Exciton effects on the refractive index and optical absorption in wurtzite quantum wells via a fractional-dimensional space approach

The optical refractive index changes and absorption coefficients of quantum wells (QWs) are theoretically investigated with considering exciton effects within the framework of the fractional-dimensional space approach. The exciton wave functions and bound energies are obtained as a function of spatial dimensionality, and the dimension increases with the well width increasing. Then optical properties are obtained by using the compact-density matrix approach and an iterative method. Numerical results are presented for wurtzite ZnO/Mg_xZn_1−xO QWs. The calculated results show that the changes of refractive index and absorption coefficients are greatly enhanced due to the quantum confinement of exciton. And the smaller the QW width (dimension) is, the larger influence of exciton on the optical properties will be. Furthermore, the exciton effects make the resonant peaks move to a lower energy. In addition, the optical properties are related to the QW width, the incident optical intensity and carrier density.     

The Exciton–Polariton Dispersion Law under the Action of Strong Pumping in the Region of the <Emphasis Type="Italic">M</Emphasis>-Band of Luminescence

The double-pulse interaction with excitons and biexcitons in semiconductors is studied theoretically. It is shown that the dispersion law of carrier wave has three branches under the action of a powerful pumping in the region of the M-band of luminescence. Values of parameters at which the dispersion law branches can intersect due to the degeneration of the exciton level energy have been found. The effect of a significant change in the force of coupling between the exciton and photon of a weak pulse with a change in the pumping intensity is predicted.     

Indirect exciton absorption and Raman scattering in GaAs-AlGaAs superlattices

Longitudinal optic (LO) phonon assisted indirect exciton creation (X_LO), hot carrier relaxation ((e-h)_LO) and Raman scattering phenomena are reported in the optical spectra of GaAs-AlGaAs superlattices. Structures of the same dimensions both with and without double heterostructure confining barriers are studied. For the structures without confining barriers, continuum transitions are suppressed in photoluminescence excitation (PLE) spectra, and as a result the X_LO, (e-h)_LO and Raman peaks are observed. The X_LO absorption peaks are identified from the observation of a clear threshold in PLE at ℏω_LO (36.4 meV) above the heavy hole exciton peak. The intensity of X_LO is a maximum at 6 meV above the threshold, probably due to dissociation into free carriers at the exciton binding energy (6meV) above ℏω_LO. The influence of non-radiative processes on incoherent (PLE) and coherent (Raman) processes is compared.     

Spike in transient photocurrent of organic solar cell: Exciton dissociation at interface

The effect of exciton interfacial dissociation on transient photocurrent (TPC) in a single-layer organic solar cell is investigated within a time-dependent device model. The spike observed in TPC experiments is attributed to exciton dissociation at the electrode/organic interface. In comparison with the observed negative signal of transient photovoltage (TPV), the spike more directly reflects the charge processes at the interface. Moreover, numerical results show that the spike of TPC is sensitive to the voltage applied on the device and the hole mobility of the organic semiconductor. Further investigation on the spike by the favorable TPC technique is suggested to provide details about the exciton and carrier processes at the interface.     

Stark effect of negatively and positively charged excitons in semiconductor quantum wells

We study the effect of an electric field applied normal to the layers on the binding energy of charged excitons (or trions) in GaAs quantum wells. We find that, in contrast to the neutral exciton, their binding energy is sharply reduced by modest electric fields. The effect is stronger for the positively charged exciton than the negatively charged one. The ionisation of the excess carrier is explained by the field-induced polarisation of the electron and hole subband wave functions.     

Electric-field control of electron–hole wave functions in a wide quantum well

The electric field dependence of the electron/hole wave function and the radiation energy of an exciton in a Be-δ-doped 80 nm quantum well (QW) is studied experimentally and compared it with variational calculation. The photoluminescence (PL) spectra show Stark shifts depending on the gate electric field and PL intensity of the exciton of the first excited state has a dip in the electric-field dependence which reflects the node of the electron wave function.     

Subnanosecond luminescence spectroscopy of Cd1−xMnxSe under high intensity optical excitation

Transient photoluminescence of Cd_1?xMn_xSe has been investigated for compositions x = 0, .05 and .10, in zero magnetic field, under high excitation intensity. The spectra yield information about the temporal evolution of inelastic scattering processes in a dense exciton gas, with a likely contribution by an electron-hole plasma. The short carrier lifetimes measured, less than 100 psec for x = .10, and a possible plasma expansion, might inhibit the generation of carrier densities sufficient for suggested exchange induced magnetic ordering.     

Shake-up intersubband transitions observed in GaAs/AlGaAs quantum wells

Shake-up transitions involving QW hole subbands have been observed as satellites in selective photoluminescence spectra of undoped GaAs/AlGaAs QWs. These shake-up transitions are explained in terms of an interaction between localized exciton and valence-band hole states attached to the QW subbands, in which holes are shaken up from the n=1 heavy hole subband to higher subbands, either the n=1 light hole subband or the n=2 heavy hole subband. The required localization is due to the interface roughness; thus these new transitions are of intrinsic origin. From the observation of the intersubband shake-up processes we derive direct information about the hole inter-subband energies. Furthermore, the satellite intensity is strikingly enhanced in the presence of a magnetic field due to an increasing exciton localization related to the compression of its wave function in the field. The exciton wave function compression continues until its radius in the plane of the well is comparable with the radius of the "flat island" characterized by constant QW width. Accordingly, from the magnetic field dependence of the shake-up satellite intensity we can roughly estimate the size of the "flat islands" and consequently probe the interface roughness.     

CdSe/ZnSe复合结构非对称量子阱的发光特性

利用MOCVD技术在GaAs衬底上外延生长了非对称量子阱结构CdSe/ZnSe材料,通过对其稳态变温光谱及变激发功率光谱,研究了其发光特性。稳态光谱表明:在82～141K时,观测到的两个发光峰来源于不同阱层厚度的量子阱激子发光,用对比实验验证了高能侧发光的来源。宽阱发光强度先增加后减小,将其归结为激子隧穿与激子热离化相互竞争的结果。通过Arrhenius拟合,对宽阱激子热激活能进行了计算。82K时变激发功率PL光谱表明:由于激子隧穿的存在,使得窄阱发光峰位不随激发功率变化而变化,宽阱发光峰位随激发功率增加发生了蓝移,并对激子隧穿进行了实验验证。     

Dielectric enhancement of the exciton energies in laser-dressed near-surface quantum wells

A theoretical study of the intense high-frequency laser field effect on the interband transitions and on the ground (1S-like) and excited (2S-like) exciton states in InGaAs/GaAs near-surface quantum wells is performed within the effective mass approximation. The carrier confinement potentials and image charge contributions to the Coulomb interaction can significantly be modified and controlled by the capped layer thickness and laser field intensity. We found that: (i) the interband and exciton transition energies monotonically enhance with the laser amplitude; (ii) for small capped layers the splitting between the 2S and 1S exciton lines are more sensitive to the dressing laser parameter, and (iii) for high enough laser intensities the dressing effects on both confining potential and Coulomb interactions can yield entirely different exciton emission spectra depending on the cap layer thickness. Our results are compared with the theoretical and experimental data obtained in the absence of the laser field and a good agreement is reached.     

Photoreflection and photoconduction spectra of CdS crystals: Excitons in the electric fields of surface states

The exciton photoreflection spectra of CdS crystals are studied. It is found that the form of the exciton photoreflection spectrum is determined by a Stark shift of the exciton energy in the electric field of surface states. The dependences of the exciton photoreflection spectrum on temperature on the intensity and wavelength of the modulating radiation, and on the processes by which the photoreflection signal relaxes is determined. An energy scheme is proposed for the surface states which explains the observed effects of photoinduced changes in the surface field. A correlation is established between the exciton photoreflection spectrum and the form of the fine structure in the photoconductivity. Fiz. Tverd. Tela (St. Petersburg) 40, 875–876 (May 1998)     

Temperature and density dependence of exciton lifetimes in GaAs/AlGaAs multiple quantum wells

The photoluminescence linewidths and excition lifetimes of free excitons in GaAs/AlGaAs multiple quantum wells were systematically investigated as a function of temperature, quantum well width, and carrier density. The experimental results showed that the exciton decay processes were strongly related to the linewidth of the exciton and the exciton binding energy.     

Photon emission induced by elastic exciton-carrier scattering in semiconductor quantum wells

We present a study of the elastic exciton-electron (X-e⁻) and exciton-hole (X-h) scattering processes in semiconductor quantum wells, including fermion exchange effects. The balance between the exciton and the free carrier populations within the electron-hole plasma is discussed in terms of ionization degree in the nondegenerate regime. Assuming a two-dimensional Coulomb potential statically screened by the free carrier gas, we apply the variable phase method to obtain the excitonic wavefunctions, which we use to calculate the 1s exciton-free carrier matrix elements that describe the scattering of excitons into the light cone where they can radiatively recombine. The photon emission rates due to the carrierassisted exciton recombination in semiconductor quantum-wells (QWs) at room temperature and in a low density regime are obtained from Fermi’s golden rule, and studied for mid-gap and wide-gap materials. The quantitative comparison of the direct and exchange terms of the scattering matrix elements shows that fermion exchange is the dominant mechanism of the exciton-carrier scattering process. This is confirmed by our analysis of the rates of photon emission induced by electron-assisted and hole-assisted exciton recombinations.     

电场调控下不对称耦合量子点中激子的发光特性

使用有效质量模型,从理论上对GaAs/A10.35Ga0.65As不对称耦合量子点在不同耦合强度下束缚态和反束缚态的能级分裂情况进行了详细分析,重点探讨了电子和空穴的耦合隧穿对量子点体系能级特征及激子发光强度的影响.研究发现:不对称耦合量子点在外电场作用下价带束缚态和反束缚态能级出现反交现象,反交处的能级分裂值和临界电...     

Lateral carrier tunnelling in stacked In(Ga)As/GaAs quantum rings

Exciton recombination dynamics in vertical stacks of InGaAs quantum rings have been studied by means of continuous wave and time resolved photoluminescence under low excitation density conditions. We have paid special attention to the effect of the carrier coupling on the exciton radiative lifetime: weak (14 nm spacer sample), intermediate (4.5 nm spacer sample), where the size filtering effects (towards small rings) compensate partially that arising from carrier coupling (towards lower energies), and strong electron and hole coupling (1.5 nm spacer sample) between layers. Experimental decay times in the latter two cases have been compared to the times simulated with a multi-quantum well based model, which accounts for the observed change of carrier coupling regime. The most important effect is observed when the hole wave function overlap along the growth direction becomes important (1.5 nm spacer sample). This situation makes important the lateral tunneling of excitons between rings, given their large lateral size, which is characterized by times around 5 ns at the emission peak energy (rings with the most probable size of the distribution).     

Temperature properties of exciton luminescence from CdTe quantum wells with different thicknesses in the CdTe/CdMnTe structure

The optical spectra of the CdTe/Cd_0.7Mn_0.3Te structure containing three CdTe quantum wells with nominal thicknesses of 16, 8, and 4 monolayers have been investigated. The temperature dependences of parameters of the exciton luminescence spectra (integrated intensity, full-width at half-maximum, position of the maximum, Stokes shift) for quantum wells with different thicknesses differ substantially. These differences are explained by a strong thickness dependence of the energy of Coulomb coupling in the exciton, the energy of localization of the exciton on bulges of the quantum well, and the degree of penetration of the exciton wave function into the barrier. At high excitation power densities, the emission contours of the quantum wells with thicknesses of 8 and 16 monolayers contain short-wavelength tails that correspond to optical transitions between excited quantum-well levels.     

Two-phonon resonant Raman scattering at the indirect exciton in silver chloride

Resonant Raman scattering in AgCl is reported for the first time. With excitation in the indirect absorption edge at 1.8K the observed scattering processes involve pairs of momentum-conserving TA(L) and LA(L) phonons with energies of 8.2 meV and 12.9 meV, respectively. The dependence of the scattered intensity on excitation energy suggests that, in contrast to AgBr, intravalley scattering by long wavelength acoustic phonons and intervalley scattering are negligibly small, the main relaxation mechanism of the free exciton being self-trapping. Some enhancement in the 2LO scattering intensity observed is probably due to resonance with the lowest direct exciton.     

共轭聚合物中受激吸收与受激辐射的量子动力学研究

基于扩展的一维SSH紧束缚模型结合非绝热的分子动力学方法,理论研究了共轭聚合物分子(PPV)在光脉冲作用下受激吸收和受激辐射的量子动力学过程.首先,设定分子初始处于基态,讨论了受激吸收过程中不同的电子受激跃迁模式与光激发脉冲的关系.通过对终态的分析,发现分子受激后只能产生电子-空穴的束缚态,包括:激子、双激子和高能激子.计算了各种激发态的产率,特别是,给出了各种激发态产率与光激发能量的定量关系.此外,基于实验,分别讨论了光激发强度对高能激子和双激子产率的影响,并与实验结果进行了比较.最后,设定分子初始分别处于激子和双激子态,研究了分子内定域能级之间的受激辐射过程,并简单讨论了激子和双激子受激辐射与光激发能量及强度的关系.     

Fluorescence quenching by charge carriers in indolo[3,2-b]carbazole-based polymer

The fluorescence quenching by the applied voltage in a new indolocarbazole-based polymer poly{5,11-di-2,3-epoxypropyl-6-pentyl-5,11-dihydroindolyl[3,2-b]carbazolyl-alt-4,4′-tiobisbenzentiole} sandwiched between ITO and Al electrodes has been investigated. Time-resolved fluorescence measurements have shown that the excitons are quenched during their entire life-time. The fluorescence intensity showed complex dependence on the applied voltage and the excitation light intensity, memory effects and dynamics on time scale of tens of seconds. These effects are explained by the exciton quenching by mobile and trapped charge carriers. Charge carrier density dynamics calculated by using a simplified model evaluating carrier injection, photogeneration, recombination and trapping was used to model variations of the fluorescence intensity induced by the periodically applied voltage. These effects point out the importance of the fluorescence quenching by trapped carriers in organic materials possessing high density of carrier traps or with broad distribution of density of states.     

Novel excitonic transitions in n-type GaAs/AlGaAs quantum wells

We report on a novel peak, the F-line, observed in photoluminescence spectra of GaAs/AlGaAs quantum wells (QWs) with various donor layer positions and concentrations. The F-line is well-defined and red shifted by approximately 1.3 meV (dependent on the experimental conditions) relatively the free exciton (FE) in a 200 Å wide QW. The F-line exhibits a strong magnetic field dependence. The enhanced intensity with increasing field is due to an increasing wave function overlap caused by the enhanced localization of the involved charge carriers. In accordance, the derived thermal activation energy for the F-line is magnetic field dependent. The F-line exhibits a diamagnetic shift as expected for an excitonic transition and splits into four components with increasing magnetic field. Another associated higher energy peak, the E-line, is observed preferably in the presence of a magnetic field, between the heavy hole- and light hole-FE in PL excitation spectra. The E-line also exhibits a striking magnetic field and temperature dependence. The observed properties of the F-line with a striking dependence on the excitation intensity, magnetic field and temperature are consistent with the observation of an exciton bound at the negatively charged D^- donor state or a negatively charged X^- exciton.