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Excitonic study of semiconductors is highly benefitted by the fast computers and numerical techniques. This effect can be better visualized by taking in to account the effect of irreducible polarization in case of electron hole interaction. Optically active excitonic states encountered when perturbed frequencies are of lower than electronic band gap. BSE(Bethe-Salpeter equation) provides the quite good results in case of semiconductors while describing the dielectric properties excluding the dynamical effects. As four point reducible polarizability is a frequency dependent term and frequently used to define excitation as two particle many body problem which further can be expanded in transition space using Hamltonian. Solution of BSE infers the information regarding the strength of electron hole interaction and why semiconducting materials are optically responsive and frequency dependent. Coulombic interaction of exciton is screened by the other electrons present in the materials. In the bulk of material excitonic problem can be resolved through MW (Mott- Wannier) equation which resembles the hydrogen like Schrodinger equation. Weak dielectric screening is responsible for large binding energy of electron. The screening effect of heterostructure environment effectively controls the exciton binding energy and radius. [ABSTRACT FROM AUTHOR]

Reverse sign spectra of the Verdet constant V(ħω) in the optical active crystals Bi12SiO20 and in the doped optical active crystals Bi12SiO20:M (M = Co, Fe) are observed in the spectral region of the excitons. The spectral region, in which the sign of the doped crystals are changed, is shifted as opposed to the one of undoped crystals and the energy of the photons ħω, at which is observed an inversion of the sign of the V(ħω) of doped crystals, is less than the one of the undoped crystals. By an exchange interaction between the localized paramagnetic ions Co2+ (or Fe3+) and the charge carriers forming the excitons is explained the reverse sign spectra of the doped crystals Bi12SiO20 in the spectral region of the excitons. For undoped crystals, this effect is described by the main internal defect (Bi3+Si+h+) in Bi12SiO20. A simplified model is proposed that explains changing of the sign of V(ħω) by one exchange interaction of the ligand ion O− with a connected exciton (e− + h+) in this defect [ABSTRACT FROM AUTHOR]

Silicene and related 2D crystals with honeycomb symmetry are of increasing interest because of their compatibility with the Si-device technology. By means of density functional and many-body perturbation theory we model their principal properties and growth on substrates. In the case of hydrogenated crystals also excitons in two dimensions are studied. [ABSTRACT FROM AUTHOR]

We report on annealing effects on photoluminescence (PL) of ZnO nanoparticles with an average size of 30 nm. The annealing has been performed at temperatures ranging from 400 to 900 °C under oxygen flow of 50~100 sccm. PL efficiencies of the annealed ZnO nanoparticles increased with the annealing temperature. Low temperature PL spectra for these annealed samples were measured. Compared to annealed samples of core-shell ZnO/SiO2 nanoparticles, we found the remarkable enhancement of the PL efficiency was mainly due to reduction of surface states arising from aggregation of nanoparticles. We also observed sharp excitonic emission peaks at 10 K under low excitation power and assigned them to be donor bound exciton (DX) and LO phonon replicas of free exciton and the DX. [ABSTRACT FROM AUTHOR]

The problem of phase transitions in a system of particles with finite value of the lifetime is analyzed. As an example the theory of condensation of indirect excitons in semiconductor double quantum wells is presented. For description of a spatial distribution of condensed and gas phases two models of phase transitions are used: the model of nucleation and growth and the model of spinodal decomposition generalized on unstable particles. It is shown that due to finite value of the exciton lifetime the sizes of condensed phases are restricted and in two-dimensional case, the regions of condensed phase have the shape of islands located within the excitonic gas. There is specific interaction between islands of the condensed phase through exciton concentration fields. This interaction causes the development of different structures of exciton condensed phases in the crystal irradiated by light. The evolution of the islands of condensed phase is studied depending on the changes of the temperature, pumping and the presence of external potential. [ABSTRACT FROM AUTHOR]

Hybrid semiconductor-metal nanoparticles monolayer of Cadmium Selenide and gold nanoparticles has been prepared, using Langmuir—Blodgett technique. The near field photoluminescence spectra from such monolayer films, shows red shift ∼75 meV with respect to CdSe QDs monolayer film and splitting ∼57 meV. The composite spectra are much broader ∼330 meV compared to the corresponding emission spectra of CdSe monolayer ∼165 meV. The possible explanation for the observed features are provided in terms of exciton—Plasmon interaction. [ABSTRACT FROM AUTHOR]

We consider exciton- and light-induced current in molecular nanojunctions. Using a model comprising a two two-level sites bridge connecting free electron reservoirs we show that the exciton coupling between the sites of the molecular bridge can markedly effect the source-drain current through a molecular junction. In some cases when excited and unexcited states of the sites are coupled differently to the leads, the contribution from electron-hole excitations can exceed the Landauer elastic current and dominate the observed conduction. We have proposed an optical control method using chirped pulses for enhancing charge transfer in unbiased junctions where the bridging molecule is characterized by a strong charge-transfer transition. [ABSTRACT FROM AUTHOR]

This contribution gives an overview of our recent study of phonon-driven exciton dissociation at semiconductor polymer heterojunctions, using a quantum dynamical analysis based on a linear vibronic coupling model parametrized for three electronic states and 20–30 phonon modes. The decay of the photogenerated exciton towards an interfacial charge transfer state is an ultrafast (femtosecond to picosecond scale) process which initiates the photocurrent generation. We consider several representative interface configurations, which are shown to exhibit an efficient exciton dissociation. The process depends critically on the presence of intermediate states, and on the dynamical interplay between high-frequency (C==C stretch) and low-frequency (ring-torsional) modes. The dynamical mechanism is interpreted in terms of a hierarchical electron-phonon model which allows one to identify generalized reaction coordinates for the nonadiabatic process. This analysis highlights that the electron-phonon coupling is dominated by the high-frequency modes, but the low-frequency modes are crucial in mediating the transition to a charge-separated state. The ultra-fast, highly nonequilibrium dynamics is in accordance with spectroscopic observations. [ABSTRACT FROM AUTHOR]

We have investigated the structural and optical properties of bulk CuCl which is a direct band-gap semiconductor with zinc blende structure, employing the time dependent density functional theory using the boot strap approximation for the exchange-correlation kernel. We have also calculated the optical properties under Random Phase Approximation. Our results show a strong signature of excitons in bulk CuCl. [ABSTRACT FROM AUTHOR]

Present paper deals with a critical and comprehensive analysis of the dependence of photo emission (PE) electronic band gap and optical absorption (OA) excitonic band gap on the size of CdSe QD, via connecting it with excitonic absorbance wavelength. Excitonic absorbance wavelength is determined through an empirical fit of established experimental evidences. Effective excitonic charge and Bohr radius is determined as a function of size. Increase in size of the CdSe QD results in greater Bohr radius and smaller effective excitonic charge. Excitonic binding energy as a degree of size of QD is also calculated which further relates with the difference in PE electronic and OA optical band gaps. It is also shown that with increase in size of CdSe QD, the excitonic binding energy decreases which consequently increases differences in two band gaps. Our results are very well comparable with the established results. Explanation for the origin of the unusual optical properties of CdSe QD has been also discussed [ABSTRACT FROM AUTHOR]

Theoretical guides to test 'macroscopic realism' in solid-state systems under quantum control are highly desirable. Here, we report on the evolution of a Leggett-Garg inequality (LGI), a combination of two-time correlations, in an out-ofequilibrium set up consisting of two interacting excitons confined in separate semiconductor quantum dots which are coupled to independent baths at different temperatures (T1 ≠ T2). In a Markovian steady-state situation we found a rich variety of dynamical behaviors in different sectors of the average temperature (TM = (T1 + T2)/2) vs. coupling strength to the reservoirs (Τ) space parameter. For high TM and Τ values the LGI is not violated, as expected. However, by decreasing TM or Τ a sector of parameters appears where the LGI is violated at thermal equilibrium (T1 = T2) and the violation starts decreasing when the system is moved out of the equilibrium. Surprisingly, at even lower TM values, for any Τ, there is an enhancement of the LGI violation by exposing the system to a temperature gradient, i.e. quantum correlations increase in a nonequilibrium thermal situation. Results on LGI violations in a steady-state regime are compared with other non-locality-dominated quantum correlation measurements, such as concurrence and quantum discord, between the two excitons under similar temperature gradients. [ABSTRACT FROM AUTHOR]

In GaAs nanowires, the effective Raman tensor is dominated by dielectric anisotropy leading to similar angular variations of allowed TO and forbidden LO phonon scattering from both zinc blende and wurtzite crystalline symmetry. Resonant Raman scattering studies give evidence of a novel excitonic transition in wurtzite wires at 90 meV above to the lowest exciton in zinc blende ones in agreement with recent theoretical predictions. [ABSTRACT FROM AUTHOR]

The chemical identification of donor bound excitons in ZnO has been studied using radioactive ions. Implantations of 117Ag - which decays to radioactive Cd and In - have enabled the identification of the I2 optical feature as being the ionized donor counterpart of I9, one of the most prominent optical features in the photoluminescence spectrum of ZnO. Both of these lines are consistent with In occupying a Zn site. [ABSTRACT FROM AUTHOR]

The optical properties of an organic polycrystalline layer of NTCDA embedded in a microcavity show exciton-photon coupling between the cavity mode and both the 0–0 and 0–1 transitions. In this work we present a theoretical microscopic model that reproduces the experimental data and explains the creation of an additional third polaritonic dispersion relation. Furthermore, a theoretical insight into the problem of polariton relaxation and photoluminescence in presence of vibronic replicas is given using a general model based on a master equation approach. [ABSTRACT FROM AUTHOR]

Optical spectra of 80 Å, 100 Å, 120 Å, and 140 Å (Cd, Mn)Te quantum wells separated by the (Cd, Mg)Te barriers were measured by means of the pulsed pump-probe method with circular polarization resolution. The energy shifts of a heavy-hole neutral exciton Xe1hh1 were recorded as a function of excitation energy for different excitation beam powers at zero time delay between pulse and probe. Photoluminescence excitation spectra for different pump mean power were also measured. The energy shifts of the Xe1hh1 exciton to higher values were observed when the excitation energy approached the resonant value from the lower energy side. The maximum of the shift was at resonance conditions. Beyond the resonance the shift weakens very rapidly despite still rising density of excitons. Creation of hot excitons—above the resonance; causes weakening of interaction between excitons. Theoretical calculations predicts also weakening of energy shift with rising temperature of created e-h pairs. [ABSTRACT FROM AUTHOR]

An effective method for computing excitonic shifts in curved quantum-wire geometries is presented. As a case example, we consider a GaAs quantum ring with infinite barriers along the cross-sectional dimensions assumed to be of square shape albeit this assumption can be easily avoided. A simple analytic expression for excitonic shifts is found using first-order perturbation theory. [ABSTRACT FROM AUTHOR]

We study the dissipative dynamics of two asymmetric quantum dots, coupled by tunneling, in the presence of a laser coherent radiation. By solving the Liouville-Von Neumman-Lindblad equation associated with the three-level model Hamiltonian, we analyze the effect of spontaneous emission as a decoherence channel. We discuss several aspects of the system, such as, population dynamics, the effect of the detuning and the appearing of electromagnetically induced transparency controlled by tunneling. Our results show that the efficiency of tunneling coupling, and consequent population of indirect excitonic state, can be controlled by tuning external physical parameters. We also show that, for certain choices of those parameters, the indirect excitonic state is robust against spontaneous emission. Finally, we determine the frequency range of applied pulse where the transparency induced by tunneling appears and the dependency of this range on the value of tunneling coupling parameter. [ABSTRACT FROM AUTHOR]

We performed time- and polarization-resolved pump-and-probe experiments on several differently strained GaN wurtzite epilayers.. Thanks to a selective spectral excitation of spin-polarized A-excitons at various temperatures and a fitting procedure considering the different A and B spin exciton states, we show that the Elliott-Yafet mechanism is the dominant spin-relaxation process. It origins from the high dislocation density in all of our GaN epilayers. [ABSTRACT FROM AUTHOR]

Third-order nonlinear optical responses in single walled carbon nanotubes have been investigated by means of pump-probe spectroscopy with various excitation conditions. In the excitation spectra of nonlinear susceptibilities probed at the E22 transition energy of the (8, 4) tube, an additional peak is observed at ∼0.2 eV above the E22 transition energy. It is found that the phonon-assisted transition is involved in the third-order nonlinear optical processes. [ABSTRACT FROM AUTHOR]

We have studied temperature dependence of the signals of optical orientation of excitons at resonant excitation of the ensemble of planar self-assembled CdSe/ZnSe quantum dots and show that spin memory in this system survives up to 100K. To describe the experimental results we apply the model that considers explicitly the effect of multi-phonon processes on the broadening of bright-exciton-phonon sublevels and demonstrate that calculated temperature dependences of spin relaxation rates are in good agreement with obtained experimental data. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

Generation mechanisms of long-lived spin coherence of two-dimensional electron gas (2DEG) have been studied theoretically and experimentally, the latter by means of a picosecond pump-probe Kerr rotation technique. CdTe/(Cd,Mg)Te quantum wells with a diluted two-dimensional electron gas have been addressed. Strong Coulomb interaction between electrons and holes, which results in large binding energies of neutral and negatively charged excitons (trions), allows selective addressing of exciton and trion states with resonant optical excitation. Different scenarios of coherence generation have been analyzed, namely, direct trion photocreation, formation of trions from photogenerated excitons and electron-exciton exchange scattering. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

We report on an exciton resonant phase coherent photorefractive effect in ZnMgSe/ZnSe single quantum wells using 30 fs light pulses. The experiments are performed in a four-wave-mixing configuration using two or three pulses. The PCP effect is attributed to the formation of an electron grating within the QW that is caused by the interference of excitons in the ZnMgSe barrier. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

The temperature dependence of the photoluminescence intensity from frozen single walled carbon nanotube solutions is reported. This is modelled assuming that it is dominated by the small energy splitting between the dark and bright states of the singlet excitons which are found to be in the region of 1–5 meV for nanotubes of 0.8–1.2nm. The luminescence is strongly enhanced by a magnetic field along the tube axis due to the mixing of the different valley states of the excitons. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

Exciton photoluminescence and reflection spectra of CdSe/CdMgSe/CdMnSe asymmetric double quantum wells with different barriers thickness were studied in strong magnetic fields for the first time. Direct excitons from the nonmagnetic CdSe and semimagnetic CdMnSe quantum wells were observed as well as a spatially indirect exciton formed by the electron in the CdSe quantum well and the heavy-hole in the CdMnSe one. The indirect exciton became observable at magnetic fields around 2 T when it was the lowest energy state of the structure. Its position shifted to lower energies with increasing magnetic field, the intensity increased drastically and the half-width decreased. Up to two LO-phonon replicas of the indirect exciton were observed. The problem of localized magnetic polarons formation is addressed. Polarization properties of the direct and indirect excitons were studied. In case of the effective tunneling of σ+ — polarized excitons from the CdMnSe well to the CdSe well, a change of circular polarization of the direct CdSe exciton from the negative to positive value was observed under magnetic field. Large anisotropy of the valence band of the CdMnSe quantum well was revealed by comparison of energy shifts in Faraday and Voigt magnetic field configurations. Calculations of exciton energies were performed. Calculated and experimental values were found to be in reasonable accordance. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

Encouraged by the latest experimental developments as well as by the theoretical interest on the near-field (NF) optics of semiconductor quantum dots (QDs), we present our most recent theoretical results on the NF optical absorption and photoluminescence (PL) of single and coupled III-V QDs subjected additionally to an external magnetic field of variable orientation and magnitude. The zero-magnetic-field “structural” QD symmetry can be destroyed varying the magnetic field orientation. The asymmetry induced by the magnetic field -except for specific orientations along symmetry axes- can be uncovered in the near-field but not in the far-field spectra. Hence, we predict that NF magnetoabsorption experiments, of realistic spatial resolution, will be in the position to bring to light the QD symmetry. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

The phonon-induced dephasing in quantum dots is due to two basic sources: real and virtual phonon-assisted transitions between excitonic states. We calculate the time-dependent optical polarization and absorption in single and coupled quantum dots. We show that, depending on the energy distance between exciton levels in quantum dots, the dephasing can be dominated either by real acoustic phonon-assisted transitions, or by virtual transitions. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

It is demonstrated that capping InAs self-assembled quantum dots with a thin GaAsSb layer allows the emission to be extended beyond 1.5μm at room temperature. This behaviour is attributed to the formation of a type-II system for Sb composition above ∼15%. Magneto-optical spectroscopy suggests that the type-II excitons remain compact and it is postulated that strain modulation of the GaAsSb layer results in hole localization immediately above the quantum dots. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

We report on novel InGaN/GaN quantum well/polyfluorene heterostructures where efficient Förster energy transfer from the well to the organic layer occurs. We show that Mott-Wannier excitons dominate the quantum well luminescence in the quantum wells in the 77 to at least 225 K range and are responsible for the efficient energy channeling to the polyfluorene films. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]