Exciton Absorption Spectra Research Articles

Theoretical Studies of the Structural, Electronic, and Optical Properties of Phosphafluorenes

Phosphafluorenes have drawn increasing attention recently in the applications of organic electronic devices due to their particular optoelectronic properties. To reveal their molecular structures, optoelectronic properties, and structure-property relationships of the newly emerged functional materials, an in-depth theoretical investigation was elaborated via quantum chemical calculations. The optimized geometric and electronic structures in both ground and exited states, the mobility of the hole and electron, the absorption and emission spectra, and the singlet exciton generation fraction of these novel phosphors-containing materials have been studied by density functional theory (DFT), single excitation configuration interaction (CIS), time-dependent density functional theory (TDDFT) methods, and the polarizable continuum model (PCM). The results show that the highest occupied molecular orbitals (HOMOs), the lowest unoccupied molecular orbitals (LUMOs), triplet energies ((3)E(g)), energy gaps (E(g)), as well as some other electronic properties including ionization potentials (IPs), electron affinities (EAs), reorganization energies (lambda), the singlet exciton generation fraction, radiative lifetime, and absorption and emission spectra can be easily tuned by chemical modifications of the phosphorus atom via methyl, phenyl, oxygen, sulfur, or selenium substitution, indicating that the phosphafluorenes are interesting optoelectronic functional materials, which have great potential in the applications of OLEDs, organic solar cells, organic storage, and sensors.

Coherent potential approximation for spatially correlated disorder

The coherent-potential approximation (CPA) is extended to describe satisfactorily the motion of particles in a random potential which is spatially correlated and smoothly varying. In contrast to existing cluster-CPA methods, the present scheme preserves the simplicity of the conventional CPA but leads to a momentum and frequency-dependent self-energy. Its accuracy is checked by a comparison with the exact moments of the Green's function and with the spectral function from numerical simulations. The scheme is applied to excitonic absorption spectra in different spatial dimensions.

Excitonic absorption spectra and ultrafast dephasing dynamics in arbitrary carbon nanotubes

AbstractWe illustrate the potential of the density matrix theory for investigation of optical properties of arbitrary single‐walled carbon nanotubes (CNTs). We have performed microscopic calculations of excitonic absorption spectra for CNTs of different chiral angles and diameters. The obtained results are in good agreement with experiments, in particular the excitonic binding energies match well both experiments and ab initio calculations. Furthermore, we show the strength of our approach by presenting calculations of the ultrafast Coulomb driven non‐equilibrium dynamics in CNTs. We find excitation induced dephasing on the picosecond time scale depending on the excitation strength. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Exciton absorption in semiconductor superlattices in a strong longitudinal THz field

We theoretically study the excitonic optical absorption in semiconductor superlattices irradiated by an intense terahertz (THz) laser polarized along the growth direction. We calculate the linear excitonic absorption spectra using the finite-difference time-domain method in real space. Our numerical results and qualitative analysis show that when tuning the frequency or intensity of the THz laser, the dynamical localization (DL) and the ac Stark effect (ACSE) are either jointly or individually responsible for the exciton peak shift. Moreover, with the onset of miniband collapse, the excitonic dimensionality crossover and the DL will counteract each other in shifting the exciton peak. The direct experimental verification of the DL through semiconductor optical spectra is still a challenge. Regarding this fact, we propose a method to single out the DL effect from the spectral shift dominated by the ACSE.

Temperature-dependent changes of the exciton absorption spectra in polar semiconductors

Using the model of dielectric continuum for phonons and effective mass approximation for electrons, the influence of longitudinal optical phonons on the formation of exciton spectra in polar semiconductors is studied with the Green function technique. The calculations are made in frame of the Mott-Vannier model for the exciton in nS-states (n = 1, 2 and 3) for a number of semiconductors of A2B6 and A3B5 groups. The results confirm that interaction of the exciton with longitudinal optical (LO) phonons manifests itself in shifting of absorption band peaks towards low-energy region. The shift differs for different states and depends on temperature. Unlike the main exciton band (n = 1), the changes in the positions and widths of the next bands (n = 2 and 3) resulted from the interaction with the LO phonons are inessential for all the crystals under analysis.

Spin-orbit coupling and crystal-field splitting in the electronic and optical properties of nitride quantum dots with a wurtzite crystal structure

We present an $sp^3$ tight-binding model for the calculation of the electronic and optical properties of wurtzite semiconductor quantum dots (QDs). The tight-binding model takes into account strain, piezoelectricity, spin-orbit coupling and crystal-field splitting. Excitonic absorption spectra are calculated using the configuration interaction scheme. We study the electronic and optical properties of InN/GaN QDs and their dependence on structural properties, crystal-field splitting, and spin-orbit coupling.

Excitonic absorption spectrum of Rb2ZnI4 thin films

The absorption spectrum of Rb2ZnI4 thin films in the spectral region 3–6eV is investigated in the temperature range 90–290K. It is found that the compound is a direct-gap insulator and that the low-frequency excitations are localized in the ZnI42− structural layers of the crystal lattice and are of a quasi-two-dimensional character. For T<200K the temperature dependence of the spectral absorption and the half-widths of the long-wavelength exciton band exhibits thermal memory effects due to precursors of the low-temperature phases.

Two-dimensional magnetoexcitons in the presence of spin-orbit coupling

We study theoretically the effect of spin-orbit coupling on quantum well excitons in a strong magnetic field. We show that, in the presence of an in-plane field component, the excitonic absorption spectrum develops a double-peak structure due to hybridization of bright and dark magnetoexcitons. If the Rashba and Dresselhaus spin-orbit constants are comparable, the magnitude of splitting can be tuned in a wide interval by varying the azimuthal angle of the in-plane field. We also show that the interplay between spin-orbit and Coulomb interactions leads to an anisotropy of exciton energy dispersion in the momentum plane. The results suggest a way for direct optical measurements of spin-orbit parameters.

Microscopic analysis of the optical and electronic properties of semiconductor photonic‐crystal structures

AbstractThe combination of dielectric photonic crystals and semiconductor nanostructures makes it possible to design many important aspects of the optoelectronic system response. A spatially‐varying dielectric environment induces modifications of the transversal and the longitudinal components of the electromagnetic field which have to be included in the self‐consistent microscopic analysis of the optical properties of hybrid semiconductor photonic‐crystal structures. In this paper, the development of a semiclassical microscopic theory is reviewed. Whereas the classical electromagnetic field is described at the level of Maxwell's equations, a full many‐body quantum theory is used for the interacting electronic excitations in the semiconductor material. Relevant examples of the numerical solutions of the resulting Maxwell semiconductor Bloch equations are presented showing, e.g., characteristic modifications of excitonic absorption spectra, the spatio‐temporal dynamics of electronic wave packets, as well as an increase of the optical gain in properly designed device geometries. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical absorption spectra in thin films of M2Ag1−xCuxI3(M=K,Rb,Cs) solid solutions

A study is made of the excitonic absorption spectra of solid solutions of the system (KI)2–(1−x)AgI–xCuI. On the basis of the dependence of the parameters of the excitonic bands on the concentration x, it is established that solid solutions of the type K2Ag1−xCuxI3 are formed in the range 0≤x≤0.2. Substitution of Ag atoms by Cu atoms in the AgI42− tetrahedra, which form chains in the orthorhombic crystal K2AgI3, promotes the formation of quasi-1D exciton bands and spectra of the persistence type, according to the Onodera–Toyozawa classification, with a splitting off of the impurity band from the exciton band. Comparison with the results of studies of the excitonic spectra in solid solutions Rb2Ag1−xCuxI3 and Cs2Ag1−xCuxI3 attests to the appearance of the Rashba effect at small x and to qualitative agreement with the Onodera–Toyozawa theory.

Exciton absorption at the initial stages of the formation of the CuCl phase in a glass

The fundamental absorption spectra of CuCl nanocrystals in glass samples are investigated in the energy range 3–4 eV at a temperature of 300 K with the aim of elucidating the kinetics of the initial stage of the formation of the CuCl phase in the glass. The CuCl phase is grown in the glass under stepwise annealing at temperatures of 500, 615, and 707°C. The kinetics of variation in the intensity and the shift of the maximum in the exciton absorption spectra of CuCl nanocrystals are studied in the course of annealing of the glass samples. It is established that, for all the temperatures under investigation, the formation of the CuCl phase begins with the transient stage that involves the fluctuation formation of supercritical nuclei of the CuCl nanomelt. At a temperature of 500°C, the transient stage gives way to the stage of a rapid increase in the number of supercritical nuclei of the CuCl phase. At temperatures of 615 and 707°C, the transient stage gives way to the stage of an intensive growth of nuclei without a considerable increase in their number. The number of nuclei formed during the transient stage at 707°C is smaller than that observed after the transient stage at 500°C by a factor of 24. However, the sizes of the nuclei formed at 707°C are larger than those observed after the transient stage at 500°C. This difference is explained by the fact that the diffusion length of Cu+ ions controlling the formation of the CuCl phase increases with increasing temperature. The experimental data on the kinetics of the formation of the new phase in the glass are in good agreement with the Zel’dovich-Frenkel classical theory of the formation of a new phase, which accounts for the stage of the formation of critical nuclei.

Coherent Absorption Spectroscopy with Supercontinuum for Semiconductor Quantum Well Structure

We suggest that supercontinuum can be used for absorption spectroscopy to observe the exciton levels of a semiconductor nano-structure. Exciton absorption spectrum of a GaAs/AlGaAs quantum well was observed using supercontinuum generated by a microstructrured fiber pumped by a femtosecond (fs) pulsed laser. Significantly narrower peaks were observed in the absorption spectrum from 11 K up to room temperature than photoluminescence (PL) spectrum peaks. Because supercontinuum is coherent light and can readily provide high enough intensity, this method can provide a coherent ultra-broad band light source to identify exciton levels in semiconductors, and be applicable to coherent nonlinear spectroscopy such as electromagnetically induced transparency (EIT), lasing without inversion (LWI) and coherent photon control in semiconductor quantum structures.

Spin characteristics of excitonic absorption in multiply charged quantum dots

The excitonic absorption spectra of negatively charged self-assembled quantum dots as a function of resident electron number $({N}_{e}=0--5)$ are theoretically studied using the configuration interaction method. Spin Pauli blockade, Coulomb (exchange and correlation) interactions, and symmetry of dot shape are identified as the three main underlying mechanisms, sensitively depending on ${N}_{e}$, in the spin characteristic spectra. In particular, we predict that polarized absorption in triply charged dots exhibits anomalous spin-dependent features, resulting from the intrinsic correlations in charged exciton ${X}^{\ensuremath{-}3}$.

Atomic scale structure and optical emission ofAlxGa1−xAs∕GaAsquantum wells

A combined study of the optical and structural properties of $\mathrm{Al}\mathrm{Ga}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ quantum wells is presented. Microphotoluminescence experiments, magnetomicrophotoluminescence, and atomically resolved cross-sectional scanning tunneling microscopy were performed on the same quantum well sample. Constant-current topographs with aluminum and/or gallium sensitivity are used to directly extract disorder potentials. Using these potentials, exciton absorption spectra, microphotoluminescence spectra, and diamagnetic shifts of individual exciton states are calculated in an envelope function approximation. Very good agreement between the theoretical and experimental results is found.

Electro-optic interband excitonic absorption of nanoring double quantum wells

The excitonic absorption spectra of nanoring double quantum wells (NDQWs) subjected to radial and lateral electric fields are theoretically investigated. The electro-optic excitonic absorption spectra show very different behaviors for different electric-field configurations. The evolution of the main excitonic peaks of the NDQW with the radial electric fields is similar with that of square double quantum wells. Since the circular symmetry of the NDQW is not destroyed, the absorption peaks associated with ring structure are insensitive to the radial electric fields. However, because of the loss of such symmetry, these absorption peaks are broadened into continuum even at low lateral electric fields. Meanwhile, due to the exciton ionization and the inhomogeneous electric-field effect, the main excitonic peaks are intensively broadened and reduced by the lateral electric fields.

Detecting coupled excitons with microphotoluminescence techniques in bilayer quantum dots

The coupling effects between excitons are examined in a pair of quantum dots to probe coupled excitons in optical measurements. The exciton photoluminescence (PL) and absorption spectrum are measured with the micro-PL excitation (micro-PLE) technique in bilayer InGaAs quantum dots. The PL peaks from the coupled exciton have similar PLE spectra for a weak excitation, which agree well with a theoretical prediction. Additional PL peaks appear for a strong excitation. A sum rule in four PL peaks is satisfied due to the energy conservation rule in the case of two exciton creation. A simple control of the coupling effect is also demonstrated using a two-color excitation technique. The coupling energy can be controlled by the intensity of a second laser. These results provide good guidelines for finding PL peaks from coupled excitons, which are crucially important to demonstrate scalable quantum gates and quantum computing with quantum dots.

Generation and Decay Dynamics of Triplet Excitons in Alq3 Thin Films under High-Density Excitation Conditions

We studied the generation and decay dynamics of triplet excitons in tris-(8-hydroxyquinoline) aluminum (Alq3) thin films by using transient absorption spectroscopy. Absorption spectra of both singlet and triplet excitons in the film were identified by comparison with transient absorption spectra of the ligand molecule (8-hydroxyquinoline) itself and the excited triplet state in solution previously reported. By measuring the excitation light intensity dependence of the absorption, we found that exciton annihilation dominated under high-density excitation conditions. Annihilation rate constants were estimated to be gammaSS = (6 +/- 3) x 10(-11) cm3 s(-1) for single excitons and gammaTT = (4 +/- 2) x 10(-13) cm3 s(-1) for triplet excitons. From detailed analysis of the light intensity dependence of the quantum yield of triplet excitons under high-density conditions, triplet excitons were mainly generated through fission from highly excited singlet states populated by singlet-singlet exciton annihilation. We estimated that 30% of the highly excited states underwent fission.

Nominally forbidden transitions in the interband optical spectrum of quantum dots

We calculate the excitonic optical absorption spectra of (In,Ga)As/GaAs self-assembled quantum dots by adopting an atomistic pseudopotential approach to the single-particle problem followed by a configuration-interaction approach to the many-body problem. We find three types of allowed transitions that would be naively expected to be forbidden. (i) Transitions that are parity forbidden in simple effective mass models with infinite confining wells (e.g. 1S-2S, 1P-2P) but are possible by finite band-offsets and orbital-mixing effects; (ii) light-hole--to--conduction transitions, enabled by the confinement of light-hole states; and (iii) transitions that show and enhanced intensity due to electron-hole configuration mixing with allowed transitions. We compare these predictions with results of 8-band k.p calculations as well as recent spectroscopic data. Transitions in (i) and (ii) explain recently observed satellites of the allowed P-P transitions.

Tight-binding model for semiconductor quantum dots with a wurtzite crystal structure: From one-particle properties to Coulomb correlations and optical spectra

In this work we investigate the electronic and optical properties of self-assembled quantum dots by means of a tight-binding model. Coulomb and dipole matrix elements are calculated from the one-particle wave functions which fully include the atomistic wurtzite structure of the low-dimensional heterostructures and serve as an input for the calculation of optical spectra. For the investigated $\mathrm{In}\mathrm{N}∕\mathrm{Ga}\mathrm{N}$ material system, the optical selection rules are found to be strongly affected by band-mixing effects for the localized valence band states. Within this framework, excitonic absorption and emission spectra are analyzed for different sizes of the investigated lens-shaped quantum dots, including the influence of the intrinsic and strain-induced electrostatic field of the wurtzite structure. A dark exciton ground state for small quantum dots is found.

Near-field spectra of quantum well excitons with non-Markovian phonon scattering

The excitonic absorption spectrum for a disordered quantum well in presence of exciton-acoustic phonon interaction is treated beyond the Markov approximation. Realistic disorder exciton states are taken from a microscopic simulation, and the deformation potential interaction is implemented. The exciton Green's function is solved with a self energy in second order Born quality. The calculated spectra differ from a superposition of Lorentzian lineshapes by enhanced inter-peak absorption. This is a manifestation of pure dephasing which should be possible to measure in near-field experiments.