Electron concentration dependence of exciton localization and freeze-out at local potential fluctuations in InN films

InN films with electron concentration ranging from n～10¹⁷ to 10²⁰ cm^?3 grown by metal–organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) were investigated by variable-temperature photoluminescence and absorption measurements. The energy positions of absorption edge as well as photoluminescence peak of these InN samples with electron concentration above 10¹⁸ cm^?3 show a distinct S-shape temperature dependence. With a model of potential fluctuations caused by electron-impurity interactions, the behavior can be quantitatively explained in terms of exciton freeze-out in local potential minima at sufficiently low temperatures, followed by thermal redistribution of the localized excitons when the band gap shrinks with increasing temperature. The exciton localization energy σ _loc is found to follow the n ^5/12 power relation, which testifies to the observed strong localization effects in InN with high electron concentrations.     

Optical properties of Tm-doped GaSe single crystals

Optical properties of Tm-doped GaSe single crystals were investigated by measurements of optical absorption and photoluminescence. The single crystals were grown by the Bridgman technique. The X-ray diffraction analysis revealed that the single crystals were in the ε-type GaSe phase. The optical absorption spectra showed a sharp absorption peak at 582 nm near the band edge, which is due to direct free exciton. The temperature dependence of the energy of the exciton absorption peak was well fitted by the Varshni relation. In the photoluminescence spectrum at 10 K, we observed a very weak emission peak at 586 nm, a relatively strong emission peak centered at 613 nm, and several sharp and narrow emission peaks in the 790-840 nm region. The two emission peaks at 586 and 613 nm were associated with intrinsic emission lines due to direct free exciton and indirect bound exciton. The emission peaks in the 790-840 nm region, which were related to extrinsic emission, were assigned as due to the ³F₄→³H₆ transition of Tm³⁺ ions with a low symmetry of D₃ in the host lattice.     

Excited states of the negatively charged exciton X−in wide CdTe/CdZnTe quantum wells

A magneto-optical study of ≈350 Å wide CdTe/CdZnTe quantum wells containing an electron gas is presented. For undoped structures the absorption spectra show lines associated with centre-of-mass quantization of the exciton in the wide CdTe well. Modulation-doped structures show absorption lines corresponding to the creation of the negatively charged exciton X⁻(two electrons, one hole). We observe not just the ground state of X⁻butalsoa series of lines which are attributed to higher states of X⁻where an electron is attached to the centre of mass quantized states of the exciton.     

High-energy frenkel cation exciton and specific features of its self-trapping in the CdI<Subscript>2</Subscript>-PbI<Subscript>2</Subscript> crystal system

It has been shown using atomic-force microscopy that the PbI₂ impurity is embedded in the CdI₂ crystal lattice in the form of nanocrystalline inclusions. The model of a high-energy cation exciton related to the ³ P ₂ state of a free Pb²⁺ ion has been considered for the impurity absorption (excitation) band at 3.23 eV. The resonance narrow photoluminescence bands with the split absorption band at 3.12 and 3.20 eV have been compared with the emission of a free Frenkel exciton. It has been demonstrated that, in the temperature range 25–45 K, there arises a self-trapped exciton state, and the main role in its formation is played by the bending vibrations of the CdI₂ crystal lattice. The potential barrier separating the self-trapped state from the free exciton is 23 meV. The photoluminescence band at 2.4 eV is assigned to the emission of the self-trapped high-energy cation exciton of PbI₂ in the CdI₂ crystal lattice.     

Induced absorption and gain from high density excitons in CdS

Induced absorption and gain in CdS at 1.8 K has been investigated under excitation densities up to 10 MW cm^?2. The absorption and gain below the free exciton energy is governed by exciton interactions and optical conversion of excitons into excitonic molecules. At the highest density, induced transparency due to excitonic molecule recombinations is observed. E_M=5.1002 eV is determined.     

Core-exciton phonon interaction in LiF

The temperature coefficients of the fundamental exciton at 12.6 eV and the Li⁺1s-2p exciton at 61.9 eV in LiF are both negative. But the Li⁺ core exciton should have a small positive temperature coefficient or, more likely, a zero one according to the point-ion model. We checked the validity of the point-ion model for LiF by measuring with great accuracy the temperature dependence of core levels binding energies by means of x-ray photoemission spectroscopy. We have concluded that the Li⁺1s-2p exciton temperature coefficient arises entirely from exciton-phonon interaction. We have determined also the Li⁺1s absorption threshold from available thermo-absorption spectra. The core-exciton phonon interaction is discussed using different models.     

Optical absorption due to paraexciton of Cu2O

In Cu₂O a new absorption line is observed at 97 cm^?1 below the n =1 of the yellow exciton (triply degenerate orthoexciton) under a strong magnetic field at 4.2 K. The line is assigned as a transition to a nondegenerate spin triplet state Γ⁺₂ (paraexciton). An analysis including the effects due to the n =1 of the green exciton yields 364 cm^?1 as the exchange energy, and 2.68 and ?1.02, or 1.02 and ?2.68 as the g-factors of the conduction and valence bands forming the yellow exciton.     

Fundamental absorption spectra of solid hydrogen

Absorption spectra of solid H₂ and D₂ are measured in the range 10–40 eV at 2K for the first time, using synchrotron radiation. The spectra can be divided into two components; exciton and interband transitions. The exciton absorption consists of bands corresponding to molecular transitions, and vibrational structures are observed in the first exciton band of solid H₂.     

Discovery of bound exciton in ZnSe : Cu

A new bound exciton with the dissociation energy of 32 meV was observed in ZnSe : Cu by electroabsorption. The dependence of the electroabsorption peak on an electric field, the absorption and the photoluminescence spectra show that the bound exciton is associated with Cu²⁺.     

Dynamics of nonlinear optical response of CuBr-doped glasses

Nonlinear absorption of the Z_1,2 exciton band in CuBr-doped glasses was observed at band-to-band excitation and described in terms of damping of the Lorentzian-shaped band. Microcrystal-radius dependent time evolution of the nonlinearity was investigated, and the full recovery of initial absorption was observed to be as short as 20 ps in the smallest microcrystals. Radiative exciton-exciton Interaction and surface recombination were claimed to be the main mechanisms of exciton density decay, and the rate constant of the first process was estimated to be equal to 5·10^-8 cm³s^-1.     

Resonant exciton trapping at impurity states in silver bromide

Excitation spectra near the indirect exciton edge of AgBr at 1.8K are reported for several luminescence lines from weakly localized excitons. Excitation below the exciton absorption threshold reveals several excited bound exciton states the energetic positions of which are determined. For excitation above the threshold, strong energy dependent structure is observed. It is interpreted in terms of resonant trapping of free excitons in both ground and excited bound exciton states associated with emission of LO(Γ), long wavelength acoustic and intervalley TA(X) and LA(X) phonons as well as combinations and overtones of these. From measurements in doped crystals two bound exciton systems are found to be correlated with Cd²⁺ and Ca²⁺, respectively.     

Exciton absorption parameters in TlGaS2 crystals

It is shown that, because the shape of the exciton absorption curve in crystalline TlGaS₂ is described by the Fano antiresonance profile, the experimentally observed exciton peak corresponds to a modified state which is the result of the configuration interaction of a discrete state (exciton) with the quasi-continuum of conduction-band states. The oscillator strength for the transition to the discrete (“pure”) exciton state is calculated as F ₀=1.22×10^?2. The exciton transition selection rules are calculated for two assumed symmetry groups, D _2h and D _4h . An analysis of the selection rules for the dipole-allowed exciton transition permits one to conclude that the symmetry group for the TlGaS₂ crystal is D _2h .     

Fine structure in the absorption edge spectrum of NbS2Y2 (Y = Cl,Br, I)

High resolution absorption spectra of single crystals of NbS₂Y₂ (Y = Cl, Br, I), obtained at temperatures between 4.2 and 300 K revealed extensive fine structure in the absorption edge. This structure has been analysed and interpreted in terms of allowed indirect interband transitions, involving phonons corresponding to SS and NbS vibrations, followed by forbidden and by allowed direct transitions. From the shape of the absorption curve associated with the phonon branches of the indirect transitions a binding energy of 23 cm^? for indirect excitons is obtained. A binding energy of 28 cm^? for direct excitons is deduced from the exciton lines observed at the long-wavelength side of the direct transitions. A detailed interpretation of the optical transitions is given in terms of a molecular orbital diagram for the Nb₂S₄ clusters, present in these crystals.     

Exciton binding energy in GaAs quantum wells deduced from magneto-optical absorption measurement

Magneto-optical absorption spectra due to exciton states and Landau-levels were measured in GaAs/AlAs multi-quantum-wells. By extrapolating the photon energies of the absorption peaks to zero magnetic field, the lowest state (1S) heavy hole exciton binding energy, E^B_$\text{h}$(1S), was obtained as a function of well size L_z in the range $\text{58}\text{A}\text{?}\text{?}\text{L}{}_{\mathrm{z}}\text{?252}\text{A}\text{?}$. The L_z dependence of E^B_h showed the change of the exciton character from three-dimensional to two-dimensional with decreasing L_z. The diamagnetic shift observed for the heavy hole exciton peak was larger than that for the light hole exciton peak, showing the anisotropic nature of the Luttinger-Kohn Hamiltonian. The diamagnetic shift of the heavy hole exciton peak became smaller as L_z was decreased, suggesting the enhancement of the two-dimensional exciton character.     

Absorption and photoluminescence of Eu2+-doped 1-D CsCdBr3 single crystal

Eu²⁺-doped CsCdBr₃ single crystals are studied by polarized variable-temperature optical absorption and luminescence spectroscopy for different excitation wavelengths. Whereas the low-energy absorption band is assigned to f→d transitions within Eu²⁺ electronic configuration, the high-energy absorption bands are assigned to Eu-trapped exciton due to the proximity of the high-energy d levels to the conduction band.     

Biexcitonic absorption in a disk-shaped GaN quantum dot

The present work investigates the nonlinear optical properties of a GaN quantum dot in the disk limit via the exciton and biexciton states using the compact density matrix formalism. Based on this model, we calculate the ground state energy of the exciton and biexciton states by the variation method, within envelope function and effective mass approximations. Linear and nonlinear optical absorption (α ⁽¹⁾, α ⁽³⁾) and oscillator strengths attributed to the optical transitions are obtained. The details of the behaviour of α ⁽¹⁾ and α ⁽³⁾ around the resonance frequencies and for different quantum dot geometries are presented. It is found that the size of quantum dot and the optical intensity have a remarkable effect on the optical absorption, and the biexcitonic two-photon absorption coefficient(K ₂) has also been calculated in this system. The results show that this parameter is strongly affected by the size of the quantum dot.     

Three-photon absorption coefficients in ZnSe/ZnS0.18Se0.82 strained-layer superlattices

Summary We provide the first experimental determination of three-photon absorption (ThPA) coefficients α⁽³⁾ in a quasi-bidimensional structure. The measurements of α⁽³⁾ have been performed on a sample of a ZnSe/ZnS_0.18Se_0.82 strainedlayer superlattice at fixed excitation energies corresponding to the exciton resonances near the direct band gap. The evidence of the ThPA process in such energy range points out the huge enhancement of exciton non-linearities in quantum-confined systems.     

Scattering of an exciton polariton by impurity centers in GaAs

Scattering of an exciton polariton by impurity centers at low temperatures has not been investigated comprehensively in spite of its significant role in processes accompanying Bose–Einstein condensation of an exciton polariton. For studying the peculiarities of the interaction of an exciton polariton with impurity centers, we have studied the integrated absorption of the ground state (n ₀ = 1) of the exciton in GaAs in thin (micrometer-thick) wafers with an appreciable optical transmission. Comparative analysis of the transmission in the vicinity of the exciton resonance performed on 15 samples of crystalline GaAs wafers with different concentrations N of impurity has revealed an unexpected regularity. The value of N increases by almost five decimal orders of magnitude, while the normalized spectrally integrated absorption of light exhibits a slight increase, following the power dependence N ^m on the concentration, where m = 1/6. It has been shown that this dependence indicates the diffusion mechanism of propagation of the exciton polaritons through the bulk of the semiconductor, which is present along with the ballistic propagation of light through the sample.     

Long-Range Energy Transfer Combined with Isotropic or Anisotropic Diffusion

Abstract

The effect of diffusion on long-range energy transfer by dipole-dipole interaction^1,2 from electronically excited donor molecules D? to (unexcited) acceptor molecules A may be important in solutions^3–7 and in molecular crystals^7–11. In solutions usually diffusion of matter occurs (the molecules perform a random walk) whereas in molecular crystals an excitation (exciton) migrates. Exciton migration can often be described in terms of an effective exciton diffusion tensor which may be highly anisotropic^12–17. Because of the restricted space only the time dependence of the donor fluorescence after δ-excitation will be given. The extension to general excitation forms and to the acceptor flouorescence is a straightforward matter^18,19.     

Excitonspektren von Silberhalogeniden und ihren Mischungen

The absorption spectra of AgBr and AgCl as well as their mixtures were investigated at liquid oxygen temperature. There are two characteristical features for exciton bands: the terms of the free atoms, and the structure of the crystal lattice. For pure and mixed crystals there exist doublets which can be derived from the terms of the free halide atoms. The wavelengths of the exciton bands shift linearly with increasing content of AgCl in AgBr. For the first band of the absorption edge this shift is 70 cm^?1 per mole percent admixture. The bands shift to a greater extent than they broaden. Thus from statistical calculations can be deduced that there is a region of about 125 anions responsible for an absorption act. The exciton bands depend considerably on thermal influences from the lattice as well as on distortions resulting from ultraviolet exposure.