High Exciton Densities Research Articles

Characterization of recombination processes in GaN by cathodoluminescence in‐depth spectroscopy

Cathodoluminescence in-depth spectroscopy (CLIS) study was made at room temperature on a thick GaN sample. In experiments, CLIS spectra for near band edge (NBE) emission and yellow luminescence (YL) were observed, and their theoretical spectra were calculated on computer. The comparison between theory and experiment indicated that the YL center is a deep acceptor which is spatially uniformly distributed within the GaN layer, that surface recombination plays an important role in the overall recombination statistics, and that the major non-radiative recombination process is that of high density excitons even at room temperature. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Dynamical Processes of High-Density Excitons in GaSe Crystals

Dynamical processes of high-density excitons and their relaxation mechanisms in GaSe crystals have been investigated in real and momentum spaces by observing resonant luminescence spectra of the excitons in the time- and space-resolved regimes. Temporal profiles of the luminescence show remarkable excitation-density dependence. The temporal profiles have been well explained by a nonlinear rate equation including a two-exciton scattering process and localization to shallow trapping states at high density. Above a certain excitation density, spatial propagation becomes remarkable owing to the suppression of localization due to site-filling effects, giving the group velocity ∼1.0×10 8 cm/s. The other characteristic features of the exciton dynamics at high density appear as inelastic scattering processes on recoil luminescence bands in the time- and space-resolved spectra. From these results, detailed scattering and relaxation mechanisms of the excitons in this material at high density have been clarified in ...

Non-linear increases in excitonic emission in synthetic type-IIa diamond

Free-exciton emissions increase non-linearly by cathodoluminescence (CL) above the threshold probe current of 30 μA in high-pressure high-temperature synthetic type-IIa diamond. A droplet of electron-hole liquid (EHL) is also observed by CL for the first time. Furthermore, boron acceptor bound-exciton emission as well as free-exciton emission also increases super-linearly in an unintentionally boron-contaminated area. The threshold probe current of non-linear increase of excitonic emission is in accordance with the emergence of the EHL peak. It is considered that the non-linear increase of excitonic emissions is due to the increase in the radiative recombination rate of free excitons under high exciton density.

Suppression of exciton super-radiance due to spatially separated scatterers

We have found that the radiative lifetime of an exciton in a GaAs quantum well (QW) increases due to high-density excitons injected in an exciton reservoir spatially separated from the QW. The photoinjected excitons in the reservoir interact with the two-dimensional exciton in the QW via the Coulomb interaction without the phase-space filling effects and the exchange interaction. The radiative lifetime increases in proportion to the exciton density in the reservoir initially, while it saturates for the higher density. It is interpreted as suppression of the exciton super-radiance due to weak localization of the exciton center-of-mass motion caused by the excitons in the reservoir. A phenomenological two-level model reproduces the experimental observation, where an exciton state is represented as the scattering-induced superposition of small wave-vector (radiative) and large wave-vector (nonradiative) states.

Excited State Dynamics of Organo-Lanthanide Electroluminescent Phosphors: The Properties of Tb(tb-pmp)3 and Gd(tb-pmp)3

The excitation energy transfer rates between the excited states of a Tb(III) complex containing the ligand 1-phenyl-3-methyl-4-(trimethylacetyl)pyrazol-4-one, are described. Energy transfer rate constants are derived from time-gated and time-correlated single photon counting measurements. Comparison with the analogous Gd(III) complex shows that there is efficient intramolecular energy transfer from a singlet state of the ligand to excited terbium f-electron states. There is no evidence of bi-exciton annihilation in these materials, even at very high exciton densities. The use of this complex as the active medium for electroluminescent device applications is addressed. We note the particular properties of the ligand which make it suitable for this application and suggest possible improvements.

Ring emission and exciton-pair scattering in semiconductor microcavities

Polariton microemitters are semiconductor devices that take advantage of the strong coupling of light and matter to produce efficient optical sources. Injecting an exciton gas into such a semiconductor microcavity can produce annular optical emission of a specific cone angle. We demonstrate that at higher exciton densities or temperatures, this ring continuously decreases in diameter due to the greater energy that can be carried off in exciton-pair scattering processes. These Coulomb-mediated exciton-exciton scatterings are shown to dominate the polariton relaxation and optical emission.

Semiconductor-cavity QED in high-Qregimes: Detuning effect

The nonresonant interaction between the high-density excitons in a quantum well and a single-mode cavity field is investigated. An analytical expression for the physical spectrum of the excitons is obtained. The spectral properties of the excitons, which are initially prepared in the number states or the superposed states of the two different number states by the resonant femtosecond pulse pumping experiment, are studied. A numerical study of the physical spectrum is carried out and a discussion of the detuning effect is presented.

High-Density Excitons in a Diluted Magnetic Semiconductor: Cd 1 − x Mn x Te

Time-resolved magneto photoluminescence in a diluted magnetic semiconductor Cd 0.9 Mn 0.1 Te has been carried out with varying exciton density from 10 14 to 10 19 cm m 3 . The reduction of the Zeeman shift and that of the magnetic polaron energy was found under strong photoexcitation. The spectral feature is interpreted in terms of the heating of the manganese spin subsystem. Polarization dependence of the spin heating is observed for the first time, revealing the contribution of the spin flip between excitons and magnetic ions to the heating process.

EVOLUTION OF ELECTRON-HOLE PLASMA AND EXCITONS IN ZnO EPITAXIAL THIN FILMS STUDIED BY FEMTOSECOND SPECTROSCOPY

Time evolution of a high density electron-hole plasma (EHP) and excitons in ZnO epitaxial thin films was studied under resonant excitation of the excitonic state using pump-probe absorption and optical Kerr gate luminescence techniques with sub-picosecond time resolution in order to elucidate dynamics of the photo-excited high density carriers. The bleaching of the exciton absorption occurs without any measurable delay time, suggesting that the excitonic state instantaneously becomes unstable because of the dissociation of the high density excitons into the EHP due to the screening of Coulomb interaction. This leads to an instantaneous bandgap reduction. Then the recovery of the renormalized bandgap as well as that of the exciton bleaching takes place with decreasing the carrier density due to radiative recombination of electrons with holes. From the time-resolved absorption and luminescence experiments, the recovery time is determined to be 25 v ps.

Instabilities and Self-Organization Phenomena in High Density Exciton Systems

The influence of final value of exciton lifetime on exciton condensed phase formation is investigated in deterministic and stochastic approach. It is shown that in some region of pumping, the periodical distribution of exciton density takes place. The numerical solution for periodical distribution of exciton density is obtained for different values of pumping. There are critical values of exciton lifetime for appearance of exciton condensed phase. The distribution density function becomes narrower with increasing pumping.

SPATIAL BEHAVIOR OF EXCITON-POLARITON MASSES IN A LAYERED CRYSTAL BiI3

Spatial behavior of the stacking fault excitons (SFE's) excited heavily in a two-dimensional space at a stacking fault interface in BiI 3 has been studied. The SFE's whose center-of-mass motion is confined at the interface show more efficient spatial expansion for higher excitation densities. In order to get phase information of the high-density SFE masses, the degnerate four-wave-mixing (DFWM) measurements with two laser beams including space-resolved regime were performed. It was found that the increase in the dephasing due to exciton-exciton collision is suppressed above a certain density. In a space-resolved regime, it turned out that the high-density exciton mass excited at one laser spot propagates coherently to the other laser-spot propagates coherently to the other laser-spot site and induces nonlinear polarization resulting in momentum-selective emission of the DFWM lights. These result suggest the possibility of a coherent collective motion of the SFE masses in a new condensate of the high-density exciton-polaritons.

PROPAGATION PROPERTIES OF HIGH-DENSITY EXCITON POLARITONS

We investigate the time evolution of the exciton wave packet by an interacting exciton polariton model. The analysis is based on the equation-of-motion for the expectation value of exciton polariton operator with respect to the macroscopic quantum state. We find that the nonlinear interaction between exciton polariton leads to the extraordinary fast propagation of high-density exciton polaritons. This result helps the proper understanding of the experiments on the unusually fast propagation of exciton polaritons in strongly excited BiI 3.

Spatial evolution of inelastic scattering of high-density excitons in GaSe crystals

Spatial evolution of heavily photo-created excitons in GaSe single crystals has been studied by observing space- and time-resolved photoluminescence. At an early stage, the free-exciton recombination band shows a blue-shift with broadening due to the phase-space filling effect and the increment of the exciton self-energy. Recoil luminescence bands due to the exciton–exciton and exciton-carrier scattering shifts to the lower and higher energy sides with increasing exciton density and by separating a detecting position from the center of the exciting laser spot, respectively. These results are consistently explained by the expansion of the high-density exciton masses in the real space and by their k-space relaxation accompanying intra-band inelastic scattering under the energy and momentum conservation law.

Coherent motion of high-density exciton masses at a quasi-two-dimensional interface in BiI 3

Spatial correlation of a quantum coherence of weakly interacting stacking fault excitons (SFEs) excited heavily at a two-dimensional stacking fault interface in BiI 3 has been studied by performing the degenerate four-wave-mixing (DFWM) measurements. It was found that the increase in the dephasing rate due to exciton–exciton scattering is suppressed above a threshold excitation density. Above such density, a propagating component of the SFE was detected on the space-resolved DFWM signals as a coherent mass of the excitons with nearly the same velocity as the group velocity of the SFE-polaritons by applying the spatially isolated two-spot excitation. In this scheme the off-diagonal element of the density matrix of the excitons induced by the exciting laser field at the one spot propagates to the other laser spot, bringing about nonlinear polarization to yield the DFWM signals. The time evolution of the space-resolved DFWM spectra evidently shows a manifestation of the macroscopic spatial expansion of a condensed quantum state of the excitons.

Spin relaxation and spin-dependent exciton interactions in ZnSe quantum wells

We demonstrate the mutual influence of the dynamics in the real-space and the spin parts of the exciton wavefunction evoked by the exchange interaction. In the regime of high exciton density, the exciton–exciton interaction in ZnSe/ZnMgSSe quantum wells is found to be strongly affected by the exciton spin orientation and the longitudinal spin relaxation via single-fermion spin flips. The exciton's homogeneous linewidth as deduced from degenerate four-wave mixing with prepulse is significantly enhanced in spin-pure exciton populations by scattering including fermionic exchange. The transverse spin relaxation in the low-density limit is deduced from the decay of spin quantum-beats in photoluminescence experiments. We find an anti-proportionality between the spin relaxation and the optical dephasing times. The temperature dependence of both relaxation times is determined by the exciton–phonon interaction. The latter gradually reduces the electron–hole exchange coupling which is the dominating term in the exciton's transverse spin relaxation.

Properties of exactly compensated semiconductors under excitonicmodulation of the charge of deep impurities

Properties of exactly compensated semiconductors have been studied underexciton-stimulated modulation of the charge on deep impurities. Theelectron and hole lifetimes are shown to depend on deep impurityconcentration in a nonmonotonic manner.Their maximum values are determined by the exciton concentration and the probability of excitonmodulation of trap charges.The results obtained are interpreted in view of the fact that, in a exactly compensatedsemiconductor,the equilibrium concentration of majority carriers decreases sharply, by several orders of magnitude.Simultaneously, the resistivity grows abruptly.The dependences of the Hall coefficient and Hall mobilityon trap concentrations are also nonmonotonic.The exciton density and the probability of exciton modulation of the charge of deep traps areestimated for CdTe crystals at T = 300 K and shallow donorconcentration of 1017 cm-3.At high exciton densities, the rate of exciton-stimulated emission of trapped carriers becomescomparable with the rate of their capture, which decreases withincreasing deep centre concentration.As a result, the lifetimes become higher.The exciton-stimulated modulation of deep trap occupancy through charge exchange dominates over thatassociated with energy exchange.

Exciton lifetime in quantum well with vicinal high-density excitons

Radiative lifetime of an exciton in a GaAs quantum well (QW) is controlled by high-density excitons, which restrict the exciton coherence through scattering. In order to circumvent the phase space filling effect of high-density excitons, we have prepared a QW structure in such a way that a reservoir for high-density excitons is separated from the QW. The lifetime increases (up to 30%) with the exciton density in the reservoir and saturates at 1×1017/cm3. The upper bound lifetime is determined by the excitonic relative motion.

Exciton–exciton interaction through barrier layer

We demonstrate that the radiative recombination of an exciton in a GaAs/AlGaAs quantum well (QW) is slowed down by the exciton–exciton scattering caused by the high-density excitons, which are injected into the exciton reservoir in the vicinity of the QW by an ultra-fast laser. Our experiment shows lifetime prolongation (up to 30% at a density of 1×10 17/cm 3 in the reservoir) and lifetime saturation of exciton in QW. The lifetime prolongation is explained by the classical dipole scattering between the excitons which disturbs the exciton superradiance. The saturation is explained by a theory which takes the momentum dependence of the scattering potential into account.

Anomalous behavior of the high-density excitons and their nonlinear optical response in a quasi-two-dimensional space in a layered crystal BiI 3

Spatial and temporal behavior of excitons heavily excited in a quasi-two-dimensional space of a stacking fault interface in BiI 3 has been studied by space- and time-resolved luminescence measurements. The luminescence spectra show the peak-energy shift due to high-density effects. Under the heavier excitation, an anomalous propagation of excitons is observed on the temporal profiles of the luminescence intensity and the peak-shift values at each detecting position. From a comparison with a model calculation, it is seen that transport mechanisms of high-density excitons are basically explained by both diffusion and polariton transport suffering the scattering, while there exist high-density exciton–polaritons which propagate with the polariton group velocity. These results are discussed in terms of a collective motion and coherent transport of excitons.

Subhertz spectral fluctuations from high-density excitons in coupled quantum wells

In 1994 Butov et al. [Phys. Rev. Lett. 73, 304 (1994)] reported large, extremely low-frequency fluctuations in the luminescence intensity from indirect excitons in a coupled quantum well. We have reproduced these fluctuations in a similar structure. Our observations indicate, however, that the fluctuations are spectral fluctuations rather than intensity fluctuations. We have eliminated several possible causes for the fluctuations and propose a possible cause.