exciton photoluminescence of an ensemble of NPs exhibits a linear polarization of 15%, demonstrating the optical alignment of exciton in zinc oxide NPs. Within the accuracy of our experimental setup, no decay time of the linear polarization is observed on the exciton lifetime scale, reflecting an exciton spin relaxation time longer than 1 ns.

Observation of Spin Relaxation in InGaAs/AlAsSb Quantum Wells

We have investigated the exciton spin relaxation in InGaAs/AlAsSb quantum wells using time-resolved spin-dependent pump and probe reflectance measurements. The spin relaxation time of 1.46 µm electron-heavy hole excitons at room temperature is obtained to be 27–54 ps for an excitation power of 20–100 mW. The carrier density dependence of the exciton spin relaxation time was clearly observed, suggesting that the spin relaxation mechanism is strongly related to the Bir–Aronov–Pikus process at room temperature.

Polarization conversion of excitonic photoluminescence under zero and nonzero magnetic fields in single InAlAs quantum dots

We demonstrated the circular to linear polarization conversion in single InAlAs/AlGaAs QDs. Since the anisotropic exchange interaction acts on the photo-created exciton spin as an in-plane effective magnetic field, the polarization of emissions can be converted from circular to linear and vice versa even under a zero external magnetic field. By using the quasi-resonant excitation, high conversion ratio up to ∼ 50 % was obtained under nonzero longitudinal magnetic field. Additionally, exciton spin relaxation time and built-in linear dichroism were estimated by exciton spin dynamics in the 3D pseudospin precession model.

Quasiresonant exciton spin orientation and alignment in a single quantum dot under zero and nonzero magnetic fields

The polarization conversion from optical orientation to alignment in single InAlAs/AlGaAs quantum dots (QDs) was studied in detail under zero and nonzero magnetic fields. Under the influence of the effective magnetic field, bright exciton doublets precess in pseudospin space, where the torque vector is composed of the external magnetic field and the anisotropic exchange field. For a number of QDs, we measured the angle difference of the polarization axes obtained with circularly polarized excitations, which is an indicator of the conversion efficiency under a zero magnetic field. By applying a longitudinal magnetic field, a high conversion efficiency of $\ensuremath{\sim}50%$ was achieved. Additionally, the exciton spin-relaxation time and the magnitude of built-in linear dichroism were estimated from the exciton spin dynamics using the three-dimensional pseudospin precession model, and a long spin-relaxation time exceeding the recombination lifetime was obtained. Finally, we discussed the influence of a nuclear magnetic field on the polarization conversion.

Spin Transport of Excitons

We report on observation of the spin transport of spatially indirect excitons in GaAs/AlGaAs coupled quantum wells (CQW). Exciton spin transport over substantial distances, up to several micrometers in the present work, is achieved due to orders of magnitude enhancement of the exciton spin relaxation time in CQW with respect to conventional quantum wells.

Exciton and hole spin dynamics in ZnO investigated by time-resolved photoluminescence experiments

The carrier spin dynamics in ZnO is investigated by time-resolved optical orientation experiments. We evidence a clear circular polarization of the donor-bound exciton luminescence in both ZnO epilayer and nonintentionally doped bulk ZnO. This allows us to measure the localized hole spin relaxation time. We find h s 350 ps at T = 1.7 K in the ZnO epilayer. The strong energy and temperature dependences of the photoluminescence polarization dynamics are well explained by the fast free exciton spin relaxation time and the ionization of bound excitons.

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

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.

Exciton-spin relaxation in quantum dots due to spin-orbit interaction

We present theoretical results for the spin relaxation of exciton-bound electrons and holes in weakly confining quantum dots. The relaxation is driven by the spin-orbit interaction in the conduction band and the linear in the momentum term in the valence band, respectively. The relaxation occurs between the optically active (bright) and inactive (dark) exciton states due to acoustic-phonon-assisted spin flips. The exchange splitting between the bright and dark states acts as a constant external magnetic field. A sequential flip of the (exciton-bound) electron and hole spins results in the spin-flip transition between the bright exciton states (i.e., an exciton-spin relaxation). We find that the spin relaxation time for an exciton-bound electron is several orders of magnitude faster than for a single electron. The resulting exciton spin-relaxation time is also several orders of magnitude faster than the one in small dots which is driven by the electron hole exchange interaction. We obtain the dependence of the exciton-spin relaxation time on dot size and temperature.

Fast exciton spin relaxation in single quantum dots

Exciton spin relaxation is investigated in single epitaxially grown semiconductor quantum dots in order to test the expected spin relaxation quenching in this system. We study the polarization anisotropy of the photoluminescence signal emitted by isolated quantum dots under steady-state or pulsed non-resonant excitation. We find that the longitudinal exciton spin relaxation time is strikingly short ($\leq$100 ps) even at low temperature. This result breaks down the picture of a frozen exciton spin in quantum dots.

Exciton spin relaxation dynamics in InGaAs∕InP quantum wells

We have investigated the exciton spin relaxation mechanism between 13 and 300K in InGaAs∕InP quantum wells using time-resolved spin-dependent pump and probe absorption measurements. The exciton spin relaxation time, τs above 40K was found to depend on temperature, T, according to τs∝T−1.1, although the spin relaxation time is constant below 40K. The clear carrier density dependence of the exciton spin relaxation time was observed below 40K, although the carrier density dependence is weak above 40K. These results imply that the main spin relaxation mechanism above and below 40K are the D’yakonov–Perel’ process and the Bir–Aronov–Pikus process, respectively.

Optically controlled magnetization of zero‐dimensional magnetic polarons in CdMnTe self‐assembled quantum dots

AbstractWe find strong optically induced magnetization of zero‐dimensional magnetic polarons confined to CdMnTe quantum dots. The spin alignment of magnetic ions at B = 0 T is created through circularly polarized resonant excitation and it is probed by polarization‐resolved photoluminescence. This observation clearly demonstrates that in the CdMnTe quantum dots the exciton spin relaxation time is significantly longer than the formation time of magnetic polaron. Importantly, the spatial confinement characteristic of quantum dots increases the stability of magnetic polarons so that significant polarizaton of the emission is observed for temperatures higher than 120 K. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)