Energy Relaxation Of Excitons Research Articles

Quenching of excitons in doped disordered organic semiconductors

Models of exciton quenching at charge-transfer centers and by F\"orster energy transfer in disordered organic materials are formulated. The models consider quenching as a diffusion-limited process with the exciton diffusion rate being controlled by energy relaxation of excitons within the inhomogeneously broadened intrinsic excitonic density of states. The calculated dependences of the radiative exciton decay rate upon the density of dopants are used in order to fit experimental data on the F\"orster-type fluorescence quenching in poly(phenylphenylene vinylene) and photoluminescence quenching at charge-transfer centers in an alkoxy-substituted poly-phenylenevinylene.

Size effect in the desorption of excited atoms and molecules from clusters of inert elements under electron bombardment

New channels of desorption of excited atoms and molecules from clusters of argon, krypton, and xenon under electron bombardment are found by the method of vacuum ultraviolet emission spectroscopy. The maximum yield of particles is registered in the interval of average sizes of 50–100 atoms/cluster. The regularities and features of the desorption of excited particles are revealed, making it possible to establish a new desorption mechanism called nonradiative excimeric dissociation. It is shown that the main stages of this mechanism are: the formation of molecular centers of the nature of highly excited diatomic excimer molecules in clusters in the process of self-trapping of high-energy p excitons (n=1); the nonradiative dissociation of these molecules to excited atoms and atoms in the ground state with large kinetic energies. It is established that the appearance of new channels of desorption of excited atoms and molecules from clusters under bombardment by electrons is due to features of the physical properties of clusters in the interval of average sizes 50–100 atoms/cluster, in particular, to features of the energy spectrum, vibrational frequency spectrum of the atoms, and exciton energy relaxation.

Exciton states and energy relaxation in ZnCdSe nano‐islands

We suggest a new concept of exciton states in nano-islands based on the study of optical spectra, namely the temperature dependence of cw photoluminescence (PL) and PL excitation (PLE), as well as PL spectra and kinetics under ps pulsed excitation. The scheme of the exciton states in nano-islands is as follows: (I) deep spatially isolated ground states, (II) excited meta-stable states the energy relaxation rate of which strongly depends on the temperature, and (III) the states extending over the whole island size. We show that a redistribution of the population between meta-stable and ground states is responsible for the anomalous temperature shift of the PL band in QWs with islands.

Fast exciton diffusion in chiral stacks of conjugatedp-phenylene vinylene oligomers

The ultrafast dynamics of photoexcitations have been studied in chiral stacks of conjugated p-phenylene vinylene molecules functionalized with hydrogen-bonding groups. The results indicate that in solution, \ensuremath{\pi}-\ensuremath{\pi} interactions between the molecules give rise to fast exciton diffusion along the stacking axis of the assemblies. The chiral nature of the assemblies is found to cause a rotation of the dipole moment of excitons propagating along the stacks as indicated by time-resolved measurements of the photoluminescence polarization anisotropy. The observed exciton diffusion and energy relaxation dynamics in the molecular stacks are shown to be very similar to those found in conjugated polymer films. Moreover, it is demonstrated that through changes in the temperature of the surrounding solvent, the stacks can be dissociated reversibly as shown by a marked reduction in the diffusivity of excitons.

Non‐classical excitonic transport in quantum wells

AbstractWe report investigations on excitonic transport in ZnSe quantum wells using far‐field nano‐photoluminescence enhanced by a solid immersion lens. The 250‐nm spatial resolution and 5‐ps temporal resolution allow the access to non‐classical transport regimes. From zero‐phonon‐line spectroscopy we deduce the spatial distribution of the PL intensity under different cw excitation condition and its temporal evolution after a pulsed excitation, respectively. The periodic quenching of the transport length as function of excitation excess energy and the nonlinear expansion of the PL spot observed in time‐resolved experiments reveal the dominance of hot‐exciton effects. The experimental data are well modelled by a Monte Carlo simulation. Spatially resolved phonon‐sideband spectroscopy is used to observe directly the coherent transport and the energy relaxation of hot excitons during transport. The coherence length is measured to be 300–400 nm at low temperatures and the excitons are found to remain hot during transport on a length scale of several micrometers.

Energy relaxation during hot-exciton transport in quantum wells: Direct observation by spatially resolved phonon-sideband spectroscopy

We investigate the energy relaxation of excitons during the real-space transport in ZnSe quantum wells by using microphotoluminescence with spatial resolution enhanced by a solid immersion lens. The spatial evolution of the LO-phonon sideband, originating from the LO-phonon assisted recombination of hot excitons, is measured directly. By calculating the LO-phonon assisted recombination probability, we obtain the nonthermal energy distribution of excitons and observe directly the energy relaxation of hot excitons during their transport. We find the excitons remain hot during their transport on a length scale of several micrometers. Thus, the excitonic transport on this scale cannot be described by classical diffusion.

TIME-RESOLVED PHOTOLUMINESCENCE OF EXCITONS IN HgI2

The time-resolved photoluminescence (TRPL) of red HgI 2 single crystal has been measured to determine the carrier lifetimes and to reveal the energy relaxation of excitons. Sharp near-bandgap luminescence lines due to free and bound excitons are observed at 530 nm, and a broad luminescence band appears at 630 nm at low temperatures. TRPL experiments of the near-bandgap luminescence have revealed that the luminescence comprise fast (30 to 200 ps) and slow (100 to 400 ps) decay components, showing several relaxation processes in free and bound exciton annihilation. TRPL of the broad band at 630 nm has shown that the luminescence is ascribed to the radiative recombination of donor-acceptor (DA) pairs.

MULTISTABLE NATURES AND PHOTO-INDUCED CHARGE-SEPARATION IN HOLE-DOPED STATES OF STRONGLY COUPLED ELECTRON-PHONON SYSTEMS

We investigate the effects of hole doping in the charge-density wave (CDW) state that has the strong electron-phonon (e-p) coupling, using the two-dimensional molecular crystal model. In calculations, we use the mean-filed theory for the interelectronic interactions and the adiabatic approximation for phonons. On the basis of this theory, we calculate e-p states of doped ground states for various values of the doping concentration of holes. From the calculated results, it is found that a multistable nature appears in the doped e-p states just before the CDW-metal phase transition. In order to see the effects of the photoexcitation in the hole-doped states, we also investigate the exciton states taking into account the electron-hole correlation. Results calculated here indicate that the separation of electron and hole occurs in the photoexcited states as a consequence of the energy relaxation of excitons.

Spin relaxation times of exciton states in ZnCdSe/ZnSe low dimensional heterostructures

Using the pulsed excitation with a femtosecond laser and streak-camera detection we have studied the processes of spin relaxation in the system of localized excitons in low dimensional superlattices formed by the insertion of CdSe submonolayers into the ZnSe matrices. Polarization of exciton luminescence in the magnetic field arises due to the thermal redistribution of population in the system of splitted exciton spin sublevels. The distribution of polarization across the PL band countour is governed by the complicated interplay of energy and spin relaxation processes. The study of PL decay kinetics in polarized light allows us to determine with the help of a simple two-level model the times of exciton spin relaxation. Spin relaxation times strongly decrease with the increase of magnetic field indicating the phonon assisted mechanism of spin relaxation. The detailed analysis of the dependence of spin relaxation times on the exciton localization energy is not possible with the model used due to the complicated character of exciton population kinetics for deeply localized states. Deeply localized states are to a great extent populated through the tunnelling relaxation of excitons with higher energies. The transfer of polarization in the processes of exciton energy relaxation has been clearly observed in the study of spectrally resolved decay kinetics in polarized light.

Acoustic Phonon Emission by Optically Excited Carriers in the InAs/GaAs Quantum Dot System

The phonons emitted by photoexcited carriers in self-organised InAs quantum dots in GaAs have been studied using a time-resolved bolometric detection technique. Measurements of the temporal, excitation power and, for the first time, angular dependence of the phonon emission have been made. The results show that carrier relaxation in the dots takes place mainly by emission of sub-THz acoustic phonons. We suggest that the results can be explained by considering the energy relaxation of excitons instead of treating the electrons and holes separately.

Exciton energy relaxation on acoustic phonons in double-quantum-well structures

The paper analyzes the rate of energy relaxation involving acoustic phonon emission between exciton states in a double quantum well. A theoretical study is made of the part played by two mechanisms, one of which is a one-step transition with emission of an acoustic phonon and the other is a two-step transition, which includes elastic exciton scattering from interface nonuniformities followed by energy relaxation within an exciton subband. The rate of the two-step transition in real double quantum wells is shown to be higher than that of the one-step transition. As follows from calculations, the fast energy relaxation between exciton states is determined by the elastic scattering of phonons from the interface.

Studies of transient stimulated emission in DOO-PPV films

We experimentally and theoretically investigated the stimulated emission band in neatly prepared 2,5-dioctyloxy poly( p-phenylene-vinylene) (DOO-PPV) films. We found that at moderately high excitation intensity spectral narrowing (SN) occurs due to the amplified spontaneous emission (ASE). A complex ASE build-up and decay dynamics as a function of excitation intensity and stripe length is observed. We also found that the exciton energy relaxation process within the inhomogenously broadened energy distribution becomes faster when ASE occurs. A theoretical model taking into account the polymer film gain and loss, the propagation effect and the time dependent exciton generation process is developed and used to explain the experimental data.

Observations of interaction-assisted hopping transport in GaAs/Ga1−xAlxAs quantum wells

We show that interaction-assisted modes of transport play a role in the energy relaxation of excitons in disordered GaAs quantum wells. Semitransparent gold films are found to induce a blue shift in the photoluminescence spectrum of disordered wells. We interpret the result in terms of a long-range electromagnetic hopping coupling suppressed by image dipoles in the gold. The results are supported by Monte Carlo simulations of electromagnetically-assisted exciton hopping using hop rates modified by the gold layer.

Exciton energy relaxation and exciton mobility edge in (CdMg)Te epilayers studied by time-resolved photoluminescence

Exciton localization is studied with time-resolved photoluminescence (PL) in Cd 1− x Mg x Te epilayers. The energy relaxation rate of excitons is found to be energy dependent and reduces drastically below an effective excitonic mobility edge. This particular property results in the saturation of the PL intensity rise time below the mobility edge and a two-exponential decay for the PL intensity and the energy relaxation of the excitons.

Frequency-domain nanosecond population-grating spectroscopy in semiconductor doped glass

We have observed coherent emission from nanosecond laser-induced population-grating in fundamental absorption edge of semiconductor doped glass at both 10 and 300 K. The third-order nonlinear response was influenced definitely by the confined exciton energy relaxation. The results on the excitation frequency dependence suggest that a low-frequency acoustic mode mediated by the laser-induced thermal grating contributes dominantly to the exciton energy relaxation.

Asymmetric luminescence line shape and exciton energy relaxation in Zn1−x−yMgxCdySe epilayers

The asymmetric line shape of the transient photoluminescence (PL) spectra of high-quality Zn1−x−yMgxCdySe epilayers is deconvoluted to a narrow and a broad Gaussian peak at each time delay. The fitted energy difference between the two peaks corresponds to one longitudinal optical (LO) phonon energy. We assign the narrow peak to the recombination of free excitons (FXs) and the broad peak to the recombination of excitons localized in the broad band tail characteristic for this quaternary compound. The broad PL peak is found to arise from the energy relaxation of the FX at the exciton mobility edge to the localized states by LO phonon emission. Therefore, the population peak of the localized excitons is formed at one LO phonon energy below the FX for all times after the laser excitation.

Theory of exciton dephasing in semiconductor quantum dots

A resonant optical excitation creates an excited state population and also induces an optical polarization. Dynamics of this optical excitation is characterized by relaxation of the population as well as decay of the induced optical polarization. In lower dimensional semiconductors, electronic confinement leads to qualitative changes in population relaxation, including spontaneous emission and exciton-phonon scattering, as shown in extensive recent studies [1]. These population relaxation processes are expected to contribute to dephasing with a dephasing rate given by half the population decay rate. Pure dephasing processes that do not involve population or energy relaxation of excitons can also contribute to dephasing. Pure dephasing, which is a well-established concept for atomic systems, remains yet to be investigated in lower-dimensional semiconductors due to a lack of direct comparison between dephasing and population relaxation and between theory and experiment. Studies of pure dephasing processes in lower-dimensional semiconductors will renew and deepen our understanding of dephasing of collective excitations in solids, although several seminal studies were done on the exciton dephasing in quantum well (QW) structures [2–6].

Multiphoton-Excited Luminescence from Diamond Nanoparticles

The highly efficient photoluminescence (PL) from diamond nanoparticles of different size (diameter) D = 5, 100, 400, 700, and 1000 nm was observed. The PL was induced by laser pulsed ultraviolet (UV) light [λexc = 354.7 nm (3.48 eV), τ = 10 ns] as well as by the infrared (IR) one [λexc = 1064 nm (1.16 eV), τ = 10 ns]. The laser light intensity dependences of the PL yield reveal multiphoton (MP) processes of the PL excitation. The PL is a result of a two-photon absorption (total energy 6.96 eV) in the case of larger size particles (100−1000 nm) excited by the UV light, but for 5 nm particles three photons (the total energy 10.64 eV) need to be provided. A five-photon absorption process occurs in the case of the IR light (total energy 5.8 eV). It is assumed that the PL results from structural defects, impurities, and the graphite-like phase excited by a relaxation of the MP-produced excitons (the band gap of diamond Eg = 5.5 eV). With the UV one-photon excitation, the intrinsic PL from the mentioned species with the same bands was observed, supporting the interpretation. The dynamics of MP excitation has been analyzed with respect to the size of particles. The amorphization and rearrangement of sp3 sites of the diamond lattice into sp2 sites of graphite due to MP absorption is assumed to be one of three stages of exciton energy relaxation and was supported by the Raman spectroscopy and TEM images. The typical Swan bands of electronically excited C2 species dominate the spectra at the UV and IR light intensity above the threshold for vaporization. It was concluded that the MP-excited PL precedes the emission accompanying the laser vaporization process.

Photoluminescence kinetics of indirect excitons inGaAs/AlxGa1−xAscoupled quantum wells

Photoluminescence ~PL! kinetics of long-lifetime indirect excitons in a GaAs/Al xGa12xAs coupled quantum well characterized by a small in-plane random potential was studied at temperatures 1.5<T<15 K for a wide range of exciton densities. Strong deviations of the indirect exciton PL kinetics from monoexponential PL rise/decay were observed at low temperatures and high exciton densities. In particular, right after the excitation is switched off, the spectrally integrated indirect exciton PL intensity increased sharply. Simultaneously, the indirect exciton energy distribution was observed to narrow significantly. The observed increase in intensity is attributed to the sharp increase of occupation of the optically active exciton states. The energy distribution narrowing is explained in terms of the phonon mediated exciton energy relaxation in momentum space and in the in-plane random potential. @S0163-1829~99!00104-6# A number of collective phenomena, in particular exciton condensation and superfluidity, have been predicted for a low-temperature two-dimensional ~2D! exciton system in semiconductor quantum well structures. 1 The principal limitation for the realization of the low-temperature exciton system is the electron hole recombination, which limits the exciton lifetime and results in the high effective exciton temperature. The latter is determined by the ratio between the exciton energy relaxation rate and the exciton recombination rate. The indirect ~interwell! excitons in coupled quantum wells ~CQW’s! are characterized by a long recombination lifetime; therefore, CQW’s present an opportunity for experimental study of a high-density low-temperature exciton system. In the general case of nonresonant excitation, the occupation of low-energy exciton states that are responsible for the formation of collective states 2,3 is determined by the exciton energy relaxation and can be revealed in the exciton photoluminescence ~PL! kinetics. In the present paper we study the PL kinetics of long-lifetime indirect excitons in a GaAs/Al x Ga 12x As CQW characterized by a small in-plane random potential. The earlier studies of the PL kinetics of indirect excitons in GaAs/AlxGa12xAs CQW’s have demonstrated a long indirect exciton recombination time, 4‐7 revealed a metastable exciton energy distribution, 8 and allowed for the measurement of the exciton energy relaxation in the hopping regime. 9

EXCITON RECOMBINATION DYNAMICS IN CdTe/CdZnTe QUANTUM WELLS

Quantum wells of CdTe/CdZnTe were grown by molecular beam epitaxy. The highest order of satellite peak of the sample is 5 from XRD spectra and excition emission linewidth is about 4.8 nm at 77 K. It was shown that our samples are very good. The recombination dynamics of exciton in high-quality CdZnTe/CdTe multiquantum wells were investigated by means of time-resolved photoluminescence(PL) spectra with different excitation power at 77 K and photoluminescence spectra with different temperature. When weaker excitation was used, radiative recombination decay time of the exciton was reduced as the excitation intensity was decreased; the results indicate that the dominant mechanism may be the quenching of exciton emission by impurities and defects. The linewidth of the exciton emission becomes broader with increasing temperature, the linewidth at low temperature is only due to the well thickness, and the broadening linewidth at high temperature is contributed by the interactions among the exciton and LO and TO phonons and ionized donor impurities. The PL intensities are reduced with increasing temperature, which is mainly due to the thermal dissociation of excitons, i.e., the electrons or holes jump into the barriers from the wells by thermal excitations. The exciton emission from n=2 heavy hole exciton at 77 K has been observed, and the n=2 heavy hole exciton luminescence decay time is shorter than that from n=1. The investigation indicates that there is the exciton energy relaxation from the n=2 to n=1 heavy hole exciton state by phonons.