Exciton Absorption Spectra Research Articles

Exciton-phonon sidebands in metallic carbon nanotubes studied using semiconductor Bloch equations

We use semiconductor Bloch equations to describe excitonic absorption spectra of metallic carbon nanotubes. In particular, we focus on the formation of exciton-phonon induced sidebands. Our approach is based on the density matrix formalism combining zone-folded tight-binding wave functions and electron-phonon coupling, allowing a straight-forward description of temporal and spectral many-body interactions. We observe clear excitonic features in the spectra of metallic carbon nanotubes in agreement with recent experimental and theoretical studies. Furthermore, depending on the temperature, we find significant exciton-phonon sidebands on both sides of the zero-phonon excitonic line. We investigate the polaronic shift and the transfer of the spectral weight to the sidebands for a variety of metallic nanotubes with different chiral angles and diameters.

Direct and indirect excitons in semiconductor coupled quantum wells in an applied electric field

An accurate calculation of the exciton ground and excited states in AlGaAs and InGaAs coupled quantum wells (CQWs) in an external electric field is presented. An efficient and straightforward algorithm of solving the Schrodinger equation in real space has been developed and exciton binding energies, oscillator strengths, lifetimes, and absorption spectra are calculated for applied electric fields up to 100 kV/cm. It is found that in symmetric 8-4-8 nm GaAs/Al(0.33)Ga(0.67)As CQW structure, the ground state of the system switches from direct to indirect exciton at approximately 5 kV/cm with dramatic changes of its binding energy and oscillator strength while the bright excited direct-exciton state remains almost unaffected. It is shown that the excitonic lifetime is dominated either by the radiative recombination or by tunneling processes at small/large values of the electric field, respectively. The calculated lifetime of the exciton ground state as a function of the bias voltage is in a quantitative agreement with low-temperature photoluminescence measurements. We have also made freely available a numerical code for calculation of the optical properties of direct and indirect excitons in CQWs in an electric field.

Exciton states and excitonic absorption spectra in a cylindrical quantum wire under transverse electric field

The effects of transverse electric field on the electronic structures, exciton states and excitonic absorption spectra in a cylindrical quantum wire are theoretically investigated in detail. The quantum wire is assumed to GaAs material surrounded by the infinite potential barrier. The results show that the external electric field removes the degeneracy of the electron or hole states. The energy levels of electron and hole, exciton binding energy, excitonic absorption coefficient and absorption energy decrease with increasing the strength of the electric field or the wire radius. The effects of the electric field become more significant for wide wires. The phenomena can be explained by the reduced spatial overlap of ground electron and hole states.

THE EFFECT OF SCREENED COULOMB INTERACTION ON THE OPTICAL PROPERTIES OF EuS/PbS/EuS FINITE CONFINING POTENTIAL QUANTUM WELL

We theoretically investigate excitonic effects on the optical properties of quasi-two dimensional realistic EuS/PbS/EuS finite confining potential quantum well. The strong contrast of material parameter’s across the well interfaces and the characteristic band-nonparabolicity effect of lead salt semiconductor are taken into account. In presence of medium polarization a feasibility of the screened Coulomb potential in quantum well is examined and appropriate screening radius is revealed for the first time. The screened binding energy investigated while a variational approach has been used. Depend on the realistic nanostructure specifics a monotonic decrease of the enhanced binding energy is received when in-plane carrier density/temperature ratio parameter increases. Exciton absorption spectra near the band edge within the effective-mass approximation is examined. Coulomb potential having a cutoff type is used, that efficiently represented the potential in the realistic quantum well. Screened exciton factor, which is the absorption intensity enhancement of the unbound (continuum) exciton states that are located above the band edge, is used. Plot of the dependence of the exciton factor on the exciton pair energy is given.

Theoretical calculation of excitonic binding energies and optical absorption spectra for Armchair graphene nanoribbons

In this paper the excitons of armchair graphene nanoribbons with layers of different width and thickness have been investigated. In this investigation, the band structure and energy gap of armchair graphene nanoribbons have been calculated using a tight-binding model including edge deformation effects (all edge atoms have been passivated with hydrogen atoms). Also, by calculating the conductance in armchair graphene nanoribbons (A-GNRs) optical absorption of armchair graphene nanoribbon in the single-electron approximation has been obtained. Finally, the binding energy of excitons in armchair graphene nanoribbons has been calculated using the Wannier model, Hartree-Fock approximation and the Bethe-Salpeter equation.

Exciton binding energy and excitonic absorption spectra in a parabolic quantum wire under transverse electric field

The effects of transverse electric field on the energy levels of electron and heavy hole, exciton binding energy and excitonic absorption spectra of GaAs parabolic quantum wire are theoretically investigated in detail. The results indicate that the electron and hole energy levels, exciton binding energy, excitonic absorption coefficient and absorption energy becomes smaller with the increase of electric field. That is more significant at the condition of weaker parabolic confinement potential. The phenomena can be explained by the separation of overlap integral of the electron and hole at the ground states.

Parallelization of the R-matrix propagation method for the study of intense-laser-driven semiconductor superlattices

Parallelization of the R-matrix propagation method for the study of intense-laser-driven semiconductor superlattices is described. We focus on photodressed excitonic electron–hole pair states related to the optical properties of the system concerned. Here, we parallelize the loop of propagation sectors, which is regarded as a rate-determining step in the whole calculation: in each sector calculated R-matrix Greenʼs functions are playing a key role. The evaluated parallelization efficiency is found sufficiently high, compared with a conventional algorithm without such parallelization. Restriction of this efficiency arising from the inter-core communications is also discussed. Further, it is shown that by virtue of the present parallelization, we readily obtain high-resolution excitonic absorption spectra revealing conspicuous dynamic Fano resonance. ► We describe a parallelization scheme of the R-matrix propagation method. ► We parallelize the loop of propagation sectors. ► The obtained absorption spectra reveal dynamic Fano resonance.

Tight‐binding model for the electronic and optical properties of nitride‐based quantum dots

AbstractTwo slightly different, efficient tight‐binding (TB) models for the description of the electronic properties of nitride‐based semiconductor quantum dots (QDs) have been developed and applied to the calculation of the electronic one‐particle spectrum of these structures. Using these one‐particle QD‐states, dipole and Coulomb matrix elements can be calculated, from which the optical properties of these systems can be obtained. These TB calculations have been performed for nitride‐based QDs with a cubic zincblende structure and those with a wurtzite crystal structure. In this paper, we discuss the general methodology used and the results obtained for the electronic one‐particle states and energies, for the dipole and Coulomb matrix elements, and for the excitonic optical emission and absorption spectra.

Aharonov-Bohm effect on impurity-bound excitons in semiconducting carbon nanotubes

We study the Aharonov-Bohm effect on exciton absorption spectra in semiconducting carbon nanotubes with an impurity in an effective-mass theory. In the presence of an impurity potential, there are two impurity-bound exciton states, one is dark and the other is bright. The dark impurity-bound exciton becomes brightened with increasing a magnetic flux through the cross section. The energy level of the bright impurity-bound exciton shows a blueshift in the magnetic flux dependence for a long-range impurity; in contrast, it shows a redshift for a short-range impurity, due to an enhancement of the coupling between intra- and inter-valley excitons. This suggests that the magnetic flux dependence can distinguish the range of the impurity potential and the strength of the inter-valley coupling of excitons.

Absorption and emission spectra of molecular excitons in single perylene nanocrystals

Nonlinear absorption spectra of single $\ensuremath{\alpha}$-perylene nanocrystals of 150--300 nm in size were measured at low temperature by multichannel phase-sensitive pump-probe microspectroscopy. For nanocrystals, free exciton (FE) absorption blueshifts while self-trapped exciton (STE) emission redshifts and FE emission is intense, compared with bulk crystals. The mechanism of the redshift is suggested to be lattice softening from a redshift in space-resolved STE emission at the edge of bulk crystals. The FE absorption band of several single nanocrystals shows a double-peak structure, suggesting that the blueshift is due to ${B}_{u}$ excitons, which are much stronger in oscillator strength than ${A}_{u}$ excitons, yet difficult to observe in bulk crystals.

Interband optical absorption in a strained Cd xZn 1− xO/ZnO quantum dot

Numerical calculations of the excitonic absorption spectra in a strained Cd x Zn 1− x O/ZnO quantum dot are investigated for various Cd contents. We calculate the quantized energies of the exciton as a function of dot radius for various confinement potentials and thereby the interband emission energy is computed considering the internal electric field induced by the spontaneous and piezoelectric polarizations. The optical absorption as a function of photon energy for different dot radii is discussed. Decrease of exciton binding energy and the corresponding optical band gap with the Cd concentration imply that the confinement of carriers decreases with composition x. The main results show that the confined energies and the transition energies between the excited levels are significant for smaller dots. Non-linearity band gap with the increase in Cd content is observed for smaller dots in the strong confinement region and the magnitude of the absorption spectra increases for the transitions between the higher excited levels.

Optical absorption and refractive index of an exciton quantum dot under intense laser radiation

An investigation of the influence of a laser field on the exciton absorption spectra and the refractive indexes has been performed by using the matrix diagonalization and the compact density-matrix methods. Electrons and holes are confined by the parabolic potential of the quantum dot. We conclude that in quantum dots the laser field amplitude provides an important effect on the linear, third-order nonlinear, and total optical absorption coefficients as well as the refractive index changes.

Excitons in single and double GaAs/AlGaAs/ZnSe/Zn(Cd)MnSe heterovalent quantum wells

Exciton photoluminescence spectra, photoluminescence excitation spectra, and magnetophotoluminescence spectra of single (GaAs/AlGaAs/ZnMnSe) and double (GaAs/AlGaAs/ZnSe/ZnCdMnSe) heterovalent quantum wells formed by molecular beam epitaxy are studied. It is shown that the exciton absorption spectrum of such quantum wells mainly reproduces the resonant exciton spectrum expected for usual quantum wells with similar parameters, while the radiative exciton recombination have substantial distinctions, in particular the additional localization mechanism determined by defects generated by heterovalent interface exists. The nature of these localization centers is not currently clarified; their presence leads to broadening of photoluminescence lines and to an increase in the Stokes shift between the peaks of luminescence and absorption, as well as determining the variation in the magnetic g factor of bound exciton complexes.

Exciton absorption spectra of Cs2CdI4 and Rb2CdI4 ferroelastic solid solutions

The exciton absorption spectra of thin films of (Cs1 − xRbx)2CdI4 solid solutions have been investigated and the refractive index n(λ) in their transparency window in the concentration range of 0 ≤ x ≤ 1 has been measured. The exciton-band parameters and optical permittivity ɛ∞(x) have been found to linearly depend on the concentration. It is established that excitons are incorporated into the CdI2 sublattice of the solid solutions and belong to intermediate-coupling ones. The characteristics of excitons in ferroelastics are compared with the corresponding parameters for CdI2, RbI, and CsI, which are used as components to synthesize ternary compounds.

Electronic states and optical properties of PbSe nanorods and nanowires

A theory of the electronic structure and excitonic absorption spectra of PbS and PbSe nanowires and nanorods in the framework of a four-band effective mass model is presented. Calculations conducted for PbSe show that dielectric contrast dramatically strengthens the exciton binding in narrow nanowires and nanorods. However, the self-interaction energies of the electron and hole nearly cancel the Coulomb binding, and as a result the optical absorption spectra are practically unaffected by the strong dielectric contrast between PbSe and the surrounding medium. Measurements of the size-dependent absorption spectra of colloidal PbSe nanorods are also presented. Using room-temperature energy-band parameters extracted from the optical spectra of spherical PbSe nanocrystals, the theory provides good quantitative agreement with the measured spectra.

Excitonic absorption spectra of metallic single-walled carbon nanotubes

We present microscopic calculations of the absorption spectra of metallic single-walled carbon nanotubes. We address the controversial question of the excitonic binding energies in metallic nanotubes as well as the excitonic character of higher transitions. In spite of the strong screening, we observe binding energies in the range of 100 meV for metallic nanotubes with diameters in the range of 1–2.2 nm. Characteristic features of the absorption spectra, such as peak splitting and an asymmetric peak shape, are observed. The splitting is due to the trigonal warping effect. The peak shoulder at high energies is a result of an overlap of the excitonic excitation with the free-particle Van Hove singularity. Furthermore, we find higher transitions to be also significantly influenced by excitons. Our approach is based on density matrix, which allows the investigation of the chirality and diameter dependence of the excitonic binding energy for the first four optical transitions for a variety of different metallic and semiconducting nanotubes. DOI: 10.1103/PhysRevB.82.035433 PACS numbers: 78.67.Ch, 73.22.f, 78.47.p

Excitonic electroluminescence at room temperature in an (In,Ga)As multiple-quantum-well diode

Electroluminescence (EL) spectra of an In0.15Ga0.85As/Al0.15Ga0.85As multiple-quantum-well p-i-n diode measured at 15–300 K are dominated by the ground heavy-hole exciton transition, as assigned from coincidence to the leading exciton resonance absorption energy. Although most of excitons thermally dissociate into free carriers populated up to the first excited confinement states at room temperature, radiative recombination is strongly enhanced at the exciton states. Simulated EL spectra based on the excitonic absorption spectra rigorously reproduce all of the excitonic EL features superposed on the exponentially tailing emission at the continuum states, showing coexistence of excitons and free carriers at room temperature.

Multi‐band theory of magnetoexcitons in ZnO/ZnMnO quantum wells

AbstractWe present a multiband magnetoexciton absorption model in ZnO/ZnMnO quantum wells which takes into account the details of the valence band structure. In our model we incorporate the k·p as well as the Coulomb coupling between valence sub‐bands. The s,p‐d interaction between the carriers and the magnetic ions in the barrier is taken into account within the mean field approximation.The effect of the electric field is accounted for using proper potential profile of the quantum well. As a result we obtain electric and magnetic field dependent excitonic absorption spectra from which the exciton binding energies and life‐times as well as the overlap with the magnetic barrier material may be deduced (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Absorption spectra of thin layers of solid solutions Cs2(Cd1 − x Zn x )I4

The excitonic absorption spectra of thin films of ferroelectric Cs2CdI4 and Cs2ZnI4 solid solutions are studied for the first time. It is found that, within the whole range of molar concentrations x, the spectra of Cs2(Cd1 − xZnx)I4 comprise two bands that originate from the bands of the two compounds. It is shown that the exciton transfer occurs most efficiently between the tetrahedrons CdI4 and ZnI4 along the b axis of the crystal. Unusual behavior of the concentration with regards to the maxima of long-wavelength excitonic bands Em(x) agrees well with the developed theory that takes into account dependence on x of the matrix element of the intertetrahedron exciton transfer and that is similar to the theory of Davydov’s splitting in molecular crystals.

Absorption spectra of thin films of the solid solutions Rb2(Cd1−xZnx)I4

The excitonic absorption spectra of thin films of the solid solutions Rb2(Cd1−xZnx)I4 have been studied for the first time. It is determined that the spectra of the solid solutions are stable and consist of two bands which genetically originate from the initial bands of the compounds Rb2CdI4 and Rb2ZnI4. It is shown on the basis of the structure of the crystal lattices of the compounds that the concentration dependence of the spectral position of the excitonic bands AI and AII is related with the transfer of excitons between CdI4 and ZnI4 tetrahedra along the b axis of the solid solutions. The experimental result is in good agreement with the calculation of the energies EAI(x) and EAII(x) on the basis of a theory similar to the theory of Davydov splitting of excitonic bands in molecular crystals.