Biexciton Formation Research Articles

Phase Diagram of Quasi Two-Dimensional Electron-Hole Gas in Single and Coupled Quantum Wells

We present the phase diagram of electron-hole plasma in single and coupled quantum wells. In the last case, electrons and holes are spatially separated. In both single and coupled quantum wells there is a Mott transition from electron-hole plasma to exciton gas. Below the Mott transition in a single quantum well there is a crossover to the biexciton gas phase which rules out the Kosterlitz-Thouless transition of excitons. In coupled quantum wells a strong direct repulsion between excitons prevents the biexciton formation. If the separation between electron well and hole well is large enough, the decrease of temperature leads to a transition to a ferromagnetic exciton phase with a spontaneous spin polarization. Further decrease of temperature leads to the Kosterlitz-Thouless transition of the exciton gas.

Low-dimensional systems: Quantum size effects and electronic properties of semiconductor microcrystallites (zero-dimensional systems) and some quasi-two-dimensional systems

This review is concerned with quantum confinement effects in low-dimensional semiconductor systems. The emphasis is on the optical properties, including luminescence, of nanometre-sized microcrystallites, also referred to as zerodimensional systems. There is some discussion on certain of the two-dimensional systems, such as thin films and layer structures. The increase in energy of excitation peaks (blue shift) as the radius R of a microcrystallite is reduced is treated theoretically, and experimental data when they are available are used to assess the reliability of the different models that have been used. These experiments normally make use of microcrystallites dispersed in a large-bandgap matrix such as glass, rocksalt, polymers, zeolites or liquids. Exciton binding energies E b are larger than for bulk semiconductors, and oscillator strengths are higher for the microcrystallites. The regimes of direct interest are as follows. Firstly there is the so-called weak-confinement regime where R is greater than the bulk exciton Bohr radius a B . Experimentally, semiconductors such as CuCl with a B , 7 Å, are suitable for study in this case. Secondly there is the moderate-confinement regime, where R , a B , and a h < R < a e , a h and a e being the hole and electron Bohr radii, respectively. Finally there is the strong-confinement regime, with R < a B , and R < a h , a e . For this case we are concerned with a ladder of discrete energy levels, as in molecular systems, rather than energy bands. The electrons and holes are treated as independent particles, and for excited states we refer to electron-hole pairs rather than excitons. Suitable materials for investigation in this regime are the II-VI semiconductors, and also GaAs and Ge, for which a B is relatively large. Although a number of different theoretical models have been used, none can be described as completely first-principles calculations, and there is room for improvement on this aspect. However, useful expressions have been developed by Brus and by Lippens and Lannoo, giving the energy of excited states as a function of R , in terms of the bulk energy gap, kinetic energy, Coulomb energy and correlation energy. Other phenomena discussed are firstly biexciton formation by the use of high intensity laser beams and secondly nonlinear optical effects. Strong nonlinearities and short decay times for excited states have been predicted, and the models developed cover both the resonant and the non-resonant cases. The possibility of using microcrystallites embedded at reasonable concentrations in a glass matrix in the field of optical communications and optical switching is also considered.

Excitons and multi-excitons in single CdTe quantum dots probed by near-field spectroscopy

A near-field optical spectroscopy study of a single CdTe/ZnTe quantum dot at low temperatures is presented. While the photoluminescence spectrum at low excitation power reveals only one single sharp peak due to the radiative recombination of excitons (X) in the single dot, several additional sharp peaks appear with increasing excitation density. The dominant features are ascribed to exciton complexes and charged exciton complexes such as negatively charged excitons (X −), neutral (2X and 3X) and negative (2X − and 3X −) biexcitons and triexcitons. Exciton charging arises due to efficient hole trapping by residual acceptors in the barrier material. This partly inhibits the formation of biexcitons and triexcitons. A spectral feature appearing close to the X − peak is tentatively assigned to X 2− negative excitons. This feature is found to shift to the red with increasing power: two possible explanations for this unexpected behaviour are proposed.

Thermal redistribution and dephasing of localized excitons in CdSe/ZnSSe quantum island structures

Temperature-dependent photoluminescence measurements of CdSe/ZnSSe quantum island structures show a blueshift of the emission maximum and a significant broadening of the emission band above 50 K. The features originate from a thermal activation of localized excitons and a redistribution of the occupation within the inhomogeneous island ensemble. The presence of exciton states with different localization at comparable transition energies is indicated by a multiexponential decay of the four-wave mixing photon echo observed at low temperatures. Upon increasing the temperature above 50 K, the decay of the four-wave mixing signal changes into a monoexponential behavior. Polarization-dependent measurements unhide strong exciton-exciton interaction and signify the formation of biexcitons with biexciton binding energies around 12 meV. The features indicate the dominance of weakly localized quantum-well-like precursor states of CdSe islands on the high-energy side of the emission band.

SUBPICOSECOND EXCITON DYNAMICS OF CuBr NANOCRYSTALS EMBEDDED IN PMMA

The temporal behaviour of the photoluminescence (PL) due to excitons and biexcitons have been observed in CuBr nanocrystals (NCs) embedded in poly methyl methacrylate (PMMA). In the excitation intensity less than 0.5 v; µJ/pulse, the decay time of the exciton PL is evaluated to be 30 v; ps. In the excitation intensity more than 0.5 v; µJ/pulse, a new fast component appears in addition to the above component, and the decay time is evaluated to be 0.8 v; ps. In'contrast, the temporal behaviour of the biexciton PL does not depend on the excitation intensity, and the biexciton formation time is evaluated to be about 0.8 v; ps. The facts suggest that two excitons are photo-generated in one NC under a strong pulse radiation, and then a biexciton is formed or one of them is annihilated radiatively without inelastic collision with each other.

BIEXCITON DYNAMICAL BEHAVIOR IN (Cd,Mn)Te/CdTe/(Cd,Mg)Te ASYMMETRIC QUANTUM WELLS

Exciton and biexciton photoluminescence in (Cd,Mn)Te/CdTe/(Cd,Mg)Te asymmetric quantum wells have been studied by time-resolved photoluminescence spectroscopy. The biexciton luminescence shows slower rise-time and longer decay as compared to the exciton luminescence. This result is explained by the biexciton localization at an unavoidable interface disorder. The biexciton luminescence intensity decreases with increasing magnetic field, while there are observed no changes of the luminescence decay rate. These results suggest that the spin-"singlet biexciton formation rate is suppressed by applying a magnetic field, whereas the biexciton decay remains unaffected.

Photo-luminescence Properties of CuCl Quantum Dots and the Dependence of Biexciton Formation Rates on Quantum Dot Sizes

We fabricated CuCl quantum dots (QDs) in a glass matrix. The blue shift of the exciton spectrum caused by quantum confinement effects was observed in the linear absorption and the photo-luminescence (PL) spectrum. Inhomogeneous line broadening was mainly induced by the size distribution of CuCl QDs and we found that the size distribution has Gaussian profiles. By solving the steady state rate equation, we also found that the intensities of biexciton PL were proportional to the square of exciton PL intensities, and the results were well accorded with the experimental results. The formation rates of biexciton were proportional to the volumes of CuCl QDs.

Spatial carrier-carrier correlations in strain-induced quantum dots

The electron-hole correlation effects on the energy levels and the wave functions of the electrons and holes in a strain-induced quantum dot containing one to ten carrier pairs have been studied using large-scale configuration-interaction calculations. The present calculations show the formation of excitons and biexcitons in the quantum dot. By increasing the number of carrier pairs, one observes a transition from a strongly correlated system to a quantum dot system for which the electron-electron and hole-hole correlations are dominated by exchange interaction and are relatively well described at the Hartree-Fock level, while for an accurate description of the electron-hole correlations configuration-interaction calculations are necessary. Ring-shaped carrier distributions emerge with increasing number of carrier pairs.

Charged Frenkel biexcitons in organic molecular crystals

It is known that the energy of the lowest electronic transition in the neutral molecules of anthracene, tetracene, and other polyacenes is blue-shifted in comparison with the corresponding transition energy in univalent molecular ions. This effect in a molecular crystal may be responsible for the attraction between a molecular (Frenkel) exciton and a charge carrier. Due to this attraction, a bound state of Frenkel exciton and free charge (charged Frenkel exciton) may be formed [5]. As we demonstrate below, the same mechanism can be responsible for the formation of a charged biexciton (bound state of two Frenkel excitons and a charge carrier). A one-dimensional lattice model is used which corresponds to J aggregates and is also a good approximation for quasi-one-dimensional crystals. Calculations are performed for molecular crystals like tetracene, where the exciton band at low temperature is much narrower than the band of the charge carrier.

Influence of the Magnetic Polaron Formation on the Exciton Kinetics in Cd1?xMnxTe/ZnTe Multiple Quantum Wells

We present optical pump–probe experiments carried out in Cd1—xMnxTe/ZnTe multiple quantum wells to study the dynamics of excitons, exchange-coupled to Mn spins in the form of magnetic polarons. From the analysis of the transient absorption spectra, we found that the magnetic polaron formation a) suppresses the formation of biexcitons (this simplifies the lineshape analysis of the pump–probe spectra) and b) retards the energy relaxation of the excitons by 14 ps in these semimagnetic structures.

Influence of the Magnetic Polaron Formation on the Exciton Kinetics in Cd1xMnxTe/ZnTe Multiple Quantum Wells

We present optical pump–probe experiments carried out in Cd1—xMnxTe/ZnTe multiple quantum wells to study the dynamics of excitons, exchange-coupled to Mn spins in the form of magnetic polarons. From the analysis of the transient absorption spectra, we found that the magnetic polaron formation a) suppresses the formation of biexcitons (this simplifies the lineshape analysis of the pump–probe spectra) and b) retards the energy relaxation of the excitons by 14 ps in these semimagnetic structures.

Dephasing and interaction of excitons in CdSe/ZnSe islands

The dephasing of excitons in self-organized CdSe/ZnSe islands grown by molecular-beam epitaxy is investigated using spectrally resolved four-wave mixing. A distribution of dephasing times is observed, indicating the existence of localized excitons with different relaxation times at comparable transition energies. Polarization-dependent measurements identify the formation of biexcitons. The observed large biexciton binding energy of 22 meV increases with decreasing exciton energy, which is attributed to an increasing three-dimensional confinement.

Dense excitonic luminescence and optical gain in ZnS-based quantum wells

Biexciton formation in ZnS epitaxial layers and Cd x Zn 1− x S–ZnS quantum wells has been studied by means of photoluminescence excitation spectroscopy. On the basis of the two-photon absorption of biexcitons, the binding energy of the biexciton in Cd x Zn 1− x S–ZnS quantum wells was estimated to be 34 meV, which was approximately 4 times larger than that in ZnS. In addition, the quantum-well biexciton was found to be localized by alloy disorder, and there existed the distribution of the biexciton binding energy. The contribution of biexcitons to optical gain formation and resultant stimulated emission was proposed in Cd x Zn 1− x S–ZnS quantum wells up to about 250 K.

Renormalized versus unrenormalized interactions of excitons in a quantum well

An effective boson Hamiltonian of 1s excitons with “spin” degrees of freedom is derived through a projection procedure. This procedure yields renormalization effect, resulting in the emergence of the interaction between excitons with opposite spins. The procedure also renormalizes the scattering amplitude between excitons with the same spins. The relevant interactions for the formation of bi-excitons are identified. The present theory gives the microscopic foundation of the phenomenology that was previously introduced for analyzing a four-wave mixing experiment in a GaAs quantum well in a high-Q microcavity.

Polarization dependence of biexciton formation in CuCl studied by pump-probe spectroscopy

We investigated the polariton effect upon the polarization selection rules for two-photon resonant transition of Γ 1 biexcitons in CuCl by pump–probe spectroscopy. It is found that, when the energy of one photon is close to the exciton resonance, the two-photon transition occurs even in the co-circular polarization configuration where the transition is usually forbidden. This is caused by the disturbance of the polarization of polaritons. We assign the origin of this disturbance to the collision between polaritons.

Kinematic Frenkel biexcitons

We consider the two-particle spectra of Frenkel excitons in molecular crystals and demonstrate that, even if the dynamical interaction is weak or absent, bound states of two excitons may be formed due to the kinematic interaction induced by their paulion nature. A simple model of a linear molecular chain with two molecules in the unit cell is studied, which may be applicable to J-aggregates. The number of exciton bands turns out to be crucial for the possibility of kinematic biexciton formation. We also estimate the biexciton contribution to the resonant part of the nonlinear optical polarizability and two-photon absorption, and analyze the stability of the kinematic biexcitons with respect to the dynamical exciton–exciton interaction.

New Transient Absorption Observed in the Spectrum of Colloidal CdSe Nanoparticles Pumped with High-Power Femtosecond Pulses

The power dependence of the transient absorption spectrum of CdSe nanoparticle colloids with size distribution of 4.0 ± 0.4 nm diameter is studied with femtosecond pump−probe techniques. At the lowest pump laser power, the absorption bleaching (negative spectrum) characteristic of the exciton spectrum is observed with maxima at 560 and 480 nm. As the pump laser power increases, two new transient absorptions at 510 and 590 nm with unresolved fast rise (<100 fs) and long decay times (≫150 ps) are observed. The energy of each of the positive absorption is red shifted from that of the bleach bands by ∼120 meV. The origin of this shift is discussed in terms of the effect of the internal electric field of the many electron−hole pairs formed within the quantum dot at the high pump intensity, absorption from a metastable excited state or the formation of biexcitons.

Exciton dephasing and biexciton binding in CdSe/ZnSe islands

The dephasing of excitons and the formation of biexcitons in self-organized CdSe/ZnSe islands grown by molecular-beam epitaxy is investigated using spectrally resolved four-wave mixing. A distribution of exciton-exciton scattering efficiencies and dephasing times in the range of 0.5--10 ps are observed. This indicates the presence of differently localized exciton states at comparable transition energies. Polarization-dependent measurements identify the formation of biexcitons with a biexciton binding energy of more than four times the bulk value. With decreasing exciton energy, the binding energy slightly increases from 21.5 to 23 meV, while its broadening decreases from 5.5 to 3 meV. This is attributed to a strong three-dimensional confinement with improving shape uniformity for decreasing exciton energy.

Interaction-induced effects in the nonlinear coherent response of quantum-well excitons

Interaction-induced processes are studied using the third-order nonlinear polarization created in polarization-dependent four-wave-mixing experiments (FWM) on a ZnSe single quantum well. We discuss their influence by a comparison of the experimental FWM with calculations based on extended optical Bloch equations including local-field effects, excitation-induced dephasing, and biexciton formation. The investigations show that, for copolarized input fields, excitation-induced dephasing is the dominant FWM mechanism, followed by the conventional density-grating FWM process, biexcitonic contributions, and local-field effects. For cross-linear polarized input fields the excitation-induced dephasing mechanism is canceled so that the conventional density-grating FWM process and biexcitonic contributions are dominating.

Pairing of vibronic small excitons due to enhanced polarizability in a crystalline polarizable nonmetallic medium

Parity breaking at a centrosymmetric site, resulting in the appearance of an inversion electric dipole moment, is shown to enhance the polarizability and thus add an additional binding energy for the pairing of vibronic excitons into biexcitons. Implications of such formation of biexcitons in polarizable media like alkali halides are discussed. The possibility of electron-hole binding in layered perovskites, where the electron and hole move in the neighboring conducting planes, while binding is mediated by a highly polarizable partner in the intercalated insulating layer, is also considered.