Exciton Parameters Research Articles

Quantum optics in dispersive and absorptive media

Using microscopic models in which both photons and excitons are treated as microscopic degrees of freedom, we discuss polaritons of two cases: one is the case where excitonic parameters are time dependent. The time dependence causes the creation of polaritons from a ‘false vacuum.’ It is shown that both the creation spectra and the creation efficiency are very different from the results of previous studies. The other case is that of polaritons in absorptive and inhomogeneous cavities. A polariton in such a system cannot be viewed as a back-and-forth oscillation between a photon state and an exciton state.

Temperature dependence of energies and broadening parameters of the band-edge excitons ofReS2andReSe2

We have measured the temperature dependence of the spectral features in the vicinity of the direct gaps ${\mathrm{E}}_{\mathrm{g}}^{\mathrm{d}}$ of ${\mathrm{ReS}}_{2}$ and ${\mathrm{ReSe}}_{2}$ in the temperature range between 25 and 450 K using piezoreflectance (PzR). From a detailed line-shape fit to the PzR spectra we have been able to determine accurately the temperature dependence of the energies and broadening parameters of the band-edge excitons. The parameters that describe the temperature variation of the transition energies and broadening function have been evaluated.

Dielectric enhancement of excitons in semiconducting quantum wires

The energy of the exciton ground state in a semiconducting cylindrical quantum wire surrounded by a dielectric has been calculated using a variational technique accounting for the effect of dielectric enhancement. The effect of dielectric enhancement in such a system has been clearly demonstrated. Exciton parameters have been calculated for an intercalated leadiodide-based quasi-one-dimensional semiconductor and GaAs wires in asbestos nanotubes.

Magneto-optical studies of excitons in Zn1-xCdxSe/ZnSe quantum wells.

Magnetoabsorption on high-quality ${\mathrm{Zn}}_{\mathit{x}}$${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$Se/ZnSe multiple quantum wells grown by molecular-beam epitaxy is used to study systematically the major excitonic properties. The small inhomogeneous width of the exciton lines allows us to observe Landau levels arising from higher heavy-hole exciton transitions as well as to analyze in detail the spin splitting and diamagnetic shift of the ground state. The Schr\"odinger equation of magnetoexcitons is solved numerically to describe their magnetic-field behavior and to deduce the relevant excitonic and band parameters in dependence on the Cd content in the well. Anticrossing features at higher Landau levels and the magnetic-field dependence of the light-hole exciton ground-state energy signify a complex valence-band structure due to heavy-light hole coupling. \textcopyright{} 1996 The American Physical Society.

Theory of polariton photoluminescence in arbitrary semiconductor microcavity structures.

A quantum theory of quantum well polaritons in semiconductor microcavities is developed. The model takes into account the coupling between the exciton level and the structured continuum of electromagnetic modes relative to the particular geometry of the microcavity. A general equation for the polariton dispersion is obtained as a function of the cavity and exciton parameters. The equation is valid in both weak and strong coupling regimes and reproduces the existing measurements of microcavity polariton dispersion. A model for the polariton luminescence is then derived from the theory. It is possible to define a polariton decay rate only when the resonances in the polariton density of states can be considered as quasimodes. The two limiting cases of very weak and very strong coupling regimes are consequently identified. In these cases the polariton radiative probabilities are derived for light emitted on the left and right sides of the microcavity separately. The influence of the microcavity structure on the polariton dispersion and radiative rates is discussed and in particular the role of the microcavity leaky modes is described in detail. A discussion of the luminescence mechanism in the intermediate coupling case is also presented. \textcopyright{} 1996 The American Physical Society.

Optical properties of ZnSe and its modeling.

Spectral data for a ZnSe film grown on GaAs(100) are obtained at room temperature by spectroscopic ellipsometry (SE) in the photon energy range between 1.5 eV and 6 eV in steps of 5 meV. The optical dielectric function for bulk ZnSe is successfully extracted from the spectral data utilizing a multilayer analysis. The resultant data are better than previous data in that (1) the real part of the dielectric function below the band gap decreases smoothly following closely Marple's data obtained by the beam deviation method, which is more reliable than SE in this energy range, (2) the data reveal four distinctive critical point structures at ${\mathit{E}}_{0}$, ${\mathit{E}}_{0}$+${\mathrm{\ensuremath{\Delta}}}_{0}$, ${\mathit{E}}_{1}$, and ${\mathit{E}}_{1}$+${\mathrm{\ensuremath{\Delta}}}_{1}$, as is expected from the band structure, and (3) the maximum value of the imaginary part of the dielectric function in the ${\mathit{E}}_{1}$ region is highest among the reported data. The resultant data are expressed as a function of critical-point parameters and photon energies using our model, which is more generally valid than other models. This enables us to calculate the optical dielectric function not only over the entire photon energy range of the given spectral data, but also below and somewhat above the given spectral range. The excitonic effects apparently present in the spectral data are incorporated in our model. This enables us to determine excitonic parameters. The excitonic binding energy at ${\mathit{E}}_{0}$ is found to be 13 meV at room temperature. \textcopyright{} 1996 The American Physical Society.

Optical properties of Zn1–xCdxSe/ZnSe quantum well excitons in an electric field: Experiment and model calculations

A set of high quality Zn 1-x Cd x Se/ZnSe quantum well structures with various well widths and Cd contents as well as Zn 1-x Cd x Se epilayers are grown by molecular beam epitaxy. Their optical properties are investigated by measuring photoluminescence, reflection, and transmission as a function of a vertically applied electric field. The quantum-confined Stark effect is observed at 5 K and a change in transmission of more than 30% achieved at 300 K. By means of numerical model calculations, the underlying electro-absorption and electro-refraction spectra are deduced and the field-induced change of the relevant exciton parameters (resonance energy, oscillator strength, broadening) derived.

Comparative analysis of the self-localization properties of excitons and F2 centers in alkali halides

A comparative analysis is presented of the energetic and temporal parameters of self-localized excitons and F2 centers in the triplet state in a number of crystals. We conclude that in crystals with different values of the Rabin-Klick parameters and self-localized excitors exhibit D2h symmetry in the triplet state.

Core excitons in silicon and silicon oxides

Using a newly developed reflectometer we have measured the reflectivity at the silicon K edge for different silicon compounds. Combining these measurements with silicon L edge reflectivity measurements carried out on a different reflectometer we determined the binding energy of core excitons at the L edge. The results show that it is possible to carry out reflectivity measurements with the resolution necessary for the determination of the exciton parameters. Comparison with literature values shows that a rigid Kramers–Kronig analysis is not needed when the angle of incidence is well below the critical angle of total external reflection.

Strain Effects in GaN on Sapphire: Towards a Quantitative Comprehension

ABSTRACTWe present here a detailed investigation of the optical properties of GaN epilayers grown by low pressure MOVPE on sapphire substrates, using 2K photoluminescence as well as 2K reflectance spectroscopy. A large series of samples has been grown under eclectic conditions (V/III ratio, growth temperature, nitridation steps,...) which allows us to propose the first semi-quantitative investigation of the sample-dependent band gap energies. This dependence of the bandgap is analyzed in terms of i) residual stress, ii) exciton parameters and iii) deformation potential characteristics of the A, B, C excitons. the residual stress cannot be simply explained in terms of the differences between the thermal expansion coefficients of the various compounds, but are strongly correlated to the V/III growth ratio imposed during the growth, and subsequently to the influence of this parameter at the scale of electronic and structural characteristics of the deposited layers.

Multiphoton 2D exciton absorption in a superlattice

Multiphoton exciton transitions induced by an intense optical wave in a semiconductor superlattice are analysed. It is assumed that the excitons can be treated as two-dimensional (2D) excitons localized in the heteroplanes and that the oscillating electric field of the optical wave is directed perpendicular to the heteroplanes. The superlattice potential barriers are modelled by a periodic chain of delta -type functions. Quasienergetic time-dependent states are used. The explicit dependence of the coefficient of the multiphoton absorption on the frequency and magnitude of the optical wave and on the superlattice and exciton parameters is obtained. The exciton absorption spectrum is shown to consist of a series of continuous absorption bands associated with the discrete levels of the exciton. The width of each band is equal to the sum of the widths of the electron and hole minibands. The distance between the neighbouring bands is equal to the separation of the exciton levels. The form of the exciton absorption essentially depends upon the parity of the number of absorbed photons and on the relationship between the separation of the exciton levels and the total width of the electron and hole minibands.

Exciton resonance reflectivity study of quantum well wires

We report on the first observation of the exciton resonant reflection from a directly grown array of isolated quantum-well-wires. Structures contained 30 GaAs/AlAs double layers were grown on (311) GaAs substrate by conventional elemental source MBE. GaAs wire-like clasters with an orientation along [233] and size of 32/10.2 A were introduced inside one of the central AlAs layers. The expected distance between nearest clusters is about 160 A. In spite of much smaller effective volume occupied by excitons in these clusters, as compared to that in a single quantum well, the quantum-well-wire structures demonstrate much lager resonance amplitude of the reflection spectra. We extracted excitonic parameters from these spectra and found that the exciton oscillator strength in the quantum-well-wires is more than 10 time higer than that in the enviroment superlattice. A 30% anisotropy of the exciton oscillator strength in the quantum-well-wire have been observed. The theory of non-local dielectric responce is extended to calculate reflection spectra from the structures with a grating of quantum-well-wires. A functional relationship between the reflection coefficient and the envelope wave function of an exciton in the structure has been established

Many-particle effects and nonlinear optical properties of GaAs/(Al,Ga)As multiple-quantum-well structures under quasistationary excitation conditions.

Quasi-two-dimensional (2D) carrier systems of GaAs/(Al,Ga)As multiple-quantum-well structures are studied under quasistationary excitation conditions using the pump and probe beam and the luminescence spectroscopy. In the low- to medium-density regime the saturation of the ${\mathit{n}}_{\mathit{z}}$=1 exciton resonances dominates the nonlinear optical properties. The low-temperature saturation density is found to be ${\mathit{N}}_{\mathit{s}}$\ensuremath{\approxeq}4\ifmmode\times\else\texttimes\fi{}${10}^{16}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$, independent of the well width ${\mathit{L}}_{\mathit{z}}$. The carrier-induced energetic shift of the 1hh-exciton resonance as a function of ${\mathit{L}}_{\mathit{z}}$ shows the dimensional dependence of the screening properties of the carriers. The 2D limit is reached at well widths smaller than 50 \AA{}, whereas the 3D behavior occurs already at ${\mathit{L}}_{\mathit{z}}$=190 \AA{}. In the high-excitation regime, the renormalization of the fundamental band gap is investigated as a function of the electron-hole plasma density. The density and the reduced band gap are determined via systematic evaluations of both gain and luminescence spectra. The observed behavior can be described by a strict 2D theory using effective exciton parameters in order to account for the finite well widths of the structures. The same theory describes very well an n-type modulation-doped quantum well if an independent shift of each subband--according only to its specific carrier density---is assumed. The correlation enhancement of the band-to-band transitions was observed in an n-type modulation-doped sample where all excitonic features were quenched by the doping density. The study of the higher subbands reveals that both exciton bleaching and subband renormalization are due mainly to a direct occupation of the specific subband; we find that the intersubband effects via Coulomb screening are negligible.

Effects of hydrostatic pressure on free and bound exciton states in silver bromide

In AgBr at low temperature, resonant Raman scattering is employed to investigate the Gamma 6+(X)L4.5- indirect free exciton state under hydrostatic pressure up to 0.7 GPa. From the pressure dependence of resonance behaviour and Raman energy shifts, the deformation potential constant of the exciton edge and mode Gruneisen parameters of various phonons involved in exciton relaxation are obtained simultaneously. Also, the effect of pressure on several bound exciton states is studied by luminescence. Donor-bound excitons of type (D0, X) with localisation energies E1=6 to 8 meV show a negative pressure shift (dE/dP=-15.1 meV GPa-1) completely parallel to the free exciton gap proving their effective-mass nature with wavefunctions identical to those of the free exciton. Similarly, the iodine-bound exciton state (E1=43 meV) is found to derive mainly from the L-point valence band states with only minor contributions from Gamma . Different contributions to the pressure-induced change in lattice relaxation occurring in this case are determined and discussed.

Optical properties of excitons in a slab. The coherent wave approach

In the framework of Stahl's coherent wave approach optical properties of semiconducting slabs in the excitonic region of the spectrum are discussed. Two integral equations are derived from which excitonic reflection and transmission spectra can be calculated. For slabs whose thickness is much greater than the corresponding excitonic Bohr radius the equations can be solved by making use of the so-called adiabatic approximation. Calculations are performed for CdS, GaAs and RbI slabs reproducing well experimental spectra. From the best fit for a RbI slab, excitonic parameters of the 1s exciton are determined.

On Concentration Dependences of the Band Parameters in AIIBVI Solid Solutions with Structural Phase Transitions

AbstractThe connection between the structure of solid solution and its spectroscopic and band parameters is discussed on the example of ZnxCd1−xSe and Zn1−xMgxS systems with the structure phase transition: zinc blende (ZB)‐wurtzite (W). It is shown that if the Eg(x) dependence is parabolic for the ZB and W pure structure ranges, it is linear in the phase transition range (for the Zn1−xMgxS system). Eg(x) is always higher for the anisotropic crystals (with stacking faults (SF) and W) than for the ZB ones depending on the more ionic character of their chemical bond. The influence of the disorder potential due to composition x and SF concentration fluctuations on the exciton and band parameters of the solid solutions is considered.

Gain measurements and band-gap renormalization in GaAs/AlxGa1-xAs multiple-quantum-well structures.

Using a pump and probe technique with nanosecond excitation pulses we measure the nonlinear optical properties of GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As multiple-quantum-well (MQW) structures under quasistationary excitation conditions. The main aspects of our work are the phenomena of optical gain, the renormalization of the fundamental band gap and of the higher subbands ${n}_{z}=2,{n}_{z}=3$), due to the electron-hole population of the lowest (${n}_{z}=1$) subbands. We present the first systematic analysis of the gain spectra to determine the renormalization of the ${n}_{z}=1$ subband and we interpret the data in terms of effective exciton parameters in combination with a pure two-dimensional theory.

Deformation potential LO-phonon Raman scattering near the EO-gap in AlxGa1−xAs-alloy. Excitonic effects

We report absolute values of dipole-allowed resonant Raman scattering by LO-phonons near the EO-gap of AlxGa1−xAs-alloys at 100 K. High purity AlxGa1−xAs n-type samples, grown by liquid-phase epitaxy and covering the range 0.4 < x < 0.7, were investigated. In the whole range of x the Raman scattering efficiency reveals resonant behavior for both GaAs- and AlAs-modes, the heights of the resonance profiles decrease with increasing x while their widths increase. The theory of deformation potential induced resonant Raman scattering by one-LO-phonon, considering as intermediate electronic states both discrete and continuous exciton states, was applied to interpret the data. Good quantitative agreement between theory and experiment, including the absolute values of the cross section, was obtained, thus illustrating the role of excitonic effects in the resonance. The dependence of the broadening parameters of heavy-hole excitons ( Γ(32) ) and light-hole ( Γ(12) ) excitons on x obtained from the fit of the theory to the experimental results, is presented. In order to achieve good agreement with the shape of the resonance profiles, a theory which considers both, excitonic and disorder effects, may be necessary.

Study of excitons in an arbitrarily shaped GaAs/Al0.3Ga0.7As single quantum well in the presence of static transverse electric field

A method is developed to calculate exciton parameters in an arbitrarily shaped quantum well by using numerical methods in conjunction with the variational method. We solve the Schrödinger equation of the relevant system both by the Monte Carlo method and by the ordinary differential equation solving method, and find that the two approaches yield similar results. Our technique is readily applied to arbitrarily shaped GaAs/AlGaAs quantum wells, and allows us to study the variation of exciton parameters with respect to transverse electric field. We report the results for exciton binding energies, electron and hole tunneling rates, and exciton volumes, which are essential to predicting electro-optical properties of quantum well. Results are presented for the square, graded, and asymmetric quantum wells, and the effects of the shape of the well on exciton properties are discussed. We find that there is only small dependence of the exciton binding and emission energy on the well shape, but a strong dependence of the electron and hole tunneling rates and hence of the dynamical aspects of excitonic properties on the well shape.

Analysis of the quasimetallic reflection band by dynamical coherent potential approximation (DCPA)

A theoretical algorithm is presented for numerical calculation of quasimetallic reflection bands of molecular excitons by a unified model which takes account of exciton–intramolecular phonon coupling by the dynamical coherent potential approximation and the polariton effect (the directional dispersion effect) by an anisotropic dielectric tensor. This method is applied to analyze experimental quasimetallic reflection spectra of DCI (1-1′-diethyl 2-2′-cyanine iodide), TCNQ (tetracyanoquinodimethane), and CTIP [γ-cyclopropyl-bis(1,2,3′-trimethyl-indolenine-2-yl) pentamethinium tetrafluoroborate] crystals. From a comparison of experimental and calculated reflectivity spectra, important exciton parameters, such as the bandwidths and shapes of low dimensional density-of-states functions of excitons, have been estimated.