Optical Properties Of Excitons Research Articles

Optical Properties of Excitons in Quantum Well Wires Under the Magnetic Field

The binding energy of excitons in quantum well wires of GaAs surrounded by Ga 1-x Al x As was calculated in an effective mass approximation with the use of the variational approach. Results obtained show that the exciton binding energies depend on the sizes of the wire and the strength of the magnetic field. The exciton theory was then used to calculate the linear absorption coefficient of HH1–C1 excitons by varying the wire dimensions and magnetic field values.

Fundamental optical properties of photonic crystal slabs in the strong coupling regime

We report on the transmission and photoluminescence (PL) properties of semiconductor-imbedded photonic crystal slabs where the photon mode is strongly mixed with the exciton mode. The samples are prepared by incorporating PbI-based inorganic–organic layered perovskites with the large oscillator strength of excitons into periodically arranged square holes on a quartz substrate. In transmission spectra, well-pronounced dips due to the excitation of quasi-guided modes are observed, when a polystyrene layer is overcoated with an appropriate thickness on the substrate. The quasi-guided and exciton modes exhibit an anticrossing with large Rabi splitting of over 100 meV, which demonstrates the formation of polaritons via the strong coupling between these two modes. In PL spectra, we found that the polariton PL is significantly enhanced where the polariton has the largest population. Thus, the optical properties of excitons can be controlled by the electromagnetic periodic boundary condition via strong coupling with a quasi-guided mode in a photonic crystal slab.

Electric-field-induced anisotropy of excitonic optical properties in semiconductor quantum dots

We report the anisotropic excitonic optical properties (polarization dependent photoemission and photoabsorption) induced by applying a lateral electric field in a single semiconductor quantum dot. The excitonic optical polarization characteristics are examined using theoretical calculation and optical measurement. The optical properties are numerically analyzed taking into account the quantum dot potential, electric field, and electron–hole Coulomb interaction. We evaluate the polarization properties from calculated exciton wave functions. The polarization properties depend strongly on the size of the quantum dots and the spatial symmetry of the hole part of the wave function. There is large electric-field-induced anisotropy in thin quantum dots to a larger lateral extent even where the polarization properties are completely isotropic without an electric field. We compare the theoretical results with experimental results for InGaAs quantum dots obtained using the microphotoluminescence technique. Qualitative agreement between the theoretical results and experimental ones is obtained.

Effects of piezoelectricity and spontaneous polarization on localized excitons in self-formed InGaN quantum dots

Exciton states confined in wurtzite InxGa1−xN/GaN strained quantum dots (QDs) are investigated within the framework of effective-mass approximation and variational approach, including three-dimensional confinement of the electrons and holes in QDs and a strong built-in electric field effect due to the piezoelectricity and spontaneous polarization. The relationship between exciton states and structural parameters of QDs is studied in detail. Our results show that the In-rich QDs-like are formed spontaneously due to In compositional fluctuations in the InxGa1−xN layer. The strong built-in electric field in InxGa1−xN/GaN strained QDs gives rise to a marked reduction of the effective band gap of QDs and leads to a remarkable electron–hole spatial separation. This effect has a strong influence on exciton states and optical properties of QDs especially for the QDs with large height (⩾5 nm) along the grown direction of the heterostructures. A good agreement has been obtained between the calculated and measured emission wavelengths for different InxGa1−xN/GaN strained QDs.

Ab initio tight-binding study of exciton optical and electro-optic properties of conjugated polymers

A simplified ab initio description of excitons in conjugated polymers is presented. Electron-hole pair states are derived from the Bethe–Salpeter equation using an expansion in density-functional based tight-binding states. The pair states, in turn, provide the optical and electro-optic response of the polymer. As examples, optical and electro-optic spectra of poly(para-phenylene vinylene) and poly(para-phenylene) are presented.

Magneto-optical properties of charged excitons in quantum dots

We present both experimental and theoretical results on the influence of a magnetic field on excitons in semiconductor quantum dots. We find a pronounced difference between the strong and weak confinement regimes. For weak confinement, the excitonic diamagnetic shift is strongly dependent on surplus charge, corresponding to a reversal in sign of the conventional diamagnetic shift for neutral excitons. In this limit, we argue that the optical properties of excitons with two or more extra electrons are fundamentally different to those of the neutral exciton and trion.

Exciton states and interband optical transitions in InGaN quantum dots

Within the framework of the effective-mass approximation, exciton states confined in wurtzite In x Ga 1− x N strained quantum dots (QDs) are investigated by means of a variational approach, including three-dimensional confinement of the electrons and holes in QDs and a strong built-in electric field effect due to the piezoelectricity and spontaneous polarization. Our results show that the strong built-in electric field gives rise to an obvious reduction of the effective band gap of QDs and leads to a remarkable electron–hole spatial separation. This effect has a significant influence on exciton states and optical properties of the QDs. The relationship between exciton states and height of QDs is studied. A comparison of the calculated and measured emission wavelengths for different In x Ga 1− x N QDs is given and a good agreement is obtained.

Solid state effects on exciton states and optical properties of PPV.

We perform ab initio calculations of optical properties for a typical semiconductor conjugated polymer, poly-para-phenylenevinylene, in both isolated chain and crystalline packing. In order to obtain results for excitonic energies and real-space wave functions we explicitly include electron-hole interaction within the density-matrix formalism. We find that the details of crystalline arrangement crucially affect the optical properties, leading to a richer exciton structure and opening nonradiative decay channels. This has implications for the optical activity and optoelectronic applications of polymer films.

Impact of polarization on quasi-two-dimensional exciton and barrier-width dependence of the exciton associated transition in wurtzite III–V nitride quantum wells

Excitonic states in Al x Ga 1− x N/GaN quantum wells (QWs) are studied within the framework of effective-mass theory. Spontaneous and piezoelectric polarizations are included and their impact on the excitonic states and optical properties are studied. We witnessed a significant blue shift in transition energy when the barrier width decreases and we attributed this to the redistribution of the built-in electric field between well layers and barrier layers. For the exciton the binding energies, we found in narrow QWs that there exists a critical value for barrier width, which demarcates the borderline for quantum confinement effect and the quantum confined Stark effect. Exciton and free carrier radiative lifetimes are estimated by simple argumentation. The calculated results suggest that there are efficient non-radiative mechanisms in narrow barrier QWs.

Electric and Magnetic Field Effects on Optical Properties of Excitons in Parabolic Quantum Wells and Quantum Dots

We show how to compute electro- and magneto-optical spectra resulting from the scattering of a plane electromagnetic wave on quantum well (QW) and cylindrical quantum dot (QD) nanostructures with parabolic confinement, in the region of the excitonic resonances. In the case of QWs we consider two configurations of the applied fields: one with both fields perpendicular to the growth planes, the other with both fields parallel to the QW planes. In the first case we observe how the diamagnetic shift and the Stark shift influence the shape and the positions of the excitonic peaks. For both fields parallel to the plane we observe Franz-Keldysh oscillations above the Landau states. In the case of cylindrical QDs we consider the effects of the magnetic field applied parallel to the symmetry axis and the electric field applied perpendicular to it. Again we observe displacements of the peaks and the variation of the peak hights depending on the relative strengths of the fields. Results are given for InxGa1—xAs/InP QWs, and cylindrical InxGa1—xAs/GaAs and GaAs/Ga1—xAlxAs QDs. Fair agreement with experiments is obtained.

Many-body effects on excitonic optical properties of photoexcited semiconductor quantum wire structures

We study carrier-interaction-induced many-body effects on the excitonic optical properties of highly photoexcited one-dimensional semiconductor quantum wire systems by solving the dynamically screened Bethe-Salpeter equation using realistic Coulomb interaction between carriers. Including dynamical screening effects in the electron-hole self-energy and in the electron-hole interaction vertex function, we find that the excitonic absorption is essentially peaked at a constant energy for a large range of photoexcitation density $(n=0--6\ifmmode\times\else\texttimes\fi{}{10}^{5} {\mathrm{cm}}^{\ensuremath{-}1}),$ above which the absorption peak disappears without appreciable gain; i.e., no exciton to free electron-hole plasma Mott transition is observed, in contrast to previous theoretical results but in agreement with recent experimental findings. This absence of gain (or the nonexistence of a Mott transition) arises from the strong inelastic scattering by one-dimensional plasmons or charge density excitations, closely related to the non-Fermi-liquid nature of one-dimensional systems. Our theoretical work demonstrates a transition or a crossover in one-dimensional photoexcited electron-hole systems from an effective Fermi liquid behavior associated with a dilute gas of noninteracting excitons in the low-density region $(n<{10}^{5} {\mathrm{cm}}^{\ensuremath{-}1})$ to a non-Fermi liquid in the high-density region $(n>{10}^{5} {\mathrm{cm}}^{\ensuremath{-}1}).$ The conventional quasistatic approximation for this problem is also carried out to compare with the full dynamical results. Numerical results for exciton binding energy and absorption spectra are given as functions of carrier density and temperature.

Correlation effects in the excitonic optical properties of semiconductors

A microscopic many-body theory for the optical and electronic properties of semiconductors is reviewed with an emphasis on the role of correlation effects. At the semiclassical level, the semiconductor Bloch equations include many-body effects via bandgap and field renormalization as well as correlation contributions representing two electron-hole pair amplitudes, excitonic populations, and coupled interband and intraband coherences. These Coulomb interaction induced carrier correlations lead to characteristic signatures in nonlinear semiconductor spectroscopy. At the fully quantum mechanical level the dominant light-matter correlations are described by coupled semiconductor Bloch and luminescence equations. Excitonic emission properties of quantum well and microcavity systems are discussed, including effects such as coherent signatures in the secondary emission and coherent control of the emitted light.

Dynamical spin properties of exciton and biexciton in CdMnTe/CdTe/CdMgTe single quantum well

Exciton and biexciton optical properties in II–VI semiconductor single quantum well have been studied by magneto-photoluminescence, and time-resolved four-wave-mixing spectroscopies. The polarization dependence of both measurements can clearly distinguish biexciton from other bound states and show the dynamical spin properties of exciton and biexciton.

Observation of multicharged excitons and biexcitons in a single InGaAs quantum dot

The effects of excess electron occupation on the optical properties of excitons (X) and biexcitons $(2X)$ in a single self-assembled InGaAs quantum dot are investigated. The behavior of X and $2X$ differ strongly as the number of excess electrons is varied with the biexciton being much more weakly perturbed as a result of its filled s-shell ground state, a direct manifestation of shell-filling effects. Good correlation is found between charging thresholds observed from s-shell recombination perturbed by p-shell occupation, and direct observation of p-shell recombination.

Optical properties of excitons in quantum dots: diffraction of an electromagnetic plane wave by a spherical quantum dot

We show how to compute the optical spectra resulting from the scattering of a plane electromagnetic wave on a quantum dot (QD) in the region of the excitonic resonances. The method uses the microscopic calculation of quantum dot eigenfunctions and the macroscopic Stahl's density matrix approach to compute the electromagnetic field within and outside the QD, taking into account the coherence between the carriers and the electromagnetic wave. Numerical results are obtained for the case of GaAs/Ga1−xAlxAs QD. Oscillator strengths and lineshapes are shown to have a typical dependence on the QD radius, and an increase in the scattering intensities near resonance is found.

Effect of degree of localization and confinement dimensionality of excitons on their recombination process inCdSe/ZnSe/ZnSxSe1−xsingle quantum well structures

We have studied the recombination dynamics of localized excitons in ${\mathrm{C}\mathrm{d}\mathrm{S}\mathrm{e}/\mathrm{Z}\mathrm{n}\mathrm{S}\mathrm{e}/\mathrm{Z}\mathrm{n}\mathrm{S}}_{x}{\mathrm{Se}}_{1\ensuremath{-}x}$ single quantum well structures (SQWS) with the CdSe well layer of 1, 2, and 3 monolayers (MLs). A time-resolved photoluminescence (TRPL) measurement and a nonlinear photoluminescence (NLPL) measurement were used. A time-integrated photoluminescence (TIPL) measurement at 20 K showed that the photoluminescence (PL) from the CdSe layers was observed in the range from the blue spectral region to the green one with increasing CdSe layer width, and that the PL linewidths were about 20--50 meV with an increase in the CdSe layer width. This is resultant from the quantum energy shift and the interfacial roughness between the CdSe and ZnSe layers, respectively. The TRPL measurement showed that excitons were localized at the tail states caused by the interfacial fluctuation. Decay times were measured and found to increase with increasing monitored photon energy, and their values increased with increasing CdSe layer width. It was found that the degree of localization of excitons was larger and the radiative lifetimes of excitons were longer with an increase in the CdSe layer width, indicating that the energy depth in the tail states was smaller and the oscillator strength of excitons was larger for the SQW with a thinner well layer. A NLPL measurement was performed for the 1 and 2 ML SQWS, which revealed that the many-body effect involving localized excitons occurred in the two samples, and that such an effect could result from the localized biexcitons for the 1 ML SQW and from the localized exciton-exciton inelastic scattering for the 2 ML SQW. The difference in the optical properties observed in the two samples is probably due to the difference in the inhomogeneity of the potentials felt by excitons. In order to study the effect of such inhomogeneity on the optical properties of excitons, we investigated the temperature dependence of the TRPL measurement for the three CdSe SQW samples, by which the radiative and the nonradiative lifetimes of localized excitons in the three SQW samples were obtained. These systematic measurements have enabled us to clarify the relationship of localized excitons to both the localization effect and the quantum confinement one. It has been deduced from these results that for a 1 ML SQW, the quantum size effect of excitons is enhanced and the biexcitonic emission is observed, and for 2 and 3 ML SQWS, the degree of localization of excitons becomes larger compared with that for the 1 ML SQW and it prevents excitons from encountering the nonradiative recombination centers.

Optical properties of excitons in semiconductor (InP)-insulator quantum wires

Features in the spectra of absorption, luminescence, and luminescence efficiency obtained under sample excitation with differently polarized laser radiation, and the nonlinear dependence of the luminescence intensity on the excitation level are explained as due to excitonic transitions in semiconductor (InP)-insulator (chrysotile asbestos) quantum wires. The measured excitonic-transition energies in the quantum wires are in quantitative agreement with calculations. The calculations took into account both the size quantization in a quasi-one-dimensional structure and the “dielectric enhancement” of excitons (the noticeable increase of the exciton binding energy and of the excitonic-transition oscillator strength associated with the increased attraction between the electron and the hole due to the large difference between the dielectric permittivities of the semiconductor and the dielectric matrix).

Magneto-Luminescence Spectroscopy of Excitonic Transitions in Homoepitaxial GaN Layers

Excitonic optical properties of homoepitaxial GaN layers have been studied by means of magneto-luminescence spectroscopy. Under magnetic field perpendicular to the hexagonal c-axis, Zeeman splittings and diamagnetic shifts were observed for the luminescence lines due to the radiative recombination of excitons bound to neutral donors and acceptors. The effective g-factors for both the donor- and acceptor-bound excitons were estimated to be 2.02 and 2.47, respectively. The diamagnetic shift of the donor-bound exciton was found to be smaller than that of the acceptor-bound exciton.

Optical properties of excitons in CdS semiconductor-insulator quantum wires

The characteristic features of the luminescence spectra of CdS semiconductor nanocrystals, crystallized in hollow channels in a dielectric template, are explained in terms of excitonic transitions in semiconductor-insulator quantum wires. The excitonic transition energies agree with the values calculated taking into account the effects of size quantization and the “dielectric enhancement of excitons” — the large increase in the electron-hole attraction as a result of the difference between the permittivities of the semiconductor and insulator. The theoretically computed binding energies of excitons in CdS quantum wires with a diameter of 10 nm reach 170 meV. It is shown that the excitonic transition energy is constant for a wide range of wire diameters.

Optical properties of excitons in semiconductor superlattices and microcavities

The optical response of Mott–Wannier excitons is investigated in semiconductor superlattices and microcavities. p-Polarized light is considered to calculate the reflectivity Rpand dispersion relation of the collective normal modes in superlattices accounting for extrinsic Morse potential wells, andRp in microcavities. Results of Rpexhibit well-defined peaks of the exciton bound states in the Morse potentials for both transverse and longitudinal modes. Comparisons ofRp with experimental reflectivity data of light for semiconductor microcavities exhibit good qualitative agreement as well as Rabi splitting.