Inelastic scattering of oppositely charged polarons in conjugated polymers

Within an one-dimensional tight-binding model, we investigate the inelastic scattering processes of oppositely charged polarons in conjugated polymers under the influence of an external electric field, by using a nonadiabatic evolution method. It is found that the polaron pair does not necessarily scatter into an entity(neutral exciton), but a mixed state composed of both polarons and excitons. The yield of the neutral exciton depends sensitively on the strength of applied fields. Additionally, effects of interchain coupling on the scattering processes are also discussed, which shows that the interchain coupling is of fundamental importance and facilitates the formation of the polaron-exciton.     

Signatures of stimulated bosonic exciton-scattering in semiconductor luminescence

We observe signatures of stimulated bosonic scattering of excitons, a precursor of Bose-Einstein-Condensation (BEC), in the photoluminescence of semiconductor quantum wells. The optical decay of a spinless molecule of two excitons (biexciton) into an exciton and a photon with opposite angular momenta is subject to bosonic enhancement in the presence of other excitons. In a gas of biexcitons and spin polarized excitons the bosonic enhancement breaks the symmetry of two equivalent biexciton decay channels leading to circularly polarized luminescence of the biexciton with the sign opposite to the circularly polarized exciton luminescence. Comparison of experiment and many body theory clearly indicates the existence of stimulated exciton-scattering, but excludes the presence of a fully condensed BEC-like state.     

Exciton-Boson Formalism in the Theory of Laser-Excited Semiconductors and Its Application in Coherent Four-Wave-Mixing Spectroscopy

By the use of a bosonization transformation and group-theoretical arguments, the Hamiltonian of an electron–hole–photon system in a laser-excited direct two-band semiconductor is transcribed into that of an exciton–photon system with the particle spins rigorously taken into consideration. It is shown that the third-order optical nonlinearities in the spectral region below the band edge have their microscopic origin in two-exciton correlations, which are expressed in terms of the effective exciton–exciton and anharmonic exciton–photon interactions. The dependence of the interparticle interactions on the spin states of quasiparticles is behind the polarization dependence of the semiconductor nonlinear optical response. On the example of the system of heavy hole excitons in quantum wells, grown from compounds with the zinc blende type of symmetry, it is demonstrated that the effective exciton–exciton interaction in two-exciton states with nonzero total spin is repulsive, while in zero-spin states it is attractive, which may result in the biexciton formation. The derived Heisenberg equations of motion for the exciton and biexciton operators form the basis for a theoretical study of the coherent four-wave-mixing in GaAs and ZnSe quantum wells. It is readily apparent from the equations that in different polarization configurations the coherent four-wave-mixing is generated by different ingredients of two-exciton Coulomb correlations: in the co-circular configuration, it is the interexciton repulsion, in the cross-linear configuration, the formation of the biexciton and its coupling to excitons, and in the collinear configuration, both of them jointly. The obtained expressions for the time-resolved and frequency-resolved four-wave-mixing signals adequately describe the main characteristics and various details of wave mixing phenomena, including a biexciton signature in the appropriate polarization configurations. Results of the work clarify the microscopic mechanism of the polarization dependence in coherent four-wave-mixing spectroscopy in semiconductor quantum wells.     

Electronic structure and absorption spectrum of biexciton obtained by using exciton basis

We approach the biexciton Schrödinger equation not through the free-carrier basis as usually done, but through the free-exciton basis, exciton–exciton interactions being treated according to the recently developed composite boson many-body formalism which allows an exact handling of carrier exchange between excitons, as induced by the Pauli exclusion principle. We numerically solve the resulting biexciton Schrödinger equation with the exciton levels restricted to the ground state and we derive the biexciton ground state as well as the bound and unbound excited states as a function of hole-to-electron mass ratio. The biexciton ground-state energy we find, agrees reasonably well with variational results. Next, we use the obtained biexciton wave functions to calculate optical absorption in the presence of a dilute exciton gas in quantum well. We find an asymmetric peak with a characteristic low-energy tail, identified with the biexciton ground state, and a set of Lorentzian-like peaks associated with biexciton unbound states, i.e., exciton–exciton scattering states. Last, we propose a pump–probe experiment to probe the momentum distribution of the exciton condensate.     

链间耦合对极化子非弹性散射性质的影响

基于一维紧束缚的Su-Schreiffer-Heeger模型, 采用非绝热动力学方法, 研究了链间耦合对聚合物中极化子对非弹性散射性质的影响: 激子的产生依赖于链间耦合, 随着耦合强度的增加, 正负极化子对的电子波函数交叠增强, 利于提高激子的产率; 当耦合区域是极化子的宽度时, 正负极化子对波函数的耦合最充分、耦合最强, 电荷跃迁更容易, 激子产率最大. 关键词： 聚合物 极化子 激子     

Effects of interchain coupling on photoexcitation in two coupled polymer chains in the presence of an electric field

Within an extended Su-Schrieffer-Heeger model including interchain interactions and the extended Hubbard model (EHM), the dynamical relaxation of photoexcitations in the presence of an external electric field is investigated using a nonadiabatic evolution method. Under the action of the interchain interactions both intrachain excitons and interchain excitons are generated after photoexcitation in two coupled polymer chains. Our results show that the field required to dissociate the excitons depends sensitively on the strength of the interchain coupling. As the interchain coupling strength increases, the dissociation field decreases. By analyzing the relation between the yield of intrachain and interchain excitons and the interchain coupling strength, we explain the dependence between the dissociation field and the strength of the interchain interactions. The theoretical results are expected to provide useful predictions concerning which polymers with properly strong interchain interactions are likely to be most suitable for use in organic solar cells.     

Impact of shell thickness on exciton and biexciton binding energies of a ZnSe/ZnS core–shell quantum dot

Impact of shell structure on the exciton and biexciton binding energies has been studied in a ZnSe/ZnS core–shell quantum dot using Wentzel–Kramers–Brillouin (WKB) approximation. For excitons, the binding is caused by the Coulombic as well as the confinement potentials while biexciton binding energy is determined by taking into account the exchange and correlation effects. The exciton binding energy was found to increase initially with increasing shell thickness which reaches saturation at larger shell thickness. On the other hand, the biexciton binding energy exhibits a crossover from the bonding to antibonding state with increasing shell thickness for smaller core radius of the quantum dot.     

The effect of interchain coupling on the reverse polarization of a high-energy exciton in conjugated polymers

By using the one-dimensional extended Su–Schrieffer–Heeger model, we investigate the effect of interchain coupling on the reverse polarization of a high-energy exciton in conjugated polymers. Our results show that, without interchain coupling, the high-energy exciton is completely localized in one chain, which could be reversely polarized by an electric field. However, when taking into account interchain coupling, the high-energy exciton tends to extend between two chains. Its reverse polarization would be reduced accordingly. We also find that the interchain coupling effect is more obvious for polymers with a stronger non-degenerate confinement. Besides this, the effect of electron–electron interaction is also discussed.     

链间耦合对聚合物中双激子态反向极化的影响

从紧束缚模型出发，研究了链间相互作用对双激子态反向极化的影响.结果发现：加链间耦合后，双激子态仍主要局域在一条链中.当简并破缺较小时，反向极化程度随耦合强度的增加大幅提高；当简并破缺较大时，反向极化程度随耦合强度的增加基本不变. 关键词： 反向极化 双激子态 链间耦合     

Self-consistent calculations of exciton, biexciton and charged exciton energies in InGaN/GaN quantum dots

We present theoretical calculations of the variation of exciton energies in truncated conical InGaN quantum dots (QDs) in a GaN matrix with dot size and indium composition. We compute the built-in strain-induced and spontaneous piezoelectric fields using a surface integral method that we have recently derived, and confirm that the built-in fields can be of the order of a few MV/cm, resulting in a spatial separation of the electrons and holes. The ground state wavefunctions of the exciton (X⁰), biexciton (2X⁰) and the two charged excitons (X⁻ and X⁺) are then calculated in the Hartree approximation, using a self-consistent finite difference method. We find that the electron–hole recombination energy is always blue-shifted for the charged excitons X⁻ and X⁺, with a further blue-shift for the biexciton, and this blue-shift increases with increasing indium content. We describe the trends in interband transition energy and the scale of the blue-shift with dot size, shape and composition. We conclude that spectroscopic studies of the exciton, charged excitons and biexciton should provide a useful probe of the structural and piezoelectric properties of GaN-based QDs.     

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.     

Excitation spectroscopy,Raman scattering and the temperature dependence of the luminescence in highly excited red HgI2

The luminescence of red HgI₂ is investigated as a function of temperature, excitation intensity and wavelength. At high excitation intensity and low temperature an “M-band” emission dominates. This M-band is assigned to biexciton decay and bound exciton scattering with acoustic phonons (“acoustic wing”), this assumption being supported by the results of excitation spectroscopy. The energy of the biexciton is determined to be (4661 ± 1) meV. From the evaluation of Raman spectra, the phonon energies (1.9, 3.1 and 14.0 ± 0.2) meV are found. At higher temperatures two lines are observed, one of which is ascribed to exciton-free carrier scattering. Position and line shape are in good agreement with theoretical results. The other emission line is found to be due to scattering involving excitons or carriers bound to lattice defects.     

Optically induced rotation of an exciton spin in a semiconductor quantum dot

We demonstrate control over the spin state of a semiconductor quantum dot exciton using a polarized picosecond laser pulse slightly detuned from a biexciton resonance. The control pulse follows an earlier pulse, which generates an exciton and initializes its spin state as a coherent superposition of its two nondegenerate eigenstates. The control pulse preferentially couples one component of the exciton state to the biexciton state, thereby rotating the exciton's spin direction. We detect the rotation by measuring the polarization of the exciton spectral line as a function of the time difference between the two pulses. We show experimentally and theoretically how the angle of rotation depends on the detuning of the second pulse from the biexciton resonance.     

Strong Exciton-Plasmon Coupling and Hybridization of Organic-Inorganic Exciton-Polaritons in Plasmonic Nanocavity

We investigate strong exciton-plasmon coupling and plasmon-mediated hybridization between the Frenkel(F)and Wannier-Mott(WM)excitons of an organic-inorganic hybrid system consisting of a silver ring separated from a monolayer WS_2 by J-aggregates.The extinction spectra of the hybrid system calculated by employing the coupled oscillator model are consistent with the results simulated by the finite-difference time-domain method.The calculation results show that strong couplings among F excitons,WM excitons,and localized surface plasmon resonances(LSPRs) lead to the appearance of three plexciton branches in the extinction spectra.The weighting efficiencies of the F exciton,WM exciton and LSPR modes in three plexciton branches are used to analyze the exciton-polaritons in the system.Furthermore,the strong coupling between two different excitons and LSPRs is manipulated by tuning F or WM exciton resonances.     

Kinematic quasi-vacancy Frenkel biexciton in a Davydov multiplet

This paper shows that, as a consequence of paulion-type Frenkel excitons in molecular crystals such as anthracene, with several molecules per unit cell, bound biexcitons exist in the absence of dynamic exciton-exciton attraction. The coupling of excitons in ideal crystals of this type is brought about by the same kinematic effect that causes excitons in nonideal crystals to be localized close to defects such as vacancies. In the excitation spectrum, the resonance peak of the resulting biexciton is found inside a band corresponding to the sum of the energies of excitons of unlike components of a Davydov doublet, located between the bands of the total energies of excitons of the like components of the doublet. The corresponding dispersion equation is obtained, and the energy and wave function of the biexciton is determined. Fiz. Tverd. Tela (St. Petersburg) 41, 423–428 (March 1999)     

高分子中双激子态对外电场的线性响应

在外电场中,非简并基态共轭高分子中的双激子具有反向极化的奇异特性.利用响应函数求得高分子链的双激子态在任意频率下的动态极化率χ(ω),并通过求ω→0极限,给出双激子态的静态极化率χ,证实了χ确实为负值.并与激子态和基态作了比较,分析了分子负极化率的起因.考察了响应函数在低频时的行为,指出激子和双激子的激发浓度增大会导致材料折射率n(ω)向不同方向变化. 关键词：     

Biexciton quantum coherence in a single quantum dot

Nondegenerate (two-wavelength) two-photon absorption using coherent optical fields is used to show that there are two different quantum mechanical pathways leading to formation of the biexciton in a single quantum dot. Of specific importance to quantum information applications is the resulting coherent dynamics between the ground state and the biexciton from the pathway involving only optically induced exciton/biexciton quantum coherence. The data provide a direct measure of the biexciton decoherence rate which is equivalent to the decoherence of the Bell state in this system, as well as other critical optical parameters.     

Crossover from exciton to biexciton polaritons in semiconductor microcavities

Pump-probe measurements in a microcavity containing a quantum well show that a population of circularly polarized ( sigma(+)) excitons can completely inhibit the transition to sigma(-) one-exciton states by transferring the oscillator strength to the biexcitonic resonance. With increasing pump intensity the linear exciton-polariton doublet evolves into a triplet polariton structure and finally into a shifted biexciton-polariton doublet. A theoretical model of interacting excitons demonstrates that the crossover from exciton to biexciton polaritons is driven by three-exciton Coulomb correlation.     

Quantum Monte Carlo simulation of exciton–exciton scattering in a GaAs/AlGaAs quantum well

We present a computer simulation of exciton–exciton scattering in a quantum well. Specifically, we use quantum Monte Carlo techniques to study the bound and continuum states of two excitons in a 10 nm wide GaAs/Al_0.3Ga_0.7As quantum well. From these bound and continuum states we extract the momentum-dependent phase shifts for s-wave scattering. A surprising finding of this work is that a commonly studied effective-mass model for excitons in a 10 nm quantum well actually supports two bound biexciton states. The second, weakly bound state may dramatically enhance exciton–exciton interactions. We also fit our results to a hard-disk model and indicate directions for future work.