Depolarization Of Photoluminescence Research Articles

Femtosecond depolarization of photoluminescence from InAs: Comparison with Cu and monolayer graphene

Femtosecond depolarization of photoluminescence from InAs: Comparison with Cu and monolayer graphene

Theory of optical excitation spectra and depolarization dynamics in bilayerWS2from the viewpoint of excimers

We investigate the optical excitation spectra and the photoluminescence depolarization dynamics in bilayer WS$_2$. A different understanding of the optical excitation spectra in the recent photoluminescence experimentby Zhu {\em et al.} [arXiv:1403.6224] in bilayer WS$_2$ is proposed. In the experiment, four excitations (1.68, 1.93, 1.99 and 2.37 eV) are observed and identified to be indirect exciton for the $\Gamma$ valley, trion, A exciton and B exciton excitations, respectively, with the redshift for the A exciton energy measured to be 30$\sim$50 meV when the sample synthesized from monolayer to bilayer. According to our study, by considering there exist both the intra-layer and charge-transfer excitons in the bilayer WS$_2$, with inter-layer hopping of the hole, there exists excimer state composed by the superposition of the intra-layer and charge-transfer exciton states. Accordingly, we show that the four optical excitations in the bilayer WS$_2$ are the A charge-transfer exciton, ${\rm A}'$ excimer, ${\rm B}'$ excimer and B intra-layer exciton states, respectively, with the calculated resonance energies showing good agreement with the experiment. In our picture, the speculated indirect exciton, which involves a high-order phonon absorption/emission process, is not necessary. Furthermore, the binding energy for the excimer state is calculated to be 40 meV, providing reasonable explanation for the experimentally observed energy redshift of the A exciton. Based on the excimer states, we further derive the exchange interaction Hamiltonian. Then the photoluminescence depolarization dynamics due to the electron-hole exchange interaction is studied in the pump-probe setup by the kinetic spin Bloch equations. We find that ......

Spin depolarization of holes and lineshape of the Hanle effect in semiconductor nanostructures

We report the statistical effect of random in-plane deformations on the magnetic field depolarization of photoluminescence (the Hanle effect) in quasi-two-dimensional semiconductor nanostructures such as quantum wells or disklike quantum dots. Provided that the optical orientation signal from the sample is due to the nonequilibrium spin polarization of holes, the lineshape of the Hanle effect becomes non-Lorentzian. This results from a scatter of hole $g$-factor values, which is immanent to the hole ensembles in such systems. Analysis of the lineshape of the Hanle effect holds promise as a tool for distinguishing between ${X}^{+}$ and ${X}^{\ensuremath{-}}$ trion states in nanostructures.

Influence of mesoscopic ordering on the photoexcitation transfer dynamics in supramolecular assemblies of oligo- p-phenylenevinylene

We have investigated the influence of molecular arrangement on the transfer rates of photoexcitations along supramolecular assemblies of hydrogen-bonded oligo- p-phenylenevinylene (OPV) molecules for two different packing geometries. For well-defined, helical stacks of monofunctional OPVs fast (≈50 ps) photoluminescence depolarization and excitation transfer to dopants was observed, in agreement with semi-coherent exciton diffusion. For disordered assemblies of bifunctional OPVs incorporating a spacer to link adjacent molecules, depolarization and energy transfer dynamics occur on a longer time scale (≈nanosecond). This strongly suggests that such spacers need to be tuned carefully as they may otherwise interfere with the π-stacking thereby reducing the intermolecular electronic coupling.

Spin-dependent recombination in GaAsN solid solutions

Room-temperature spin-dependent recombination in a series of GaAs1−x Nx solid solutions (x = 2.1, 2.7, 3.4%) has been observed as manifested by a more than threefold decrease in intensity of the edge photoluminescence upon switching from circular to linear polarization of the exciting light or upon the application of a transverse magnetic field (∼300 G). The interband absorption of the circularly polarized light is accompanied by the spin polarization of conduction electrons, which reaches 35% with an increase in the pumping level. The observed effects are explained in terms of the dynamic polarization of deep paramagnetic centers and the spin-dependent trapping of conduction electrons on these centers. The electron spin relaxation time, as estimated from the dependence of the edge photoluminescence depolarization in the transverse magnetic field (the Hanle effect) on the pumping intensity, is on the order of 1 ns. According to the adopted theory, the electron spin relaxation time in the presence of spin-dependent recombination is determined by a slow spin relaxation of localized electrons. The sign (positive) of the g factor of localized electrons has been experimentally determined from the direction of the magnetic-field-induced rotation of their average spin observed in the three GaAsN crystals studied.

Ultrafast depolarization of the fluorescence in a conjugated polymer

The effect of the extent of pi electron conjugation on the primary photophysics in semiconducting polymers is reported. A rapid depolarization of photoluminescence and transient absorption, which indicates a reorientation of the transition dipole moment by similar to 30 degrees on a sub-100 fs time scale, is observed in the fully conjugated polymer poly[2-(2'-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] (MEH-PPV). In contrast, partially conjugated polymers exhibit a much slower depolarization. The results reveal rapid changes of exciton delocalization in the fully conjugated MEH-PPV driven by structural relaxation.

Hot Electron Scattering Rate in p-Doped MQW Structures GaAs/AlAs

We have studied inelastic hot-electron scattering due to interaction with neutral acceptors accompanied by their excitation and ionization. Comparative studies of narrow GaAs/AlAs multiple quantum wells and bulk GaAs films were performed in the concentration range of neutral acceptors 1018 to 1019 cm—3. The method of hot photoluminescence depolarization in an applied magnetic field was utilized. The scattering cross-section in multiple quantum wells was shown to be four times smaller than in the bulk samples. A theoretical model of hot-electron scattering by acceptors in a quantum well was developed that explains satisfactorily the experimental results.

Inelastic hot electron scattering by neutral acceptors in [formula omitted] multiple quantum well structures

We have studied inelastic hot electron scattering due to interaction with neutral acceptors accompanied by their excitation and ionization. Comparative studies of narrow GaAs AlAs multiple quantum wells and bulk GaAs films were performed in the concentration range of neutral acceptors 10 18–10 19 cm −3. The method of hot photoluminescence depolarization in an applied magnetic field was utilized. The scattering cross-section in multiple quantum wells was shown to be four times smaller than in the bulk samples. A theoretical model of hot electron scattering by acceptors in a quantum well is developed that explains satisfactorily the experimental results.

High-Temperature Reorientation Of Distortions In The Excited State Of The VGaTeAs Complexes In n-Type GaAs

AbstractWe have investigated polarization of photoluminescence from the VGaTeAs complexes in n-GaAs induced through resonant excitation by polarized light. Experimental data in temperature range from 77 to 240 K were described by classic one-dipole approximation within the model of the VGaTeAs complex subjected to the Jahn-Teller distortion in the ground and excited states. It is shown that depolarization of photoluminescence at temperatures over ∼ 120 K may be explained by concurrent action of i) thermal emission and back capture of holes bound to the complexes in the excited state and ii) reorientation of complex distortion during a life of this state. The model parameters have been estimated.

Intervalley Γ-X scattering rate in gallium arsenide crystals

Abstract From hot photoluminescence depolarization the electron lifetime (18±2 fs) at electron energy e o =0.57 eV has been measured in GaAs crystals. From these data the Γ -X intervalley scattering time is estimated as 30± 10 fs.

Depolarization of Photoluminescence of Isotropic Solutions Produced by Excitation Energy Migration

A theory of the excitation energy transfer between like molecules in isotropic solution based on a centre or shell model of a primarily excited luminescent molecule and on the extended Förster “excitation master equation” has been elaborated. Fluorescence and phosphorescence depolarization are shown to be governed by singlet-singlet energy migration and described by the same expression. The comparison of the theoretical curve with the experimental data obtained by Gondo et al. (1975) for benzo[f]quinoline in ethanol glass at 77 K results in the following critical distance R0 for the excitation energy migration: 21.5 Å for fluorescence and phosphorescence, respectively.

On the prediction of selfdepolarization of photoluminescence in two-dimensional systems

Based on the theory of concentrational depolarization of photoluminescence (CDPL) worked out for three-dimensional systems [8] an extension has been made for two-dimensional systems. In the theory the excitation energy remigration and photoluminescence quenching by non-luminescent dimers have been taken into account as well. The expression obtained for emission anisotropy has been compared with the experimental results ofTrosper et al. [18] concerning CDPL of chlorophyll-a in two dimensional solutions. Good agreement of theory with experiment has been found for the values of critical distancesR0 smaller than those obtained in [18] as upper limits.

Further generalization of the theory of concentration depolarization of photoluminescence

The theory of concentration depolarization is generalized to include the effects of non-luminescent dimers which belong to the centres. The theory is applied to Na-fluoresceine and to rhodamine 6G.

Energieübertragung zwischen gleich- und ungleichartigen Molekülen in Lösung. I. Theorie / Energy Transfer Among Like and Unlike Molekules in Solution

A theory presenting uniform description of the excitation energy transfer among like (concentration depolarization of photoluminescence) and unlike (quenching by foreign absorbing substances) molecules in solutions is given. Based on the shell model of a luminescent centre, expressions for the emission anisotropy as well as the quantum yield of photoluminescence have been obtained. The dependence of the probability of resonance excitation energy transfer on the mutual orientation of the transition moments of the interacting sensitizer and acceptor molecules, as well as the fluctuations of the concentration of the acceptor molecules have been taken into account

Theory of concentration depolarization of photoluminescence and intermolecular energy transfer in rigid solutions

A new model of a luminescence centre is presented, consisting of a molecule D 0 which obtains excitation energy directly by absorption of light and molecules D 1, D 2, …, D n among which energy can be transferred in a non-radiative way with high probability. The molecules D 0, D 1, …, D n can be successive points of localization of excitation energy. Several expressions are given for the emission anisotropy which correspond to various degrees of taking into account remigration of excitation energy. The necessity of taking into consideration the remigration of excitation to molecules D 0, at least from molecules D 1 and D 2, is stated. In the theory, concentration quenching arising from non-luminescent dimers present in solution is included. The theory is compared with experimental data concerning concentration depolarization of rhodamin 6G in glycerin-water solutions. A discussion of the results follows.

Theory of the influence of concentration on the luminescence of solid solutions

In the theory presented the following phenomena have been analysed from a uniform point of view: 1) concentration quenching 2) quenching by foreign absorbing substances 3) concentration depolarization of photoluminescence. The participation of primarily unexcited molecules of the donor in the process of energy transfer to the molecules of the acceptor as well as the remigration of excitation energy have been considered in the theory. It has been accepted that in case 1) the role of the molecules of the acceptor is carried out by dimers. Expressions for the quantum yield as well as emission anisotropy of photoluminescence have been obtained. It has been proved that the expression for yield, describing the process of quenching by foreign absorbing substances in the particular case whenγ D ≪γ A turns into the expression which was previously obtained byForster. The expression for emission anisotropy which also takes into account the self-quenching of photoluminescence describes the effect of repolarization appearing in the range of large concentrations. A discussion of the results obtained is presented