Mixing of singlet and triplet exciton states in highly excited CdS

Fluorescence spectra due to the free excitons have been studied in CdS at 4.2 K under various excitation levels. It has been found that the triplet-exciton emission intensity relative to the singlet line is enhanced remarkably with the excitation power density. This effect is reasonably explained by the mixing of the singlet with the triplet through the many body interaction. The density of optically generated excitons is determined from the magnetic field dependence of the triplet emission intensity.     

Pico-second spectroscopy of highly excited CdSe and CdSI high density exciton system

Time-resolved luminescence spectra are measured at 4.2 K most in detail for CdSe under 531 nm pico-second pulse excitation. The M, P, and A-LO lines from high density excitons rise in intensity after pulse excitation, reach maxima, and then fall, the rise time being in a range of 100–600 psec and becoming faster with increasing excitation density. This time dependence is due to the light amplification effect by stimulated emission.     

Phonon laser and indirect exciton dispersion engineering

Engineering of dispersion of indirect excitons by normal electric and in-plane magnetic fields is proposed to be used for controlling the state of many-exciton system (e.g., coherent state) and its photoluminescence and for producing an inverse population in the excitonic system. The possibility of phonon laser creation on the basis of the latter effect is discussed. Phonon number distribution appears to be a fingerprint of that in exciton system. Numerical estimations for the proposed scheme are made for GaAs/AlGaAs quantum wells.     

Surface photovoltage in exciton absorption range in CdS

The high resolution, intrinsic spectra of surface photovoltage are reported for semiconducting n-type CdS single crystals. At reduced temperatures (120–160 K) the spectra exhibit three sharp maxima due to A, B and C free exciton transitions. Energy positions of these lines and valence band parameters (spin-orbit and crystal field splittings) estimated from surface photovoltage are in good agreement with values obtained by other methods. The excitonic transitions are very sensitive to surface treatment, i.e. polishing, etching, background illumination and surface doping. The mechanism of direct interaction of free excitons with surface states is proposed to explain exciton lines in surface photovoltage.     

Free exciton emission in forward-biased CdS MIS diodes

The radiative decay of free excitons with the emission of 0, 1, or 2LO phonons has been observed in electroluminescence in Cds MIS diodes operated in forward bias at temperatures in the range 40 – 77K. The line shapes and temperature dependence of the ILO and 2LO phonon assisted replicas are discussed in terms of the distribution of kinetic energy of the excitons with an effective temperature equal to that of the lattice.     

Phonon–replica transitions in InGaN/GaN quantum well structures

A side-bump feature in a photoluminescence (PL) spectrum of an InGaN compound was widely observed. With reasonable fitting to PL spectra with three Gaussian distributions, the temperature variations of the peak positions, integrated PL intensities, and peak widths of the main and first side peaks of three InGaN/GaN multiple quantum well samples with different nominal indium contents are shown and interpreted. The existence of the side peaks is attributed to phonon–replica transitions. The variations of the peak position separations and the decreasing trends of the first side peak widths beyond certain temperatures in those samples were explained with the requirement of phonon momentum condition for phonon–replica transitions. In the sample with 25% nominal indium content, the phonon–replica transition could become stronger than the direct transition of localized states.     

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.     

The excited states of an intersubband exciton

The energy of intersubband collective spin-and charge-density excitations is calculated for a system of quasi-two-dimensional electrons at certain values of the angular-momentum component perpendicular to the plane of the electron free motion. The calculation is carried out within the Hartree-Fock approximation. It is shown that the excited states correspond to the electron transitions in the vicinity of the Fermi momentum. The dispersion of the collective excitations is considered.     

Resonant Brillouin scattering near the a exciton in CdS

We have studied resonant Brillouin scattering in CdS by tuning a narrow-band cw dye laser through the A-exciton transition at helium temperatures. We observed besides the resonance enhancement a pronounced asymmetry of the Stokes and anti-Stokes shift and a dramatic increase of the Brillouin shifts as a function of laser frequency. These observations are consistent with a model of exciton-polariton scattering by longitudinal acoustic phonons. For incident frequencies higher than the longitudinal exciton frequency additional features are found which cannot be explained by the two-branch model for exciton-polaritons.     

Picosecond studies of highly excited CdS

Results on picosecond luminescence and excite-and-probe transmission as well as transient grating measurements for highly excited CdS measured at a bath temperature of 5 K will be presented. The luminescence and optical gain both due to electron-hole plasma and excitonic molecule recombination are observed. The electron-hole plasma decays very fast by bimolecular recombination of electrons and holes in the plasma and diffusion of the carrier toward the low density regions, and transforms into excitons and excitonic molecules within 100–200 ps. The possibility of electron-hole liquid formation is definitely excluded. The exciton and excitonic molecule decay rather slowly and govern the optical properties for times longer than 200 ps.     

Phonon heating of two-dimensional exciton gases in GaAs/AlGaAs quantum wells

We report the first measurements of the interaction of non-equilibrium phonons with two-dimensional exciton gases (2DExGs). The rise in the effective temperature of the 2DExG produced by the phonons depends on the width of the quantum well and the exciton sheet density and hence on the ratio τ^?1 (ex-ph)/τ^?1 (ex-ex). The dependence of the effective temperature rise on this ratio is attributed to the non-equilibrium frequency distribution of the phonons incident on the 2DExG.     

Effects of optical activity on exciton luminescence of CdS

The luminescence spectra have been investigated in the vicinity of the B_n=1 exciton resonance in CdS at T = 2 K. The effect of “weak” optical activity on luminescence has been observed for the first time. The observed spectra are compared with the frequency dependences of the polariton transmission coefficients.     

Two-photon absorption in the vicinity of the exciton absorption in CdS

The two-photon absorption spectrum of CdS in the wavelength region of the free and bound exciton lines has been measured using a pulsed dye laser and a differential transmittance technique. An intense absorption band is observed which is possibly due to the interference of the contributions of different intermediate states to the interband two-photon absorption. No absorption is observed which can be attributed to the formation of free excitonic molecules.     

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.     

Detection of exciton - Phonon bound state in a single crystal of CdTe

Experimental evidence on the presence of the quasi-particle exciton-phonon complex was found on a Cd Te single crystal. It's identification was achieved with the help of high resolution photoconductivity. From the corresponding spectrum, it was obtained that excitonic line lies at 30 meV below the conduction band, and that the peak corresponding to the exciton-phonon complex is located at 19.3 meV above the excitonic peak. With this data, along with the well established value of the Longitudinal phonon energy for this material, the bound energy for the exciton-phonon quasi-particle was found to be 2.3 meV. This value matches quite well the expected one. Furthermore, this quantity was approximately confirmed by obtaining the KT value for which the corresponding peak dissapears. An explanation is proposed in order to explain the presence of the excitonic line well below expected.