Real-time analysis of nonlinear transient response of weakly confined excitons

By means of the nonlocal transient-response theory, we elucidate the characteristics of the femtosecond transient response of thin films with a thickness beyond the long wavelength approximation (LWA) regime. In this regime, the contribution of higher excitonic states with a nondipole-type spatial structure becomes dominant and the interplay between the spatial structures of excitonic wavefunction and the radiation field plays an important role, causing an anomalous enhancement of nonlinear signal at specific size-energy resonant conditions. In addition, in the femtosecond pulse excitation, the interference of the signals from the multiple excitonic states, which are excited simultaneously by the incident pulse with a wide spectral width, can generate a greater diversity of optical response than expected by the steady-state analysis. This suggests the possibility that we can control the optical function of the nano-materials by the selective excitation of the aimed excitonic states using the laser pulse. This study serves as a theoretical basis for the nonlinear transient response by ultrashort pulse excitation of excitons confined in nano-structures.     

Properties of excitons in a wide CdTe-based quantum well with irregular insertions of a fractional monolayer of ZnTe

The irregular short period CdTe/ZnTe superlattice structure is investigated by both stationary and time-resolved optical spectroscopy with and without an external magnetic field as a perturbation. This study is aimed to emphasize the properties of radiative excitonic recombination in a superlattice of this type in comparison with the excitons confined in a single QW structure. The decay time of the excitons is about 400 ps which is deduced from the time-resolved measurements. Theg-factors of electrons and holes are obtained by the spin quantum beat measurements combined with Zeeman measurements. The experimental results show that theg-factors of holes in the irregular short period CdTe/ZnTe superlattice become dramatically different in comparison with the single CdTe/CdMgTe quantum wells.     

Excitonic processes at organic heterojunctions

Understanding excitonic processes at organic heterojunctions is crucial for development of organic semiconductor devices. This article reviews recent research on excitonic physics that involve intermolecular charge transfer (CT) excitons, and progress on understanding relationships between various interface energy levels and key parameters governing various competing interface excitonic processes. These interface excitonic processes include radiative exciplex emission, nonradiative recombination, Auger electron emission, and CT exciton dissociation. This article also reviews various device applications involving interface CT excitons, such as organic light-emitting diodes (OLEDs), organic photovoltaic cells, organic rectifying diodes, and ultralow-voltage Auger OLEDs.     

Stark shifts of charged particles in semiconductor quantum wells

 We calculate the effect of a homogeneous electric field on electrons, holes and excitons confined in a quantum well structure consisting of alternate thin layers of well and barrier material. The electric field which acts perpendicular to the quantum well is taken as a perturbation on the quantum well structure confining the charges. The electron and hole energies in the conduction and valence subbands are calculated by solving a one-dimensional Schr?dinger equation. The exciton binding energy is calculated using an improved excitonic model. Results obtained indicate the importance of higher-order excitons in optical transitions at high electric fields. Received: 29 February 1996/Accepted: 19 August 1996     

Magnetophotoluminescence study of GaAs/AlGaAs coupled double quantum wells with bimodal heterointerface roughness

This work reports on the results of magnetophotoluminescence (MPL) measurements carried out in a sample containing two Al_0.35Ga_0.65As/GaAs, coupled double quantum wells (CDQWs), with inter-well barriers of different thicknesses, which have the heterointerfaces characterized by a distribution of bimodal roughness. The MPL measurements were performed at 4 K, with magnetic fields applied parallel to the growth direction, and varying from 0 to 12 T. The diamagnetic shift of the photoluminescence (PL) peaks is more sensitive to changes in the confinement potential, due to monolayer variations in the mini-well thickness, rather than to the exciton localization at the local potential fluctuations. As the magnetic field increases, the relative intensities of the two peaks in each PL band inverts, what is attributed to the reduction in the radiative lifetime of the delocalized excitons, which results in the radiative recombination, before the excitonic migration between the higher and lower energy regions in each CDQW occurs. The dependence of the full width at half maximum (FWHM) on magnetic field shows different behaviors for each PL peak, which are attributed to the different levels and correlation lengths of the potential fluctuations present in the regions associated with each recombination channel.     

Excitonic polaron and phonon assisted photoluminescence of ZnO nanowires

The coupling strength of the radiative transition of hexagonal ZnO nanowires to the longitudinal optic (LO) phonon polarization field is deduced from temperature dependent photoluminescence spectra. An excitonic polaron formation is discussed to explain why the interaction of free excitons with LO phonons in ZnO nanowires is much stronger than that of bound excitons with LO phonons. The strong exciton-phonon coupling in ZnO nanowires affects not only the Haung-Ray S factor but also the FXA-1LO phonon energy spacing, which can be explained by the excitonic polaron formation.     

Interaction between the exciton and nuclear spin systems in a self-organized ensemble of InP/InGaP size-quantized islands

Magnetic interaction between spin-polarized nuclei and optically oriented excitons in a self-organized ensemble of size-quantized InP islands in an InGaP matrix has been studied in a magnetic field in Faraday geometry. The effective magnetic fields generated by polarized nuclei at excitons have been measured. The strengths of these fields were found to be different for active and inactive excitons because of the difference between the excitonic g factors. The heavy-hole g factor has been determined. The active and inactive excitonic states were found to be coupled through cross-relaxation. Fiz. Tverd. Tela (St. Petersburg) 41, 2193–2199 (December 1999)     

Two-dimensional excitonic phase in strong magnetic fields

The excitonic phase of the coupled electron-hole system which consists of p- and n-channel inversion layers separated by a thin insulating layer and subjected to a strong magnetic field, is investigated in the mean field approximation. The spectrum of the symmetry-restoring collective excitation mode is shown to be consistent with the possible superfluidity of excitons.     

Effects of excitation spectral width on decay profile of weakly confined excitons

We report the effect due to a simultaneous excitation of several exciton states on the radiative decay profiles on the basis of the nonlocal response of weakly confined excitons in GaAs thin films. In the case of excitation of single exciton state, the transient grating signal has two decay components. The fast decay component comes from nonlocal response, and the long-lived component is attributed to free exciton decay. With an increase of excitation spectral width, the nonlocal component becomes small in comparison with the long-lived component, and disappears under irradiation of a femtosecond-pulse laser with broader spectral width. The transient grating spectra clearly indicates the contribution of the weakly confined excitons to the signal, and the exciton line width hardly changes by excitation spectral width. From these results, we concluded that the change of decay profile is attributed not to the many-body effect but to the effect of simultaneous excitation of several exciton states.     

Efficient radiative recombination of multinode-type excitons up to room temperature in CuCl thin films

We report the observation of anomalous temperature dependences of degenerate four-wave mixing spectra in CuCl thin films with high crystalline quality. The observed temperature dependence is in good agreement with the phase-decay-constant dependence of calculated induced-polarization spectrum. An excitonic state with large radiative width can be observed at high temperatures as superradiance is faster than the dephasing process. We succeeded in observing the DFWM signal up to room temperature based on the extremely large radiative width peculiar to the thickness region beyond the long-wavelength approximation regime.     

Indirect excitons and double electron-hole layers in a wide single GaAs/AlGaAs quantum well in a strong electric field

The recombination spectra of indirect excitons and double electron-hole layers in a wide single quantum well in an electric field are studied. It is found that electrons and holes in the wide well become spatially separated in a sufficiently strong electric field. This leads to a substantial reorganization of the radiative recombination spectrum and to a significant increase in the carrier lifetime. It is shown that the total charge of the electron-hole system can be changed by varying the photoexcitation frequency and the applied electric field, thus passing from the neutral case of indirect excitons to the case of charged double electron-hole layers. The concentration of excess carriers in the well is measured as a function of the electric field strength. The behavior of the excited states of indirect heavy-hole and light-hole excitions is studied for a neutral excitonic system in a strong electric field. It is shown that the electric-field dependences allow the excited states of indirect excitons with a light hole to be distinguished from the excited states with a heavy hole.     

Theoretical study of indirect excitons’ lifetime in coupled AlGaN/GaN quantum wells in the presence of an electrostatic trap

The lifetime of electrostatically trapped indirect excitons in a field-effect structure based on coupled AlGaN/GaN quantum wells has been theoretically studied. Within the plane of a double quantum well, indirect excitons are trapped between the surfaces of the AlGaN/GaN heterostructures and a semitransparent metallic top gate. The trapping mechanism has been assumed to be a combination of the quantum confined Stark effect and local field enhancement. In order to study the trapped exciton lifetime, the binding energy of indirect excitons in coupled quantum wells is calculated by finite difference method in the presence of an electric field. Thus, the lifetime of trapped excitons is computed as a function of well width, AlGaN barrier width, the position of double quantum well in the device and applied voltage.     

Exciton-assisted tunneling transport in heterojunction microstructures

Longitudinal tunneling transport in the low-dimensional heterojunction structures induced by the excitonic Coulomb interaction has been formulated and discussed in the framework of Fermi's golden rule. We have investigated the tunneling transition of free carriers to quantum-well Wannier–Mott excitons incorporated in the sequential tunneling Hamiltonian. The modeling is evaluated by a set of coupled rate equations involving subband states of electron, hole and exciton. The exciton-assisted tunneling (EAT) phenomenon has its characteristic fingerprint causing tunneling current prior to the resonance electric fields, and a significant modulation of the I–V characteristics. It is also found that the bias offset and the FWHM of the EAT current spectrum can be comparable to that of resonant tunneling (RT) current, depending both on the 2D hole density of the confined subband and the excitonic properties in the active region. The EAT effect has a different I–V spectral line shape, compared to that of the RT or its replica, tailing off in the resonance regime induced by the exciton binding energy.     

镓铟磷三元化合物中的自由激子和杂质N发光中心的结构位形

我们研究了GaxIn1-xP:N(x=0.99,0.98,0.96)的发光光谱,用Ar+离子激光器的458nm线激发。N的浓度为5×1017cm-3。图1示出了在6K下Ga0.99In0.01P:N的发光光谱。Mariette和Chevallier[1,2]以及Nelson和Holonyak,Jr[3]。认为Nx0带起源于束缚在孤立N中心上激子的辐射复合。     

Time-resolved spectroscopy of excitonic transitions in ZnO/(Zn, Mg)O quantum wells

We report on the optical spectroscopy of a series of ZnO/(Zn, Mg)O quantum wells of different widths, using time-resolved photoluminescence. The samples were grown by molecular beam epitaxy on ZnO templates, themselves deposited on sapphire substrates. The barriers consist of Zn_0.78Mg_0.22O layers. The presence of large internal electric fields in these quantum wells results in a competition between quantum confinement and the quantum confined Stark effect as the quantum well width is varied. A transition energy lying 0.5 eV below the ZnO excitonic gap was observed for the widest of our wells. The PL spectra of the wide quantum wells were obtained using time-integrated photoluminescence, taking a great care with screening effects induced by their very slow dynamics. The effect of the built-in electric field on the excitonic properties was investigated. The excitonic fine structure is shown to depend strongly on the enhancement or suppression of the exchange interaction as a function of the quantum well width.     

Rabi oscillations of excitons in single quantum dots

Transient nonlinear optical spectroscopy, performed on excitons confined to single GaAs quantum dots, shows oscillations that are analogous to Rabi oscillations in two-level atomic systems. This demonstration corresponds to a one-qubit rotation in a single quantum dot which is important for proposals using quantum dot excitons for quantum computing. The dipole moment inferred from the data is consistent with that directly obtained from linear absorption studies. The measurement extends the artificial atom model of quantum dot excitonic transitions into the strong-field limit, and makes possible full coherent optical control of the quantum state of single excitons using optical pi pulses.     

Relaxation of hot excitons in CdZnSe/ZnSe quantum wells and quantum dots

The relaxation dynamics of hot excitons was studied in (Zn,Cd)Se/ZnSe quantum wells and quantum dots. A fast population of the radiative excitonic ground state occurs for an excitation excess energy corresponding to an integer number of optical phonon energies. This is indicated by a spectrally narrow photoluminescence peak observed immediately after the exciting laser pulse. Spatial diffusion of excitons, controlled by the interaction between excitons and acoustic phonons, causes a distinct linewidth broadening with increasing delay time in quantum wells. In contrast, this process is found to be strongly suppressed in quantum dots.     

纳米结构ZnO晶体薄膜室温紫外激光发射

文章综述了纳米结构的氧化锌半导体薄膜在室温下自由激子的自发辐射以及由自由激子引起的受激发射的特性，阐述了在不同激发密度下室温紫外受激发射的机理．纳米结构氧化锌半导体薄膜是用激光分子束外延(L-MBE)技术生长在蓝宝石衬底上的．薄膜由密集而规则排列的纳米尺度的六角柱组成．这些纳米六角柱起着限制激子运动的作用，激子的量子尺寸效应，使激子的跃迁振子强度大幅度增强．同时六角柱之间的晶面组成了一个天然的激光谐振腔．室温下用三倍频的YAG脉冲激光激发，可从这些纳米结构的氧化锌薄膜中观测到很强的紫外激光发射．研究发现，在中等激发密度下，紫外受激发射是由于激子与激子间碰撞而引起的辐射复合．在高密度激发条件下，由于激子趋于离化，紫外受激发射主要由电子-空穴等离子体的辐射复合引起．由于纳米结构中激子的跃迁振子增强效应，在室温下测量到的光学增益高达320cm^-1，这比在同样条件下测量到的块状氧化锌晶体的光学增益要高一个量级以上．与传统的电子-空穴等离子体激光辐射相比，激子引起的受激发射可在较低的激发密度条件下实现．这在实际应用上很有价值．     

Magnetic-field-effects on photoluminescence polarization in type II GaAs/AlAs superlattices

Optical orientation and alignment in the presence of a magnetic field have been applied to study the fine structure of excitons in type II GaAs/AlAs superlattices. We have developed a theory of polarized photoluminescence taking into account the anisotropic exchange splitting of the radiative excitonic doublet. The observed effects of the longitudinal and transverse magnetic fields on the polarization of the exciton emission unambiguously confirm that the actual symmetry of the exciton is lower that D_2d and that there exist two classes of excitons with opposite signs of the anisotropic exchange splitting.