Biexciton System Research Articles

Effects of exciton–biexciton coherent coupling on exciton absorption in quantum dots

We report unusual excitonic absorption spectra with dip-shaped structures caused by exciton–biexciton coherent interactions in quantum dots. The exciton absorption spectrum is measured with the micro-photoluminescence excitation technique in a single InGaAs quantum dot. The spectrum changes from Lorenztian-shaped to dip-shaped with increasing excitation intensity. A theoretical analysis with the density matrix method reveals that exciton–biexciton coherent interactions create dressed states in the exciton–biexciton system and lead to the unusual absorption spectra.

Quantum states and optical responses of low-dimensional electron–hole systems

Quantum states and their optical responses of low-dimensional electron–hole systems inphotoexcited semiconductors and/or metals are reviewed from a theoretical viewpoint,stressing the electron–hole Coulomb interaction, the excitonic effects, the Fermi-surfaceeffects and the dimensionality. Recent progress of theoretical studies is stressed andimportant problems to be solved are introduced. We cover not only single-exciton problemsbut also few-exciton and many-exciton problems, including electron–hole plasma situations.Dimensionality of the Wannier exciton is clarified in terms of its linear and nonlinearresponses. We also discuss a biexciton system, exciton bosonization technique, high-densitydegenerate electron–hole systems, gas–liquid phase separation in an excited state andthe Fermi-edge singularity due to a Mahan exciton in a low-dimensional metal.

Renormalization Effects in the Spectrum of a Coherently Driven Exciton–Biexciton System

Starting from the total Hamiltonian of an excited exciton–biexciton system, nonresonant renormalizations in the electronic spectrum of a coherently driven direct semiconductor are considered. Stringent group-theoretical inclusion of the particle spin in the Hamiltonian allows one to account for the dependence of different renormalization effects on polarizations of the incident laser fields. On the example of circularly polarized driving and probing pulses it is shown that the kind of observed renormalization is defined by the pump-and-probe polarization geometry. Thus, the exciton optical Stark effect must appear in the case of co-circular pump-and-probe, whereas a mixing of the polariton and biexciton spectra is possible only in the case of counter-circular pump-and-probe. The polariton--biexciton dispersion renormalization may manifest itself as synchronous splittings of the exciton--polariton and biexciton spectra under resonant pumping at a frequency of the polariton--biexciton transition, or in their shifts in opposite directions under near-resonant pumping. The mechanisms of both kinds of renormalization effects are analyzed, and the dependence of their characteristics on the pump parameters and microscopic parameters of the exciton–biexciton–photon system is established. An evaluation of the characteristics shows that the effect of polariton–biexciton dispersion renormalization dominates in the spectra of semiconductors with stable biexciton formation. Results of the theoretical study provide an adequate explanation of available experimental data.

Bose–Einstein statistics behaviors of exciton–biexciton photoluminescence decay processes in a GaAs/AlAs type-II superlattice

We have investigated the Bose–Einstein (BE) statistics behavior of the exciton–biexciton (EX–BEX) system during the photoluminescence decay processes at 5 K in a (GaAs) 12/(AlAs) 12 type-II superlattice. Owing to the long lifetime of the type-II exciton, which is of the order of microseconds due to the indirect transition nature, we have precisely evaluated the density relation between the EX and BEX from the line-shape analysis of time-resolved photoluminescence spectra. At an EX density around 1×10 10/cm 2, the BEX density suddenly increases with a threshold-like profile. This behavior is quantitatively explained by the framework of BE statistics of the EX–BEX system. It is demonstrated that the threshold-like increase of the BEX density is recurred by the incidence of a second-excitation pulse with a time delay, which leads to the dynamical control of the BE statistics behavior.

Optical coherent control and phase shift of excitonic and biexcitonic four-wave-mixing polarization

The optical coherent control of the exciton–biexciton system is studied in four-wave-mixing (FWM) experiments on a ZnSe single-quantum well. Coherent control is achieved by applying a pair of phase-locked laser pulses which produce an FWM signal in combination with an additional third pulse. Oscillations as a function of the interpulse delay time are observed in the FWM signal at the energetic position of the exciton and biexciton resonance, respectively. A phase shift between these oscillations is found for certain orders of arrival of the excitation pulses at the sample. This clearly demonstrates that the coherent control acts in different ways: if the phase shift is observed the FWM polarization is coherently controlled. Otherwise, the coherent control just involves the polarization induced by the two phase-locked pulses.

Superfluidity of indirect excitons and biexcitons in coupled quantum wells andsuperlattices

The collective properties of indirect excitons in coupled quantum wells (CQWs)are considered. The energy of the ground state of the exciton liquid as afunction of the density of electrons e and holes h at different separationsDbetween e and h layers is analysed. The quantum gas–liquid transition asDdecreases is studied. The superfluidity appearance temperatures in the system(Kosterlitz–Thouless transition temperatures) have been estimated at differentseparations Dbetween e and h layers. For the anisotropic two-dimensional e–h system in CQWsthe Mott metal–insulator quantum transition is considered. The instability of theground state of the system of interacting two-dimensional indirect excitons in aslab of superlattice with alternating e and h layers is established. The stablesystem of indirect quasi-two-dimensional biexcitons, consisting of indirect excitonswith opposite directed dipole moments, is considered. The radius and the bindingenergy of the indirect biexciton are calculated. The collective spectrumof a rare system of two-dimensional indirect biexcitons, interacting aselectrical quadrupoles, is studied. The density of the superfluid componentns (T)and the Kosterlitz–Thouless phase transition to the superfluid state in this systemare analysed.

Boson characteristics of the exciton–biexciton system in a GaAs/AlAs type-II superlattice

We report on the appearance of Bose–Einstein statistics of the exciton–biexciton system in a (GaAs) 12/(AlAs) 12 type-II superlattice. The density relation of the exciton and biexciton is estimated from time-resolved photoluminescence spectra at various excitation powers and bath temperatures. The long lifetime of the type-II exciton, e.g., 2 μs at 5 K, enables us to obtain the precise information of the density relation. In a relatively low exciton-density region, the biexciton density obeys a well-known square law. At an exciton density around 1×10 10 cm −2, the biexciton density suddenly increases with a threshold-like nature. This behavior, which is realized at temperatures up to 8 K, can be understood from Bose–Einstein statistics.

Biexcitons in CdMnTe/CdTe/CdMgTe single quantum well

We have performed two-pulse and three-pulse time-integrated as well as spectrally resolved four-wave-mixing measurements on a single CdMnTe/CdTe/CdMgTe quantum well. The polarization dependence of both four-wave-mixing measurements can be explained well by the five-level model of the exciton–biexciton system including two-biexciton states with parallel spin and anti-parallel spin orientations.

BIEXCITON kth POWER AMPLITUDE SQUEEZING DUE TO OPTICAL EXCITON–BIEXCITON CONVERSION

We extend a previous paper on biexciton normal squeezing to the case of k th power amplitude squeezing in a quantum interacting system of photons, excitons and biexcitons. We find that the k th power amplitude squeezing depends on the characteristics of the initial coherent biexciton while the normal squeezing does not. Both the direction of maximal squeezing and the squeezing degree are derived as explicit functions of k. Especially, the squeezing degree always decreases with increasing k regardless of the initial conditions. All these properties are examined numerically.

Self-reflection in a system of excitons and biexcitons in semiconductors

The characteristic features of the self-reflection of a powerful electromagnetic wave in a system of coherent excitons and biexcitons in semiconductors were investigated as one of the manifestations of the nonlinear optical skin effect. It was found that a monotonically decreasing standing wave with an exponentially falling spatial tail is formed in the surface region of a semiconductor. Under the influence of the field of a powerful pulse, an optically homogeneous medium is converted into one with distributed feedback. The appearance of spatially separated narrow peaks of the refractive index, extinction coefficient, and reflection coefficient is predicted.

Superfluidity of indirect biexcitons in superlattices

Instability in a system of interacting quasi-two-dimensional excitons in a type II superlattice of a finite thickness due to attraction between oppositely-directed excitonic dipoles in neighboring layers has been discovered. A stable system is that of indirect quasi-two-dimensional biexcitons formed by indirect excitons with dipole moments oriented in opposite directions. The radius and binding energy of indirect biexcitons has been calculated. A collective spectrum of a system of such biexcitons with a weak quadrupole interaction between them has been studied. Feasibility of Bose condensation, the density n s(T) of the superfluid component, and a phase transition to the superfliud state in a low-density system of indirect biexcitons have been analyzed.

Biexciton-biexciton interaction in semiconductors

For a semiconductor with single isotropic conduction and valence bands, the effective biexciton-biexciton interaction is derived starting from the second-quantization representation for the Hamiltonian of the system of interacting excitons and for the biexciton wave function within the framework of the adiabatic approximation. The interaction is found to be an average of the sum of effective interactions between excitons forming interacting biexcitons over the envelope functions of these biexcitons. Depending on the momentum transfer and on the difference between the momenta of interacting biexcitons, the interaction admits an analytical study when the first vanishes. In this case, the interaction has in the r space the form of a function of the interbiexciton distance consisting of a strong repulsive and a weak attractive part. At low temperatures the function describes main features in the behavior of the interaction among biexcitons, in which repulsion predominates over attraction. It is shown that while biexcitons remain stable quasiparticles they weakly attract each other for any value of the distance. A quantitative analysis of obtained results for the CuCl crystal shows that the biexciton system in this model substance is a weakly nonideal Bose gas with positive scattering length ${\mathrm{a}}_{\mathrm{s}}$\ensuremath{\simeq}${3\mathrm{a}}_{\mathrm{x}}$, which closely approaches the ideal one at excitation densities n\ensuremath{\leqslant}${10}^{18}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$.

Gain spectroscopy of excitonic molecules and its dynamics in a ZnSe single quantum well.

We demonstrate spectroscopically how optical gain is formed in an interacting system of excitons and biexcitons in a single ZnSe quantum well. The dynamics of gain formation are studied on a subpicosecond time scale in pump-probe experiments, revealing the influence of coherent effects and the impact of exciton-exciton and exciton-biexciton interactions. Spectrally resolved, polarization-dependent degenerate four-wave-mixing experiments further demonstrate how the excitonic many-body phenomena profoundly influence the nonlinear optical response in this II-VI semiconductor heterostructure. \textcopyright{} 1996 The American Physical Society.

An experimental progress in the Bose-Einstein condensation of biexciton system in CuCl

The Bose-Einstein statistics and the Bose-Einstein condensation(BEC) of biexcitons in CuCl are examined by the spectral shape analysis of the M-band and the phase-conjugation (PC) spectroscopy. It is found that the PC signal intensity increases when the total density of biexcitons exceeds the critical density for the BEC. 100 % increase of the PC signal intensity is established under the supply of biexcitons incoherently.

Bose-Einstein condensation of biexciton system in CuCl

An experiment is described which aims at detecting the coherency associated with the presence of a Bose condensate of biexcitons in CuCl. The optical phase-conjugation signal at the two-photon resonance of theK = 0 biexciton is detected in the presence and in the absence of a high density pool of randomly injected biexcitons. An increase in the magnitude of the phase-conjugate signal together with a small blue shift of the biexciton resonance are attributed to the Bose condensate.

Excitonic Molecules in Nonlinear Optical Response

AbstractA model which allows to treat self‐consistently many‐body exciton effects, including biexciton formation, is proposed for the theoretical description of an excitonic polarization induced by an intense electromagnetic field. In the presence of a pump field the normal spectrum of electromagnetic waves in the vicinity of the nonlinear biexciton resonance is shown to be composed of four polariton branches, where each is the superposition of two photons and two excitons. In the self‐consistent field approximation, the structure of the interconnected, strongly nonlinear exciton and biexciton resonances is studied, including optical Stark shift, multistable behaviour, etc. For a certain range of the exciton interaction parameters at sufficiently strong pump wave the system of excitons and excitonic molecules transforms abruptly to a dense coherent electron–hole “liquid”. Appearance and temporary evolution of an exciton–biexciton system after switching on a pump is also considered.

Periodic and Stochastic Self‐Pulsations in the System of Coherent Excitons, Photons, and Biexcitons

AbstractPossible appearance of quasiperiodic and chaotic self‐pulsations in a system of coherent excitons and photons as well as in the processes of one‐ and two‐photon exciton–biexciton conversions are established for the first time. In the last two cases the generalized Lorenz equations are obtained. Lyapunov analysis of the stationary state stability and computer experiments are made. The main bifurcations in the three considered systems of quasiparticles are found.

Optical Bistability and Multistability in a System of High Density Excitons and Biexcitons in Semiconductors

AbstractResults are given on the investigation of optical bistability and multistability phenomena both, in the exciton range of the spectrum due to exciton‐photon and exciton‐exciton interactions and in the range of two‐photon generation of the biexcitons (excitonic molecules). For each case the steady‐state equations are obtained.

The Lorenz attractor in a system of coherent excitons, photons and biexcitons

It has been shown that a system of equations describing the dynamic evolution of coherent excitons, photons and biexcitons has a strange attractor of Lorenz type. The conditions for the appearance of stochastic instability have been found and numerical estimations for the CdS crystal are presented.

Local-field effect in the biexciton system in CuCl.

We show that the inclusion of the local-field effect in the calculation of the nonlinear dielectric function of CuCl near the biexciton resonance does not yield bistability on reflection as suggested by Abram and Maruani.