Shallow acceptor bound exciton and free exciton states in high-purity zinc telluride

We investigated excitons bound to shallow acceptors in high-purity ZnTe and measured excitation spectra of two-hole luminescence lines at 1.6 K using a tunable dye-laser. The electron-hole coupling in the bound exciton (BE) states appears to be very different for the various acceptors even for almost identical exciton localisation energies. Three different types of BE are reported. For the Li-acceptor BE we observe three sub-components separated by 0.22 and 0.17 meV and interpreted as J = $\text{1}\text{2}$, $\text{3}\text{2}$, $\text{5}\text{2}$ states. The Ag-acceptor BE exhibits a strong ground state and a weak excited state at 1.3 meV higher energy. For the as yet unidentified k-acceptor we observe a single BE level, degenerate with the Ag-acceptor BE ground state. Dips in the excitation spectra due to absorption into free exciton 1S, 2S, and 3S states yield an exciton Rydberg R₀ = 12.8±0.3 meV and a free exciton binding energy FE(1S) = 13.2±0.3 meV.     

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.     

Magneto-reflectance of the 1S exciton ground states in InP and GaAs

The reflectance due to the 1S exciton of InP and GaAs is studied in magnetic fields up to 12 T in the Faraday and Voigt configurations. Electron-hole exchange interaction is taken into account in a perturbation model on degenerate exciton states in cubic crystals to explain the splitting and the field dependence of the oscillator strengths of the J = 1 and J = 2 exciton states. Electron and hole g-values are estimated.     

Ionic polarization effects on a shallow exciton at a finite temperature

The effective Hamiltonian for the interaction of a shallow exciton with longitudinal optical (LO) phonons at a finite temperature is derived to first order in the electron-phonon coupling α and E_B/?ω.     

Characteristics of exciton polaritons in a ZnO microcavity

We have investigated the characteristics of exciton polaritons in a ZnO microcavity with HfO₂/SiO₂ distributed Bragg reflectors. The results of the angle-resolved reflectance spectra were analyzed by calculating the cavity polariton dispersions with a phenomenological Hamiltonian for the coupling between the cavity photon and three kinds of excitons labeled A, B, and C peculiar to ZnO. The vacuum-Rabi-splitting energy is estimated to be ∼80 meV. The reflectance dips originating from the cavity polaritons shift to lower energy side with an increase in temperature. We discuss the temperature dependence of the cavity-polariton energies of the ZnO microcavity on the basis of the phenomenological Hamiltonian taking account of the temperature dependence of the exciton energies with Varshni’s law.     

Energy of exciton in quantum dot for a potential containing Coulomb and quadratic terms

We theoretically study the energy levels of an exciton in a quantum dot. We take in to account both quadratic and Coulomb terms. Next, we use the method of series to solve the Schrödinger equation exactly. Using this formalism, we have calculated the exciton energy in both ground and excited states. The results are comparable to those of variational exact diagonolization, full configuration interaction, Hartree-Fock and 1/N methods. Our approach could be fitted for any desired material.     

Self-trapped exciton in alkali fluorides: ODMR study

The optical detection technique is used to observe magnetic resonance in the triplet state ³B_1u of the self-trapped exciton in LiF, NaF, KF. The strong anisotropy of the hyperfine interaction in the x?y plane is related to the more localized character of the electronic wavefunction. The magnetic dipolar contribution to the fine structure can be described by a Mollwo-Ivey type law in alkali fluorides and chlorides.     

Magnetic polaron contribution to donor bound exciton in Cd1−xMnxSe

Photoluminescence spectra associated with the donor bound exciton has been studied in Cd_1-xMn_xSe as a function of composition (x = .10, .05, and 0.0) and temperature in zero magnetic field. In comparison with the previously studied neutral donor, the bound exciton shows a large enhancement in its binding energy and broadening, attributed to the exchange interaction with the Mn⁺⁺-ion 3d electrons. In particular, a transition from a bound polaron to a fluctuation dominated regime is in evidence for x = .10 in this multiparticle system.     

The 2s exciton in intermediate dimensionality structures

We report the observation of the 1s and 2s ground state excitons in small-offset superlattices with alternating layers of non-magnetic (ZnSe) and diluted magnetic (Zn_xMn_1 − xSe) semiconductors. Due to the large Zeeman splitting of the electron states in ZnMnSe, valence and conduction band offsets in these structures can be tuned by external magnetic field over the scale of tens of meV. Two series of superlattices were investigated, with Mn concentrations close to 10 and 15 atomic percent, respectively. Each series consisted of five superlattices with different layer thicknesses. The 2s exciton of the ground state is observed in all samples, and exhibits a diamagnetic shift that is consistent with earlier studies. The ability to determine the energy separation between the 1s and 2s exciton states allows us to analyze our results in the framework of thefractional dimension analysis , and to extract the values of exciton binding energies for the superlattices from the analysis.     

The trion as an exciton interacting with a carrier

The X⁻ trion is essentially an electron bound to an exciton. However, due to the composite nature of the exciton, there is no way to write an exciton-electron interaction potential. We can overcome this difficulty by using a commutation technique similar to the one we introduced for excitons interacting with excitons, which allows to take exactly into account the close-to-boson character of the excitons. From it, we can obtain the X⁻ trion creation operator in terms of excitons and electrons. We can also derive the X⁻ trion ladder diagram between an exciton and an electron. These are the basic tools for future works on many-body effects involving trions.     

Pool-Frenkel thermoelectric modulation of exciton photoluminescence in GaSe crystals

Effect of external field on the exciton photoluminescence of GaSe crystals has been investigated and it has been observed that the PL is quenched with the applied field. The changes observed in the PL spectra have been analyzed with impact exciton, Franz-Keldysh and Pool-Frenkel effects. From the analyses of the experimental data, it has been found that the intensity of direct free, indirect free and bound exciton peaks decreased exponentially with the square root of applied field as I∼exp-β√E. The energy positions of emission peaks were found to shift to longer wavelength with the applied field as ΔE∼β√E. From these findings, the Pool-Frenkel thermoelectric field effect is seen to be the dominant mechanism in the variation of exciton PL with the applied field even though the impact exciton and Franz-Keldysh effects contribute.     

Optical activity of CdS crystals in exciton spectral region

Natural optical activity of wurtzite CdS crystals has been studied for the first time both theoretically and experimentally in the spectral region of the B_n=1 exciton resonance. It has been shown that the observed optical activity is due to mixing the longitudinal Γ_5L exciton state with the transverse Γ₁ state by the energy terms linear in wave vector.     

Shape of recombination lines for exciton complexes in quantum dots with in-plane electric field

We study electron–hole recombination lines of exciton (X) and exciton complexes (X−, X+, 2X) in planar quantum dots with the electric field oriented within the plane of confinement. A model of a two-dimensional circular infinite quantum well is applied and the ground state of the complexes is found using an exact diagonalization method. We demonstrate that for each of the exciton complexes the recombination lines become non-monotonic for some material and sample parameters as a result of Coulomb interactions. A phase diagram for the line shape is presented. The relation of the exact results to the mean field approach is also discussed.     

Electro-optical effects at the discrete and continuum exciton states in GaAs

We report electroreflectance spectra from a GaAs-Au Schottky barrier interface, in the region of the fundamental energy gap, at a temperature of ～ 1.8 K. Both the ground exciton level and the continuum of states are investigated separately, by operating in the small modulation and in the on-off field limit, respectively. The field profile is monitored by I–V, C-V, and photovoltage measurements. It is found that, even at ～ 1.8K, a high surface field is generally present. This moves the actual reflecting boundary of the exciton-polariton away from the surface and affects the ER lineshape through field-induced inhomogeneity effects and interference across the high field layer below the surface. The spectra corresponding to the states of the continuum are discussed in terms of the recent calculations of photon-assisted tunneling with coulomb interaction.     

Triplet exciton in a non-reactive bis-triphenyllead (or tin) diacetylene crystal. A spectroscopic study

The lowest triplet exciton state of crystals formed by (C₆H₅)₃?X?C ≡ C?C ≡ C?X?(C₆H₅)₃ where X is Pb or Sn and an equimolecular amount of CH₂Cl₂ solvent molecules, is studied through the analysis of absorption and of phosphorescence emission and excitation spectra at low temperatures. In the lead compound, the origins of absorption and emission are identical, showing the dominance of free exciton emission and the absence of crystal lattice relaxation in the excited state; the splitting is accounted for by symmetry considerations in the different crystalline phases; the triplet exciton band is several cm^-1 wide, although the nearest diacetylene-diacetylene distance is about 10 Å. There is evidence for triplet exciton motion, through the detection of trap (presumably X-trap) emission in the tin compound, and through the analysis of the excitation spectrum lineshape, showing the effect of exciton-exciton interaction in the lead compound.     

Coherence effects in the exciton transfer in LH2 unit

The LH2 subunit ofRps. acidophila consists of a ring of 9αβ heterodimers to which BChl’s are associated. We apply the strong exciton coupling model in which allαβ BChl₂ pairs are organized in a cyclic unit cell and strongly excitonically coupled. We investigate the exciton energy structure with heterogeneity both in the local BChl850 energies and in transfer integrals. We calculate the pure coherent memory functions of the LH2 subunit which enter the time nonlocal Generalized Master Equations in various regimes of the exciton transfer. We discuss the influence of the exciton interaction with a bath (phonons) on the time dependence of the memory functions and we show limits of the use of the time local Pauli Master Equations. We study also the influence of the heterogeneity in the local BChl850 energies and in transfer integrals on the delivery time of the exciton inside of the LH2. Presented at the Czech-Israeli-German Symposium “Dynamical Processes in Condensed Molecular Systems”, Prague, Czech Republic, 26–30 May 1997. This work was supported by the Grants 105/95 of the Charles University and 1235/96 of the Development Fund.     

Influence of magnetic field on exciton luminescence

The magnetic field dependence of the exciton emission intensity I_ex(H) has been investigated in Ge crystals stressed along the direction near 〈100〉. In the low field limit the magnetic field correction has been evaluated to the wave functions of the ground and some excited states of an isotropic exciton. The calculated dependence I_ex(H) in the case of Ge is in a good agreement with the experimental one at H ? 0.5 T.     

Stimulated self-trapped exciton emission in CsI:Pb

Defects of the type of V_K and Pb⁺ centres were created in CsI:Pb under the 4.03 eV XeCl laser line irradiation at 10 K. After irradiation, the self-trapped and localized exciton emission excited by the same XeCl laser line was observed as a result of the recombination of electrons, optically released from Pb⁺, with the V_K centres. A strongly superlinear dependence of the emission intensity on the excitation intensity was found for the 3.65 eV emission of the self-trapped exciton. A much weaker superlinearity was observed for the visible localized exciton emission. Optical amplification of the exciton emission was considered as the most probable reason of the observed phenomenon. At 10 K, optical gain G=3.74 was calculated for the self-trapped exciton emission.     

Crossover from strong to weak exciton confinement and third-harmonic generation on one-dimensional quantum dots

We study the effects of exciton confinement on the nonlinear optical susceptibility of one-dimensional quantum dots. We use a direct numerical diagonalization to obtain the eigenenergies and eigenstates of the discretized Hamiltonian representing an electron–hole pair confined by a semiparabolic potential and interacting with each other via a Coulomb potential. Density matrix perturbation theory is used to compute the nonlinear optical susceptibilities due to third-harmonic generation and the corresponding nonlinear corrections to the refractive index and absorption coefficient. These quantities are analyzed as a function of ratio between the confinement length L and the exciton Bohr radius a₀. The Coulomb potential degrades the uniformity of the level separation. We show that this effect promotes the emergence of multiple resonance peaks in the third-harmonic generation spectrum. In the weak confinement regime β = L/a₀ ? 1, the third-order susceptibility is shown to decay as 1/β⁸ due to the prevalence of the hydrogenoid character of the exciton eigenstates.     

Fine structures of indirect exciton absorption in phosphorus-doped silicon

A new indirect exciton absorption structure has been observed in phosphorus-doped silicon crystals by using a wavelength modulation method. The structure is interpreted in terms of two-electron transitions involving a free exciton and the valley-orbit states of a phosphorus donor, in which the donor is left in the 1s (A₁) singlet state.