玻璃中CdSSe纳米晶体的光谱性能

对掺有镉、硒、硫的玻璃在650—800℃退火4?h，生长了不同尺寸的CdS0.13Se0.87纳米晶体，测量了纳米晶体的吸收光谱、光致发光（PL）谱和电调制光谱，确定了纳米晶体部分电子态的能量，讨论了CdSSe纳米晶体的光学性质与其尺寸之间的依赖关系.随着纳米晶体尺寸的增大，对应激子的吸收峰、PL峰及电吸收信号发生红移，表现出明显的量子尺寸效应.小尺寸纳米晶体的电吸收表现为量子受限的Stark效应，而大尺寸纳米晶体的电吸收线形与体材料的相似；随着纳米晶体尺寸的增大，电吸收信号增强.所有尺寸的纳米晶体都表现 关键词： CdSSe纳米晶体 吸收光谱 光致发光谱 电光响应     

Charged excitons in quantum dots: novel magnetic behavior and Auger processes

We describe theoretically multiply-charged excitons interacting with a continuum of delocalized states. Such excitons exist in relatively shallow quantum dots and have been observed in recent optical experiments on InAs self-assembled dots. The interaction of an exciton and delocalized states occurs via Auger-like processes. To describe the optical spectra, we employ the Anderson-like Hamiltonian by including the interaction between the localized exciton and delocalized states of the wetting layer. In the absence of a magnetic field, the photoluminescence line shapes exhibit interference effects. When a magnetic field is applied, the photoluminescence spectrum demonstrates anticrossings with the Landau levels of the extended states. We show that the magnetic-field behavior of charged excitons is very different to that of diamagnetic excitons in three and two-dimensional systems.     

A unified approach to long-range and short-range many-body corrections to optical spectra of semiconductors

Long-range Coloumb - and exchange interactions - leading to bound exciton states at the onset of the optical spectra of semiconductors and to Fano-type excitonic resonances in the continuum - and short-range Coulomb - and exchange interaction, that lead to a red shift in the low-energy part of the spectra as compared to one-particle calculations, are treated in a unified approach. Almost quantitative agreement between theoretical absorption spectra is obtained without any fitting parameter in the case of sc TlCl.     

Optical absorption of triplet molecular excitons in alkali cyanides

The optical absorption spectra of alkali cyanides in the UV region present a set of weak absorption bands which are identified as triplet a'³Σ⁺ molecular excitons. The nature of the molecular exciton transitions in the ionic-molecular crystals is discussed and the existence of an admixture between molecular exciton and charge transfer exciton states is suggested.     

Theory of multi-exciton states in semiconductor nanocrystals

In this article, the fundamental physics of multi-exciton states in semiconductor nano-crystals is reviewed focusing on the mesoscopic enhancement of the excitonic radiative decay rate and the excitonic optical nonlinearity and the mechanism of their saturation with increase of the nanocrystal size. In the case of the radiative decay rate the thermal excitation of excited exciton states having small oscillator strength within the homogeneous linewidth of the exciton ground state is essential in determining the saturation behavior. The weakly correlated exciton pair states are found to cause a cancellation effect in the third-order nonlinear optical susceptibility at the exciton resonance, providing the first consistent understanding of the experimentally observed saturation of the mesoscopic enhancement of the excitonic optical nonlinearity. The presence of the weakly correlated exciton pair states is confirmed convincingly from the good correspondence between theory and experiments on the induced absorption spectra from the exciton state in CuCl nanocrystals. Furthermore, ultrafast relaxation processes of biexcitons are discussed in conjunction with the observed very fast rise of the biexciton gain in nanocrystals. In prospect of future progress in research, the theoretical formulation to calculate the triexciton states as one of the multi-exciton states beyond the biexciton is presented for the first time including the electron-hole exchange interaction.     

Discovery of bound exciton in ZnSe : Cu

A new bound exciton with the dissociation energy of 32 meV was observed in ZnSe : Cu by electroabsorption. The dependence of the electroabsorption peak on an electric field, the absorption and the photoluminescence spectra show that the bound exciton is associated with Cu²⁺.     

Size-dependent photo-induced shift of the first exciton band in CdTe quantum dots in glass prepared by a two-stage heat-treatment process

CdTe nanocrystals were grown from commercially available RG850 Schott filter glass by two-step heat-treatment process which almost doubles the particle to matrix volume fraction. A calculation shows that a quantized-state effective mass model in the strong confinement regime might be used to deduce the average radius for the nanocrystals larger than 2 nm in radius from the energetic position of the first exciton peak in optical absorption spectrum. Size-induced shift of ∼360 meV in the first exciton peak position was observed. The steady state photoluminescence spectra exhibit a broad band red shifted relative to the first exciton band, which indicates the existence of shallow trap states. The non-linear optical properties of CdTe nanocrystals were studied by room temperature resonant photoabsorption spectroscopy. The differential absorption spectra had three-lobed structure whose size-dependent evolution was explained by bleaching of the absorption, red shift and broadening in the Gaussian absorption band used to fit the first exciton peak. A maximum red shift of 2.32 meV for the average nanocrystal radius of 4.65 nm was estimated by fitting the photomodulation spectra with a combination of first and second derivative Gaussian absorption bands. We presume that the red shift is induced by the electric field of trapped charges in surface states. Internal electric field strengths of 23 and 65 kV/cm were predicted for the average nanocrystal radii of 3.95 and 4.65 nm, respectively, with the help of second-order perturbation theory in the strong confinement limit.     

Assessment of the coherent potential approximation for the absorption spectra of a one-dimensional Frenkel exciton system with a Gaussian distribution of fluctuations in the optical transition frequency

We investigate the accuracy of the coherent potential approximation (CPA) for the optical absorption spectra of a one-dimensional Frenkel exciton system with nearest-neighbor interactions and a Gaussian distribution of fluctuations in the optical transition frequency (diagonal Gaussian disorder). Our earlier studies have established that the CPA gives highly accurate results for the integral of the density of states of this system. In this paper we compare the CPA results for the normalized optical absorption with the finite-array calculations of Schreiber and Toyozawa and Schreiber for the same model. It is found that the CPA results for the absorption are in good agreement with their findings. It is pointed out that an expansion of the density of states in terms of the eigenstates of the ideal (no disorder) array contains a term proportional to the normalized absorption. Since the density of states is accurately approximated by the CPA, the presence of this term explains the success of the CPA in reproducing the absorption spectra. Our findings support the use of the Gaussian disorder model to interpret the absorption spectra of one and quasi-one dimensional exciton systems.     

Initial-state distortion and final-state interactions in the 2H(3He, 3He p)n and 2H(3He, 3H p)p reactions

The predictions of two fully antisymmetrized reaction theories (DWBA and PWBA-FSI) are compared with absolute coincidence cross sections for the ²H(³He, ³He p)n and ²H(³He, ³H p)p reactions exhibiting final-state interactions (FSI) and quasi-elastic scattering (QES) both with and without charge exchange. The DWBA theory takes into account both the initial ³He-d and the final N-N interactions, while the PWBA-FSI theory includes only the latter. New QES data at E_He = 35.9 MeV, as well as previously reported 26.8 and 35.9 MeV data, are fitted. The DWBA theory gives good fits, both in shape and magnitude, to spectra showing N-N final-state interactions but gives somewhat poorer fits to QES spectra whose predicted magnitudes are two to ten times too large. The PWBA-FSI theory always predicts cross sections that are too large; however the predicted shapes are about as good as those from the DWBA. The initial-state interaction is shown to affect both the width and position of QES peaks from these reactions.     

Spectral differences between monomers and trimers of photosystem I depend on the interaction between peripheral chlorophylls of neighboring monomers in trimer

The spatial position of the long-wavelength chlorophylls in trimer of pigment-protein complex of photosystem I (PSI) have been determined bymodeling the optical fluorescence absorption and emission spectra for two hypothetical models of PSI trimer. The calculation has been performed using X-ray diffraction data on the spatial position of chlorophylls in PSI monomer; the pigment site energies were taken from the studies of other researchers, while interactions between monomers in trimer are considered as fitting parameters. The interaction energy between the chlorophylls spaced by a distance smaller than 10 Å was estimated based on the concept of extended dipole?dipole interaction. The model under study allowed us to evaluate the influence of the exciton interaction between peripheral pigments on the optical response of PSI trimer. The intensity and shape of stationary fluorescence line turned out to be sensitive to the PSI monomer packing in trimer. A visualization of the density matrix for low-energy exciton states has made it possible to estimate the localization of long-wavelength chlorophylls in PSI trimer.     

Influence of infrared radiation on the excitonic absorption edge of CuCl

We report changes in the excitonic absorption edge of CuCl caused by intense CO₂ laser radiation at 10.6 m, a wavelength which lies in the infrared (ir) transparency region of CuCl. With an ir intensity of 0.4 GW/cm² we observe a 100% absorption increase for the Kr⁺ laser probe wavelength of 406.7nm. The effect scales linearly with ir intensity but does not depend on relative polarization. We explain the effect by laser field induced electroabsorption of the exciton. The magnitude of the effect is closely related to electroabsorption induced by static external fields and by internal electric fields from optical phonons.     

Exciton and biexciton binding and vertical Stark effect in a model lens-shaped quantum box: Application to InAs/InP quantum dots

The problem of excitonic and biexcitonic binding is studied in the system of parabolic coordinates for a lens-shaped quantum box. The exciton wavefunction is expanded in terms of electron–hole configurations made from electron and hole single-particle states. Configuration interaction method and perturbative calculations are used to study the competition between confinement and correlation effects. Biexcitonic binding energy is calculated in the strong confinement regime and a comparison to the case of a spherical box is made. Absorption spectra with and without correlation effects are computed for InAs/InP quantum dots. Excitonic binding energy and enhancement factor are estimated to be equal to about 20 meV and 1.5, respectively. The excitonic absorption is finally studied in the presence of a uniform vertical electric field. A weak vertical Stark effect is predicted for lens-shaped quantum box described within this model.     

Oscillations of the Degree of Circular Polarization in the Optical Spin Hall Effect

The optical spin Hall effect appears when elastically scattered exciton polaritons couple to an effective magnetic field inside of quantum wells in semiconductor microcavities. Theory predicts an oscillation of the pseudospin of the exciton polaritons in time. Here, we present a detailed analysis of momentum space dynamics of the exciton polariton pseudospin. Compared to what is predicted by theory, we find a higher modulation of the temporal oscillations of the pseudospin. We attribute the higher modulation to additional components of the effective magnetic field which have been neglected in the foundational theory of the optical spin Hall effect. Adjusting the model by adding non-linear polariton-polariton interactions, we find a good agreement in between the experimental results and simulations.     

Interband absorption in square and semiparabolic near-surface quantum wells under intense laser field

The exciton effects on the interband absorption spectra in near-surface square and semiparabolic quantum wells under intense laser field are studied taking into account the correct dressing effect for the confinement potential and electrostatic self-energy due to the repulsive interaction between carriers and their image charges. We found that for near-surface quantum wells with different shapes the laser field induces significant effects on their electronic and optical properties. The numerical results for the InGaAs/GaAs system show that the red-shift of the absorption peak induced by the increasing cap layer can be effectively compensated using the blue-shift caused by the enhanced laser parameter. In square quantum well without laser field our theoretical values for the absorption peak position are in good agreement with the available experimental data. As a key result, we conclude that the optical properties in near-surface quantum wells can be tuned by tailoring the heterostructure parameters: well shape, capped layer thickness and/or dielectric mismatch as well as the external field radiation strength.     

Excitonic optical absorption in wurtzite InGaN/GaN quantum wells

Within the framework of the effective-mass and envelope function theory, exciton states and optical properties in wurtzite (WZ) InGaN/GaN quantum wells (QWs) are investigated theoretically considering the built-in electric field effects. Numerical results show that the built-in electric field, well width and in composition have obvious influences on exciton states and optical properties in WZ InGaN/GaN QWs. The built-in electric field caused by polarizations leads to a remarkable reduction of the ground-state exciton binding energy, the interband transition energy and the integrated absorption probability in WZ InGaN/GaN QWs with any well width and In composition. In particular, the integrated absorption probability is zero in WZ InGaN/GaN QWs with any In composition and well width L > 4 nm. In addition, the competition effects between quantum confinement and the built-in electric field (between quantum size and the built-in electric field) on exciton states and optical properties have also been investigated.     

Electronic Correlation Effects on the Optical Properties of SiC,BeO and Boron Nitride Nanotubes and Monolayers

We present numerical calculation of the impact of electron-electron interaction on the behavior of density of states and optical properties of BeO,SiC and Boron-Nitride nanotubes and sheets.Hubbard model hamiltonian is applied to describe the dynamics of electrons on the lattice structure of theses compounds.The excitation spectrum of the system in the presence of local electronic interactions has been found using mean Seld approach.We find the band gap width in both optical absorption and density of states reduces with local Hubbard electronic interaction parameter.The absorption spectra exhibits the remarkable peaks,mainly owing to the divergence behavior of density of states and excitonic effects.Also we compare optical absorption frequency behavior of BeO,SiC and Boron-Nitride nanotubes with each other.Furthermore we investigate the optical properties of BeO and SiC sheets.A novel feature of optical conductivity of these structures is the decrease of frequency gap in the optical spectrum due to electronic interaction.     

Excitons and many-electron effects in the optical response of single-walled boron nitride nanotubes

We report first-principles calculations of the effects of quasiparticle self-energy and electron-hole interaction on the optical properties of single-walled boron nitride nanotubes. Excitonic effects are shown to be even more important in BN nanotubes than in carbon nanotubes. Electron-hole interactions give rise to complexes of bright (and dark) excitons, which qualitatively alter the optical response. Excitons with a binding energy larger than 2 eV are found in the BN nanotubes. Moreover, unlike the carbon nanotubes, theory predicts that these exciton states are comprised of coherent supposition of transitions from several different subband pairs, giving rise to novel behaviors.     

Electromodulation spectroscopy of confined systems

The convolution formalism of Aspnes and Rowe for the dielectric function in the presence of an electric field is applied to electromodulation spectroscopy of confined systems. It is shown that for isolated confined systems where excitonic effects are not important, electromodulation spectroscopy leads to first derivative like optical features of two dimensional critical points. For superlattices in weak fields (neglecting excitonic interactions), the familiar third derivative forms of Aspnes and Rowe are applicable. In contrast, when moderate and high electric fields are applied to superlattices, electromodulation spectroscopy will exhibit optical features characteristic of first derivative line shapes. This phenomenon arises because the electric field causes the envelope functions of the electron and hole states to become spatially localized. The importance of excitons and their inhomogeneous broadening to the experimentally observed line shapes are considered.     

Electromodulation spectroscopy of excitons: High field regime

The effects of electric field on excitons have been investigated experimentally by means of electroabsorption (EA). The behavior of excitons with ionization fields F_I of the same order of magnitude as or smaller than typical applied fields is reported. EA spectra associated with excitons in TlBr, PbI₂, BiI₃, trigonal Se, and α-monoclinic Se will illustrate this case. In this situation the EA signal arises principally from the effect on the exciton ground state, and no finer structure could be detected. The exciton ground state undergoes a shift to higher energies with the applied field. Characteristic dependences with field of the sizes, energy position of the peaks, and zero crossing points of the EA signals have been found. Estimates of exciton parameters can be readily obtained from the EA spectra. Unusual features of the exciton EA spectra PbI₂ are discussed. Qualitatively, at least, theoretical predictions are obeyed.