Exciton

About: Exciton is a research topic. Over the lifetime, 31603 publications have been published within this topic receiving 810642 citations.

Theory of optically excited intrinsic semiconductor quantum dots

Abstract: The influence of the Coulomb interaction on one and two electron-hole-pair excitations in semiconductor quantum dots is analyzed. Using a numerical matrix-diagonalization scheme, the energy eigenvalues and the eigenfunctions of the relevant states are computed. Significant deviations from the strong-confinement approximation are observed. It is shown that the biexciton binding energy increases with decreasing dot size. This result is verified using third-order perturbation theory for small quantum dots. The optical properties of the quantum dots are computed, and it is shown that the Coulomb interaction significantly influences the allowed dipole transitions, causing increasing two-pair absorption on the high-energy side of the decreasing one-pair absorption. Surface-polarization effects are studied for quantum dots embedded in another dielectric medium.

Dielectric disorder in two-dimensional materials

Abstract: Understanding and controlling disorder is key to nanotechnology and materials science. Traditionally, disorder is attributed to local fluctuations of inherent material properties such as chemical and structural composition, doping or strain. Here, we present a fundamentally new source of disorder in nanoscale systems that is based entirely on the local changes of the Coulomb interaction due to fluctuations of the external dielectric environment. Using two-dimensional semiconductors as prototypes, we experimentally monitor dielectric disorder by probing the statistics and correlations of the exciton resonances, and theoretically analyse the influence of external screening and phonon scattering. Even moderate fluctuations of the dielectric environment are shown to induce large variations of the bandgap and exciton binding energies up to the 100 meV range, often making it a dominant source of inhomogeneities. As a consequence, dielectric disorder has strong implications for both the optical and transport properties of nanoscale materials and their heterostructures.

Superlattices and Other Heterostructures: Symmetry and Optical Phenomena

Abstract: 1 Quantum Wells and Superlattices.- 2 Crystal Symmetry.- 2.1 Symmetry Operations, Groups.- 2.2 Point-Group Classification.- 2.3 Space Groups.- 2.4 Group Representations, Characters.- 2.5 Point-Group Representations.- 2.6 Spinor Representations.- 2.7 Representations of Space Groups.- 2.8Invariance Under Time Inversion.- 2.9 Selection Rules.- 2.10 Determination of Linearly Independent Components of Material Tensors.- 3 Electron Spectrum in Crystals, Quantum Wells and Superlattices.- 3.1 The k-p Method.- 3.2 The Effective-Mass Method Deformation Potential.- 3.3 Method of Invariants.- 3.4 Electron and Hole Spectrum in Diamond-and Zincblende-Type Cubic Crystals.- 3.5 Electron Spectra of Quantum Wells and Superlattices.- 3.6 Hole Spectrum in Quantum Wells and Superlattices for Degenerate Bands.- 3.7 Deformed and Strained Superlattices.- 3.8 Quantum Wells and Superlattices in a Magnetic Field.- 3.9 Spectrum of Quantum Wells and Superlattices in an Electric Field.- 4 Vibrational Spectra of Crystals and Superlattices Electron-Phonon Interaction.- 4.1 Normal Vibrations: Distribution in Irreducible Representations.- 4.2 Vibrational Spectra of Superlattices.- 4.3 Electron-Phonon Interaction.- 5 Localized Electron States and Excitons in Heterostructures.- 5.1 Shallow Impurity Centers.- 5.2 Localized States at Superlattice Defects.- 5.3 Excitons.- 5.4 Exchange Splitting of Exciton Levels.- 6 Interband Optical Transitions.- 6.1 Optical Superlattices.- 6.2 Interband Transitions and Dielectric Susceptibility of a Periodic Heterostructure.- 6.3 Coulomb Interaction Between the Electron and the Hole.- 6.4 Exciton Polaritons in an Optical Superlattice.- 6.5 Light Reflection.- 6.6 Electro-Optical Effects in Interband Transitions.- 6.7 Magneto-Optical Spectra.- 7 ntraband Transitions.- 7.1 Cyclotron Resonance and Effective Electron Mass.- 7.2 Intersubband Absorption.- 7.3 Electron-Spin Resonance.- 7.4 IR Reflection in an Undoped Superlattice.- 8 Light Scattering.- 8.1 Theory of Light Scattering in Semiconductors.- 8.2 Scattering by Intersubband Excitations.- 8.3 Scattering by Acoustical Phonons with a Folded Dispersion Law.- 8.4 Scattering by Optical Phonons in Heterostructures.- 8.5 Acceptor Spin-Flip Raman Scattering.- 9 Polarized Luminescence in Quantum Wells and Superlattices.- 9.1 Luminescence as a Tool to Study Electronic Spectra and Kinetic Processes in Two-Dimensional Systems.- 9.2 Luminescence in the Quantum Hall Regime, Quantum Beats.- 9.3 Optical Spin Orientation and Alignment of Electron Momenta.- 9.4 Optical Orientation and Alignment of Excitons.- 9.5 Polarized Luminescence of Excitons and Impurities in an External Magnetic Field.- 10 Nonlinear Optics.- 10.1 Two-Photon Absorption.- 10.2 Photoreflectance.- 10.3 Diffraction from a Light-Induced Spatial Grating.- 10.4 Third-Harmonic Generation.- 10.5 Linear and Circular Photogalvanic (Photovoltaic) Effects.- 10.6 Current of Optically Oriented Electrons.- 10.7 Photon Drag Current.

Fluorescence-line narrowing in CdSe quantum dots: Surface localization of the photogenerated exciton

Abstract: The electronic properties of shallow band-edge surface traps in nanometer-size CdSe quantum dots are probed using fluorescence-line-narrowing spectroscopy. We find large changes in electron-hole-pair radiative lifetimes and couplings to LO phonons as the temperature is changed from 1.75 to 10 K. We attribute these changes to the localization of the photogenerated hole at the surface of the dots, accompanied by thermally activated motion between these surface localized states. A simple model based on the observed exciton--LO-phonon couplings is constructed to estimate the extent of hole localization in the luminescing state. A size-dependent study (20--80 \AA{} diameter) indicates that surface effects diminish rapidly with increasing size.

Optical transient bleaching of quantum‐confined CdS clusters: The effects of surface‐trapped electron–hole pairs

Abstract: We studied the optical transient bleaching of ∼40 A, ammonia‐passivated CdS clusters in a polymer with nanosecond and picosecond pump‐probe techniques. The transient bleaching spectra behave differently in different time regimes. Within the 30‐ps pump laser pulse width, we tentatively attribute the bleaching to the exciton‐exciton interaction, and the magnitude can be enhanced by surface passivation. On time scales of tens of picoseconds and longer following the pump pulse, when only trapped electron‐hole pairs remain from the pump excitation, the bleaching is due to the interaction between such a trapped electron‐hole pair and a bound exciton produced by the probe light. Experimentally we determined that roughly one trapped electron‐hole pair can bleach the excitonic absorption of the whole CdS cluster. We developed a theoretical model which considers the effects of the trapped electron‐hole pair on the energy of the exciton transition and its oscillator strength. We found that, when a trapped electron and hole are present, the lowest exciton absorption is red‐shifted from the original exciton absorption, and this transition has a weak oscillator strength, which explains the observed efficient bleaching. The model also predicts that a trapped electron is more efficient than a trapped hole for bleaching the excitonic absorption of CdS clusters in the size regime considered here. This is confirmed by pulse radiolysis results. Finally, we discuss the possible effects of charged surface defects on the linear absorption spectra of semiconductor clusters.