We calculate with the variational technique the fine structure of a biexciton in wurtzite crystals with the effective electron-hole exchange interaction taken into account. The values of the electron-hole exchange integrals are taken from the free exciton Γ_{5}-Γ_{6} splitting. We calculate the biexciton dissociation energy and the ratio of mixing of the symmetric and antisymmetric envelopes which arises from the electron-hole exchange interaction. Results are presented for CdS, CdSe and ZnS crystals.
We present a general method for the calculation of the energy band structure for a one-dimensional Kronig-Penney potential in the presence of a strong laser field. Numerical illustrations show that the energy band structure is strongly modified by the intense radiation field. In particular, it is also shown that the laser field creates new laser-induced forbidden gaps, widths of which increase with increasing intensity of radiation
The electronic structure of a biexciton is discussed while taking into account the detailed band structure of CuCl. The fine structure of the excitonic molecule is clarified with the effective electron-hole exchange effect taken into account beyond the effective mass approximation. The electron-hole exchange interaction mixes the states of opposite parities with respect to the permutation of electrons and holes. Two trial envelope functions, symmetric and antisymmetric under the permutation of two electrons or two holes, were used in the numerical minimization of the ground state energy of the biexciton. The obtained binding energy of the biexciton, as well as, the ratio of the mixing of the trial envelope functions of opposite parities are presented.
Charge transfer induced local/resonance Frenkel exciton states in doped naphthalene crystals are investigated in terms of the cluster model, proposed in Paper I of this series. The effect of cluster size is studied in order to assess the physical relevance of the results. Possible methods of accounting for the random arrangement of the impurities are suggested and discussed.
The ground state energy for a chain of donor and acceptor molecules (mixed-stack architecture) is calculated within the three-state model. The model describes the intermolecular electron transfer and, in particular, stresses the role of the diagonal coupling of the electron to symmetry breaking molecular displacements and the local electric field. The modulation of the intermolecular Coulomb interaction is shown to have important consequences for the ground state and its dynamics. In particular, the ground state energy as a function of the displacement may show one, two or three minima with varied molecular ionicity. An analysis of the function gives a phase diagram which indicates a possibility for the coexistence of neutral (undistorted) and ionic (distorted) chains of molecules in the ground state. The function is illustrated by numerical calculations with parameters appropriate for the tetrathiafulvalene-chloranil crystal which undergoes a neutral-to-ionic phase transition induced by either temperature or pressure. The effect of the electron transfer on the lattice dynamics of the mixed-stack system is briefly considered. It is suggested that the thermodynamical phase diagram for tetrathiafulvalene-chloranil system can be understood as a result of two effects: pressure induced quantum mixing between diabatic states which determine a nature of components and temperature stimulated classical mixing of the components.
Reflectivity and Faraday rotation measurements were carried out on Cd_{1-x}Mn_{x}Te bulk crystals in the complete zinc-blende range of composition 0 < x ≤ 0.71. From a comparison of the results of these two experiments for samples with x ≤ 0.3, we demonstrate that the Faraday angle Θ, measured in limited spectral range, is proportional to the energy splitting of exciton states ΔE, measured at the same temperature (2 K) and magnetic field B (up to 5 T). The determined proportionality constant was used to calculate ΔE values also for x > 0.3. This allowed us to find an empirical description of ΔE values for the whole range of compositions as a function of B and x.
Binding energy of muonium hydride is calculated variationally with wave function dependent exponentially on three interparticle distances. The lower bound for the dissociation energy into the hydrogen atom and muonium is obtained as 3.853 eV. Expectation values of the interparticle distances are also calculated.
Photoreflectance spectra were measured at room temperature for energies in the vicinity of the E_{0} critical point for p-type as well as n-type doped GaAs/SI-GaAs structures. Depending on the doping concentration the existence of two photoreflectance subsignals was observed; the first one arises from the surface space charge region while the second one from the interface region. The decomposition of photoreflectance spectrum into surface and interface subsignals was based on the photoreflectance measurements carried out for different wavelengths of the laser pump beam.
Exciton magnetic polarons are studied in CdTe/Cd_{1-x}Mn_{x}Te (0.4 ≤ x ≤ 0.8) quantum wells. The magnetic polaron formation leads to the appearance of an additional line in the photoluminescence excitation spectra, which can be employed to determine the Zeeman splittings more exactly than by using the free exciton peak. We find an overall increase in the polaron energy with increasing x in the whole range of Mn contents studied.
The paper gives an overview of theory and presents several examples of application of some holographic techniques: four-wave mixing, self-scattering and beam coupling to study nonlinear optical properties of solids. Among them excitation energy transfer in chromium-doped vibronic laser materials and picosecond study of undoped CdTe are reported.
The dynamic response of an electron Fermi sea to the presence of optically generated holes gives rise to an enhanced interaction of correlated electron-hole pairs near the Fermi level, resulting in an enhanced oscillator strength for optical transitions, referred to as the Fermi-edge singularity. We studied this effect in modulation-doped quantum wells which provide confined dense Fermi sea, spatially separated from dopant atoms, easily accessible for investigations under low excitation conditions. The Fermi-edge singularity was observed in both photoluminescence and photoluminescence excitation experiments, although in the case of photoluminescence the samples had to be either co-doped with acceptors in the wells to provide necessary localization of holes or designed to allow for nearly resonant scattering between the electronic states near the Fermi energy and the next unoccupied subband of the 2D electron gas.
We present absorption measurements on free-standing ZnSe, ZnS and ZnS_{x}Se_{1-x} films (d = 0.4...2 μm) under hydrostatic pressure up to 15 GPa. The refraction index n(λ,P) of ZnS and ZnSe in the transparent region up to 800 nm and the pressure shift of the E0 absorption edge of ZnSe and some ZnS_{x}Se_{1-x}-compositions was investigated at 293 K. At 2 K free exciton states near the E_{0-} and E_{0} + Δ_{0}-gap are visible in absorption. Increase in the Rydberg energy under pressure was found, which is explained with k • p-theory in the framework of the hydrogen model.
The mechanisms of irregular photoluminescence intensity oscillations, as observed in optically detected cyclotron resonance experiments, are discussed. Two possible scenarios are analyzed, both requiring impact ionization of the center(s) by electric field accelerated free carriers. The first assumes coexistence of dielectric and energy relaxation processes. The second assumes a subsequent impact ionization of two different centers.
The optically created exciton will be self-trapped if its coupling to phonons is strong enough, and will moreover be self-decomposed if the electron and the hole couple to phonons in an opposite way. The bistability between the parity-conserved and parity-broken self-trapped excitons was observed in alkali halides. The situation is most dramatic if the bistability between the parity-broken self-trapped exciton and the ground state (with no exciton) comes into play since the electron-hole pairs may then be spontaneously generated at every lattice site, resulting in the electronic and structural phase transition. The neutral to ionic phase transition observed in a few organic charge transfer compounds under applied pressure or decreasing temperature can be considered as an example. Recent experiment revealed that TTF-chloranil, among others, is subject to photo-induced transient phase change over hundreds of unit cells per one photon. The dynamics of this process can be described in terms of self-trapping and self-multiplication of a photo-generated charge transfer exciton along the chain through the attractive dipolar interaction. This description of phase transition in terms of exciton dynamics will provide a new paradigm of materiology.
Exciton properties in growth interrupted quantum wells of GaAs/AlGaAs are compared with those observed for structures grown without growth interruption during the molecular beam epitaxy process. We report observation of quasi-localized excitons in quantum well structures grown without growth interruptions. Quasi-localized excitons drift towards the states of a lower potential energy in the quantum well. For growth interrupted MBE structures islands with a constant quantum well thickness become large compared to the exciton radius. Free or lightly localized excitons are observed in that case.
A magnetic field induced coupling is observed between the Landau levels with different quantum number of two GaAs quantum wells separated by a thin (Ga,Al)As tunnel barrier using magneto-photoluminescence-excitation spectroscopy. This coupling cannot be due to a resonance between single particle energy levels, and therefore an explanation is proposed in terms of the Coulomb interaction of magneto-excitons across the thin barrier.
The dominant mechanism responsible for the optical detection of the Mn^{2+} magnetic resonance in Cd_{1-x}Mn_{x}Te (x = 0.095, 0.007) is explained. By either change of the external magnetic field or by setting the conditions for the Mn^{2+} magnetic resonance, we could change the relative efficiencies of the two competing excitonic recombination processes. By lowering magnetization at the magnetic resonance, recombination via the acceptor bound exciton channel, which is mainly nonradiative, is enhanced. Then, a large up to 50% decrease in the total photoluminescence efficiency was observed in the optically detected magnetic resonance experiment. Such observation allows for verification of the large efficiency of the Auger-type transition responsible for the nonradiative decay of the acceptor bound exciton.
Photoluminescence properties of CeF_{3} have been investigated. Two kinds of emitting centers have been identified: regular Ce^{3+} sites exhibiting two close emission bands (285 and 300 nm) and a fast fluorescence (20 ns) and several physically different perturbed Ce^{3+} sites giving rise to a broad emission band near 340 nm and a longer fluorescence (30-40 ns) the decay time of which varies with wavelength. An energy transfer occurs between these two kinds of centers. Scintillation properties have been studied under UV and X-ray excitation. The additional fast decay always observed at short time under ionizing radiation is interpreted by a temperature dependent luminescence quenching phenomenon due to high excitation density.
In the paper the energy states of structured isoelectronic impurities of transition metals and rare earth elements, donor and acceptor excitons of 3d and 4f impurities, the role of donor and acceptor excitons of 3d and 4f impurities in energy transfer from the matrix to impurities are discussed. It is shown that structured impurities may be classified as "open" and "closed" isoelectronic impurities. The number of electrons in the 3d or 4f shells is changed at hω < E_{g} not changed structure levels generated by forbidden gap properties donor acceptor excitons those energy transfer mechanisms from matrix to an impurity (a capture carriers into 3d or 4f shells open and auger process essentially distinguished two kinds structured isoelectronic for closed impurities a model is discussed in frame which the spectra of electroabsorption photoluminescence cathodoluminescence are described.
Optical properties of a series of CdTe/CdMnTe multi quantum well structures grown with MBE and ALE (CdTe quantum wells only) methods are compared. Based on the results of the photoluminescence experiments we conclude that the ALE growth leads to a different lateral scale of quantum well width fluctuations, which results in different exciton properties in two multi quantum well systems studied. In the wells grown with ALE method excitons are less localized. They can migrate in a quantum well plane between quantum well regions varying in thickness by 1 monolayer.
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