Showing papers on "Exciton published in 1982"

Exciton binding energy in quantum wells

Abstract: Variational calculations are presented of the ground exciton state in quantum wells. For the GaAs-GaAlAs system, the results obtained from a trial wave function not separable in spatial coordinates are shown to be valid throughout the entire well-thickness range, corresponding in the thin and thick limits to two- and three-dimensional situations, respectively. For the InAs-GaSb system, in which electrons and holes are present in spatially separated regions, the exciton binding is substantially reduced. In the limit of thin wells, the binding energy is only about one-fourth of the two-dimensional value.

Large room‐temperature optical nonlinearity in GaAs/Ga1−x AlxAs multiple quantum well structures

Abstract: We report the first measurements of optical absorption saturation in GaAs/GaAlAs multiple quantum well (MQW) structures at room temperature near the heavy hole exciton peak. Linear absorption shows distinct exciton peaks at room temperature in the MQW and we deduce this is because the confinement increases exciton binding energy without increasing LO phonon coupling. This room‐temperature MQW absorption also saturates more readily than that in a comparable GaAs sample; the measured saturation intensity is 580 W/cm2 with a recombination time of 21 ns in a MQW with 102‐A GaAs layers. From this we predict a nonlinear refraction coefficient n2∼2×10−5 cm2/W. This large nonlinearity should permit room‐temperature optical devices compatible with laser diode wavelengths, materials and power levels.

Numerical Experiments on the Absorption Lineshape of the Exciton under Lattice Vibrations. I. The Overall Lineshape.

Abstract: A standardized formulation is introduced for the lineshape problem–the effect of lattice vibrations on the exciton absorption, and used in a Monte-Carlo calculation of the density of exciton states and the absorption spectra in the regime of weak scattering. For the direct edge, we obtain the asymmetric lineshape with a strongly decaying low-energy side while the high-energy side can be approximated by a life-time broadened Lorentzian. The linewidth is found to depend on temperature as T 2/3 , T 1 and T 3/2 for one-, two-, and three-dimensional lattices in agreement with renormalized perturbation theory in one and two dimensions. The discrepancy of the perturbational result ( T 2 ) in three dimensions is ascribed to the density of states which is most sensitive to disorder in this case. The shift of the absorption peak is found to be linear in T for low T , the origin of its sublinear behaviour for higher T is clarified.

Exciton Bose condensation : the ground state of an electron-hole gas - I. Mean field description of a simplified model

Abstract: We consider an electron-hole gas in a simple model semiconductor, with direct gap and isotropic, non degenerate bands. We study the Bose condensed ground state of that system as a function of density, using a mean field variational ansatz. In a first stage, we ignore screening as well as the spin structure of the carriers. We thus describe the smooth transition between Bose condensation of atomic excitons at low densities, and the « excitonic insulator » state and ultimately electron-hole plasma at high densities. As compared to previous treatments, our approach includes the effect of electron-hole pairing on the ground state, within a simple realistic ansatz.

Exciton–surface plasmon coupling: An experimental investigation

Abstract: Langmuir–Blodgett monolayer assemblies, which contained dye molecules, have been deposited on silver films. Exciton–surface plasmon interactions have been studied with attenuated total reflection (ATR) spectroscopy. Reflectivity and dispersion curves for plasmon surface polaritons (PSP’s) at the metal interface are reported for both angular and wavelength scans. In agreement with theory dispersion curves from angle scans exhibit a double ‘‘back bending’’ at the transverse exciton frequency ωT (due to PSP interaction with the in plane component of the dye transition dipole moment) and at the longitudinal frequency ωL (due to PSP interaction with a perpendicular component). Correspondingly, dispersion curves from wavelength scans break into separate branches at these frequencies.

Biexcitons in GaAs quantum wells

Abstract: Careful examination of the exciton edge of the photoluminescence from a number of high-quality multiple-GaAs-quantum-well samples grown by molecular-beam epitaxy reveals at low temperatures a double peak whose splitting of approximately 1 meV decreases somewhat with increasing GaAs well width $L$. The higher-energy peak is due to the $n=1$ free-heavy-hole-exciton transition while the excitation intensity, temperature, and polarization dependences of the lower-energy peak suggest that it is due to biexcitons with a binding energy $B$ of about 1 meV. In support of the biexciton interpretation a theoretical calculation of $B(L)$ is presented. This calculation gives two-dimensional biexciton binding energies more than an order of magnitude larger than the three-dimensional calculated values.

Optical properties of copper in silicon: Excitons bound to isoelectronic copper pairs

Abstract: Copper doping of silicon crystals results in an intense emission at 1.014 eV. The photoluminescence spectrum exhibits a characteristic structure consisting of a narrow nophonon line and equispaced lower-energy resonant-mode phonon replicas. The typical phonon energy is 7.0 meV. We observe isotope shifts of the lines which conclusively show that copper is incorporated in the luminescent defect. Combination with the observed quadratic dependence of the emission intensity on copper concentration leads us to suggest copper pairs as recombination centers. The symmetry of the pair as determined from uniaxial stress and Zeeman data is that of a $〈111〉$ configuration. The no-phonon line structure and the splitting in external fields indicate an exciton localized at an isoelectronic trap. The exciton is discussed in terms of an isoelectronic donor combining the present data with recent deep-level transient-spectroscopy results.

Exciton migration and trapping in photosynthesis

Abstract: — Mobile electronic excited states, excitons, undergo random walks through the antenna chlorophyll arrays of photosynthetic organisms. The time interval from exciton creation, by photon absorption, until its first arrival at a reaction center (RC) is called the “first passage time” (FPT) of the random walk. A theory of exciton migration and trapping presented here predicts that the exciton lifetime, as measured from chlorophyll fluorescence decay in chromatophores or P700 complexes, is a linear function of the fractional number of quanta absorbed directly by the antenna, not by the RC. The slope of this line is the FPT, and its intercept is the exciton's lifetime as limited only by photoconversion at the RC. This photoconversion-limited lifetime is simply related to the in situ photoconversion rate constant via two parameters, each of which is experimentally accessible. It is also possible to obtain values of individual FoUrster rate constants, at least approximately, from measurements of exciton lifetime as functions of temperature and excitation wavelength. This new theory, based on lattice random walk models, receives some support from fluorescence measurements done on Rhodopseudomonas sphaeroides R26 chromatophores. In its present form the theory is only applicable to one-antenna-component systems, like Rp. sphaeroides R26 or Rhodospirillum rubrum chromatophores or P700 complexes, but should be readily extendible to multi-antenna-component systems including whole chloroplasts.

Transient Optical Spectra of a Dense Exciton Gas in a Direct-Gap Semiconductor

Abstract: The observation is reported of a remarkable persistence of the $1s$ exciton resonance in transient picosecond absorption spectra of GaAs under conditions of resonant optical excitation up to pair densities two orders of magnitude above the Mott density. The stability (excess linewidth 0.8 meV at $n=6\ifmmode\times\else\texttimes\fi{}{10}^{16}$ ${\mathrm{cm}}^{\ensuremath{-}3}$) of this dense exciton gas against decay into the energetically more favorable continuum of electron-hole pairs is due to the relatively weak self-screening of discrete excitons.

Exciton Bose condensation : the ground state of an electron-hole gas - II. Spin states, screening and band structure effects

Abstract: We first generalize the approach of the previous paper by including spin degrees of freedom. We classify the various spin states and we discuss the effect of interband exchange interactions. We then introduce screening, in the framework of a generalized RPA which incorporates Bose condensation of bound electron-hole pairs. We discuss in detail the low density limit : screening corrections do not change the sign of the compressibility, which remains positive, in contrast to previous estimates. We show that such RPA corrections reduce to an approximate form of the Van der Waals attraction between excitons. Viewing this RPA approach as an interpolation procedure at intermediate densities, we propose several interpolation schemes that should account for the Mott transition, and we give some preliminary very rough numerical estimates. Finally, we discuss the effect of band degeneracy on the ground state : different degeneracies in the two bands should lead to a normal plasma at high density while at low densities bound excitons « Bose condense », with a breakdown of their internal symmetry; we expect a first order transition with a liquid-gas phase separation.

Numerical Experiments on the Absorption Lineshape of the Exciton under Lattice Vibrations. II. The Average Oscillator Strength per State.

Abstract: We introduce the average oscillator strength per state (AOSPS) as the ratio of the exciton absorption lineshape and the density of exciton states at a given energy. The numerical investigation of the AOSPS spectra and their dependence on temperature and dimensionality shows contrasting behaviour on the two sides of the absorption peak. Unambiguously we can ascribe the high-energy side to indirect transitions and the low-energy tails to momentarily localized states with giant oscillator strength. The calculated spectra of the participation ratio lend further support to this viewpoint.

Wannier-Mott excitons in semi-infinite crystals: Wave functions and normal-incidence reflectivity

Abstract: A method is developed to obtain Wannier-Mott --- exciton wave functions in semiinfinite crystals in the framework of the effective-mass approximation. An analytical approximation is shown to agree well with numerical wave functions and is used to compute exciton nonlocal polarizability (in closed form) and $s$-wave reflectivity. The results are compared with normal-incidence reflectivity experiments in CdS, ZnSe, GaAs, and InP, and the experimental line shapes are well reproduced. The existence of an intrinsic dead layer is confirmed and a new additional boundary condition is derived. It is shown that the spike, frequently observed at the longitudinal-exciton frequency, is due to extrinsic dead layers.

Influence of magnetic fields up to 20 T on excitons and polaritons in CdS and ZnO

Abstract: From an invariant expansion, we construct the exciton Hamiltonian for $A$ and $B$ excitons in CdS and ZnO represented by an 8\ifmmode\times\else\texttimes\fi{}8 matrix including the influences of the finite wave vector and an external magnetic field. We diagonalize the Hamiltonian matrix to obtain the exciton states. Then the excitons are coupled to the electromagnetic radiation field thus giving the polariton states. The theoretical dispersion curves are fitted to the results of two-photon Raman scattering, transmission, and reflection experiments in magnetic fields up to 20 T. From this fit we deduce precise values for eigenenergies, $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}$-linear terms, exciton masses, $g$ factors, and diamagnetic shifts.

Thermodynamics of excitons in semiconductors

Abstract: During the few microseconds between its laser‐induced creation in a pure semiconductor crystal and its destruction by recombination, a bound electron–hole pair, or exciton, is very active. Like the negative and positive charge carriers from which it forms, the exciton exhibits great mobility, whether it is coaxed by an applied force or simply moves under its own thermal energy. And like an atom in free space, it is bound by Coulomb forces, has discrete energy levels and may combine with other excitons into molecules or even condense into a liquid‐like state. In a semiconductor crystal such as silicon, all of this occurs at temperatures below about 30 K, for these weakly bound neutral particles ionize easily into electrons and holes at higher temperatures. The study of these particles and their products has occupied many physicists over the last couple of decades, and the investigations continue to uncover interesting new phenomena.

X‐ray induced processes in SrTiO3

Abstract: The x‐ray induced visible luminescence of SrTiO3 has been studied in the range 15–300 °K An apparently single‐emission band at 500 nm has been observed whose efficiency abruptly decreases at ∼30 °K This luminescence band is proposed to be intrinsic and associated with the decay of a relaxed Ti+3O−53 exciton After 15 °K irradiation, a visible absorption spectrum which increases towards the infrared is produced and anneals out at ∼30 °K It is suggested that the absorption is due to electron transition from a Ti+3 center to the conduction band, in accordance with recent ultraviolet photoemission spectroscopy data The thermoluminescence spectrum after 15 °K irradiation has also been determined The first glow peak at 30 °K appears to correlate well with the thermal decay of the irradiation‐induced absorption

Elastic and inelastic mean-free-path determination in solid xenon from electron transmission experiments

Abstract: Electron transmission experiments in the (0-20)-eV energy range have been performed on Xe films (0-3000 \AA{}) deposited at various temperatures on different metal substrates [Nb,Pt,W(100)]. A theoretical model is used to extract from these experiments the elastic and inelastic mean free paths in the (0-8.5)-eV energy region. Some structures in the transmitted current arise from inelastic processes and yield information on electronic excitations (excitons and electron-hole-pair creation) and the energy of the bottom of the conduction band. Other structures present in the elastic mean free path appear to be caused by structural effects and thermal disorder of the deposited film.

Electric field induced fluorescence quenching and photocarrier generation in x‐metal‐free phthalocyanine

Abstract: The electric field induced fluorescence quenching and transient photoconductive response of x‐H2Pc particles dispersed in an inert polymer matrix have been measured for different applied electric fields and temperatures between 22 and 70 °C. The results of the measurements show that fluorescence quenching and carrier generation are the different aspects of the same process, namely, the depopulation of the first excited singlet state by exciton dissociaton into electron hole pairs. The linear correlation between fluorescence quenching and relative photoresponse enables the determination of absolute quantum efficiency by performing only relative measurements. The temperature dependent studies showed consistency between carrier generation and fluorescence quenching, and established an activation energy for the generation of charge carriers.

Quantum-mechanical-model calculations of radiative properties of a molecular crystal. I. Polaritons and abnormal decays of excitons in one- and two-dimensional systems

Abstract: An analysis of the interaction of an electronic collective excitation, of dimensions 1 and 2, with the radiation field is presented. The use of translation symmetry in infinite lattices allows us to reduce the interaction problem to a coupling of a discrete matter state to an effective continuum of photons presenting a low-energy edge. The resulting states (radiatively unstable excitons, polaritons) are investigated as a function of the electronic excitation energy relative to the edge of the continuum, with emphasis on the spectral properties and the dynamical regimes in the intermediate region, when the energy of the electronic excitation sweeps the edge of the continuum. Contact with the three-dimensional polariton is made while application to radiative dynamics in layered crystals is suggested.

The effect of arsenic vapour species on electrical and optical properties of GaAs grown by molecular beam epitaxy

Abstract: Hall effect, DLTS and low-temperature photoluminescence measurements were used to study the effect of dimeric (As2) vs tetrameric (As4) vapour species on the electrical and optical properties of nominally undoped and of Ge-doped GaAs layers grown by molecular beam epitaxy (MBE). The arsenic molecular beam was generated from separate As2 and As4 sources, respectively, and from a single source providing an adjustable As2/As4 flux ratio. The occurence of the previously described defect related bound exciton lines in the luminescence spectra at 1.504–1.511 eV was found to be directly correlated with the presence of three deep states (M1, M3, M4) which are characteristic of MBE grown GaAs. The intensity of the extra luminescence lines and simultaneously the concentration of the deep electron traps can be reduced substantially simply by decreasing the As4/As2 flux ratio. The incorporation of defect related centers as well as of amphoteric dopants like Ge strongly depends on the surface chemistry involved. Therefore, a considerably lower autocompensation ratio in Ge-dopedn-GaAs is obtained with As2 molecular beam species which provide a higher steady-state arsenic surface population.

Experiment investigation on the complex dielectric function of CdS in the exciton and in the plasma limit

Abstract: The real and imaginary parts of the complex dielectric function of CdS are experimentally investigated for low temperatures and for various excitation conditions Strong changes of the dielectric function are observed at high excitation in the spectral region below the band edge Emphasis is put on the variations of the real part at the transition from a low density exciton phase to a high density plasma phase It is shown, that the Kramers-Kronig relations are also valid for highly excited semiconductors The experimental results are compared with recent theoretical predictions

Self-trapping of excitons and lattice defect production in solid rare gases

Abstract: A comparison of the calculated fluorescence spectra of solid and liquid rare gases with the experimental data makes it probable that in solid rare gases the exciton self-trapping is accompanied by defect production, here the defect production being the fundamental mechanism of the relaxation of exciton to its ground state. The comparison of the fluorescence spectra of solid and liquid phases makes it possible to forecast the production of radiation damages. A conjectural relationship between exciton cavities and nucleation processes proceeding in bubble chambers are briefly discussed.

Free Exciton Luminescence and its Sample Dependence in Alkali Iodides

Abstract: The spectrum and the quantum yield of the 1s free exciton luminescence in alkali iodides have been studied as a function of temperature. These results are strongly dependent on the samples used. On the basis of these results the intrinsic nature of the luminescence process and the self-trapping process of the 1s free excitons has been studied.

Resonant degenerate four‐wave mixing in GaAs multiquantum well structures

Abstract: We have observed degenerate four‐wave mixing, on a picosecond timescale, at the lowest two‐dimensional exciton resonance line of GaAs‐Al0.25Ga0.75As layer structures, grown by molecular beam epitaxy. Mixing is observed at exciton densities as low as 109 cm−2 per layer and is strongly resonant at the heavy exciton line, where the diffraction efficiency reaches a maximum of 12% for a 2×10−7 J/cm2 pump pulse.