Showing papers on "Exciton published in 1998"
TL;DR: In this article, an organic semiconductor microcavity that operates in the strong-coupling regime was shown to have characteristic mixing of the exciton and photon modes (anti-crossing), and a room-temperature vacuum Rabi splitting.
Abstract: The modification and control of exciton–photon interactions in semiconductors is of both fundamental1,2,3,4 and practical interest, being of direct relevance to the design of improved light-emitting diodes, photodetectors and lasers5,6,7. In a semiconductor microcavity, the confined electromagnetic field modifies the optical transitions of the material. Two distinct types of interaction are possible: weak and strong coupling1,2,3,4. In the former perturbative regime, the spectral and spatial distribution of the emission is modified but exciton dynamics are little altered. In the latter case, however, mixing of exciton and photon states occurs leading to strongly modified dynamics. Both types of effect have been observed in planar microcavity structures in inorganic semiconductor quantum wells and bulk layers1,2,3,4,5,6,7,8. But organic semiconductor microcavities have been studied only in the weak-coupling regime9,10,11,12,13,14,15,16,17,18. Here we report an organic semiconductor microcavity that operates in the strong-coupling regime. We see characteristic mixing of the exciton and photon modes (anti-crossing), and a room-temperature vacuum Rabi splitting (an indicator of interaction strength) that is an order of magnitude larger than the previously reported highest values for inorganic semiconductors. Our results may lead to new structures and device concepts incorporating hybrid states of organic and inorganic excitons19, and suggest that polariton lasing20,21,22 may be possible.
770 citations
TL;DR: In this paper, the emission spectrum of high quality ZnO epilayers is studied from room temperature up to 550 K. At room temperature and low excitation power, a single emission peak is observed which may be identified with the free exciton from its peak energy and dependence on temperature.
Abstract: The emission spectrum of high quality ZnO epilayers is studied from room temperature up to 550 K. At room temperature and low excitation power a single emission peak is observed which may be identified with the free exciton from its peak energy and dependence on temperature. However, when excitation intensities exceed 400 kW cm−2 a sharp peak emerges at lower energy which we attribute to exciton-exciton scattering. At higher excitation intensities (>800 kW cm−2) a second stimulated emission peak emerges at even lower energies: we attribute this peak to be stimulated emission of an electron hole plasma. Similar features are observed for all temperatures up to 550 K.
741 citations
TL;DR: In this paper, the Bethe-Salpeter equation for the two-particle Green function is solved in reciprocal space and the resulting coupled electron-hole excitations are evaluated.
Abstract: We present a new {ital abthinspthinspinitio} approach to calculate the interaction between electrons and holes in periodic crystals and to evaluate the resulting coupled electron-hole excitations. This involves a novel interpolation scheme in reciprocal space in solving the Bethe-Salpeter equation for the two-particle Green{close_quote}s function. We apply this approach to the calculation of the entire optical absorption spectrum, as well as of the energies and wave functions of bound exciton states in GaAs and LiF. Very good agreement with experiment is observed. {copyright} {ital 1998} {ital The American Physical Society}
690 citations
TL;DR: In this article, a model for the donor VXO and acceptor (VO, VGa) microstructure is proposed, where donors would be assembled in shallow clusters responsible for delocalized electron behavior with minority acceptors between.
670 citations
TL;DR: In this article, the authors provide a detailed review of the state-of-the-art properties of the group III nitrides (AlN, GaN and InN).
Abstract: The group III nitrides (AlN, GaN and InN) represent an important trio of semiconductors because of their direct band gaps which span the range 1.95-6.2 eV, including the whole of the visible region and extending well out into the ultraviolet (UV) range. They form a complete series of ternary alloys which, in principle, makes available any band gap within this range and the fact that they also generate efficient luminescence has been the main driving force for their recent technological development. High brightness visible light-emitting diodes (LEDs) are now commercially available, a development which has transformed the market for LED-based full colour displays and which has opened the way to many other applications, such as in traffic lights and efficient low voltage, flat panel white light sources. Continuously operating UV laser diodes have also been demonstrated in the laboratory, exciting tremendous interest for high-density optical storage systems, UV lithography and projection displays. In a remarkably short space of time, the nitrides have therefore caught up with and, in some ways, surpassed the wide band gap II-VI compounds (ZnCdSSe) as materials for short wavelength optoelectronic devices. The purpose of this paper is to review these developments and to provide essential background material in the form of the structural, electronic and optical properties of the nitrides, relevant to these applications. We have been guided by the fact that the devices so far available are based on the binary compound GaN (which is relatively well developed at the present time), together with the ternary alloys AlGaN and InGaN, containing modest amounts of Al or In. We therefore concentrate, to a considerable extent, on the properties of GaN, then introduce those of the alloys as appropriate, emphasizing their use in the formation of the heterostructures employed in devices. The nitrides crystallize preferentially in the hexagonal wurtzite structure and devices have so far been based on this material so the majority of our paper is concerned with it, however, the cubic, zinc blende form is known for all three compounds, and cubic GaN has been the subject of sufficient work to merit a brief account in its own right. There is significant interest based on possible technological advantages, such as easier doping, easier cleaving (for laser facets) and easier contacting. It also appears, at present, that the cubic form gives higher electron and hole mobilities than the hexagonal form. The dominant hexagonal structure is similar to that found in a number of II-VI compounds such as CdS and they can therefore be taken as role models. In particular, the lower symmetry gives rise to three separate valence bands at the zone centre and exciton spectra associated with each of these have been reported by many workers for GaN. Interpretation is complicated by the presence of strain in many samples due to the fact that most material consists of epitaxial thin films grown on non-lattice-matched substrates (bulk GaN crystals not being widely available). However, much progress has been made in understanding the physics of these films and we discuss the current position with regard to band gaps, effective masses, exciton binding energies, phonon energies, dielectric constants, etc. Apart from a lack of knowledge of the anticipated valence band anisotropy, it can be said that GaN is now rather well documented. Less detail is available for AlN or InN and we make no attempt to provide similar data for them. The structure of the paper is based on a historical introduction, followed by a brief account of the various crystal growth methods used to produce bulk GaN and epitaxial films of GaN and the ternary alloys. This is then followed by an account of the structural properties of hexagonal GaN as measured by x-ray diffraction and electron microscopy, phonon properties from infrared and Raman spectroscopy, electrical properties, with emphasis on n- and p-type doping, and optical properties, measured mainly by photoluminescence. A brief comparative account of cubic GaN properties follows. Discussion of alloy properties in the context of their use in quantum well and superlattice structures forms an introduction to the device sections which close the paper. These include details of the technology necessary for etching, contacting and forming laser facets, as an introduction to recent results on LEDs and laser diodes. Having described the current position, we speculate briefly on likely future developments.
464 citations
TL;DR: In this article, the neutral-donor bound excitons were observed in the form of rotator states analogous to rotational states of the ${\mathrm{H}}_{2}$ molecule.
Abstract: Neutral-donor--bound-exciton transitions have been observed in ZnO. The isolated neutral donors are made up of defect pair complexes. The neutral-donor nature of these pair complexes was determined from magnetic-field measurements and from two-electron transitions. Excited states of the neutral-donor bound excitons were observed in the form of rotator states analogous to rotational states of the ${\mathrm{H}}_{2}$ molecule.
448 citations
TL;DR: In this paper, the emission mechanisms of strained InxGa1−xN quantum wells (QWs) were shown to vary depending on the well thickness, L, and x. The absorption edge was modulated by the quantum confined Stark effect and quantum confined Franz-Keldysh effect (QCFK) for the wells, in which, for the first approximation, the product of the piezoelectric field, FPZ and L exceed the valence band discontinuity, ΔEV.
Abstract: The emission mechanisms of strained InxGa1−xN quantum wells (QWs) were shown to vary depending on the well thickness, L, and x. The absorption edge was modulated by the quantum confined Stark effect and quantum confined Franz-Keldysh effect (QCFK) for the wells, in which, for the first approximation, the product of the piezoelectric field, FPZ, and L exceed the valence band discontinuity, ΔEV. In this case, holes are confined in the triangular potential well formed at one side of the well producing the apparent Stokes-like shift. Under the condition that FPZ×L exceeds the conduction band discontinuity ΔEC, the electron-hole pair is confined at opposite sides of the well. The QCFK further modulated the emission energy for the wells with L greater than the three dimensional free exciton Bohr radius aB. On the other hand, effective in-plane localization of carriers in quantum disk size potential minima, which are produced by nonrandom alloy compositional fluctuation enhanced by the large bowing parameter and...
441 citations
TL;DR: In this paper, a theory for four-wave mixing signals from molecular aggregates, which includes effects of two-exciton states, static disorder, and coupling to a phonon bath with an arbitrary spectral density, is developed.
Abstract: A theory for four-wave-mixing signals from molecular aggregates, which includes effects of two-exciton states, static disorder, and coupling to a phonon bath with an arbitrary spectral density, is developed. The third-order polarization is rigorously partitioned into a coherent and a sequential contribution. The latter is given by a sum of an exciton-hopping and a ground state (bleaching) terms, both expressed using the doorway-window representation. Applications are made to photon-echo and pump-probe spectroscopies of the B850 system of the LH2 antenna in purple bacteria.
404 citations
TL;DR: In this paper, the electron-hole interaction was statically screened using a model dielectric function, which was used to calculate the optical absorption of LiF and MgO insulators.
Abstract: We present a computationally efficient scheme to calculate the optical absorption of insulators from first principles, including the electron-hole interaction. Excited states of the solid are chosen to consist of single electron-hole pairs. The electron-hole interaction is statically screened using a model dielectric function. Only two pieces of input are required, the crystal structure of the material and the macroscopic dielectric constant. We apply this scheme to two wide-gap insulators, LiF and MgO, and obtain excellent agreement with experimental measurements of their UV reflectance spectra.
348 citations
TL;DR: In this article, a study of the elastic exciton-exciton Coulomb scattering in a semiconductor quantum well is presented, including the interexciton exchange of carriers and the spin degrees of freedom.
Abstract: A study of the elastic exciton-exciton Coulomb scattering in a semiconductor quantum well is presented, including the interexciton exchange of carriers and the spin degrees of freedom. The theoretical results show that electron-electron and hole-hole exchanges are the dominant mechanisms of interaction, while the classical direct term is negligible. The density-dependent homogeneous linewidth is calculated within the Born approximation and good agreement with the existing experimental data is obtained. Owing to the interexciton exchange of carriers, collisions lead to spin relaxation as actually observed in recent time-resolved photoluminescence experiments. [S0163-1829(98)06736-8].
345 citations
TL;DR: In this article, photoluminescence at low temperature is studied for a CdTe-based microcavity tuned to resonance with a quantum well exciton, and two distinct stimulation effects are observed with increasing excitation.
Abstract: Photoluminescence at low temperature is studied for a CdTe-based microcavity tuned to resonance with a quantum well exciton. Two distinct stimulation effects are observed with increasing excitation. The first one is associated with the lower polariton state in the strong exciton-photon coupling regime. This effect, whose physical origin has not yet been identified, could be favored by the higher stability of exciton in CdTe. The second stimulation, obtained for much higher excitation, can be assigned to the electron-hole plasma in the weak coupling regime.
TL;DR: The photophysical properties of thin films of poly[2-methoxy-5-(2′-ethylhexyloxy), para -phenylene vinylene] (MEH-PPV) on TiO 2 substrates have been investigated in this paper.
TL;DR: In this article, it was shown that modification by electronic excitation can be achieved in a far wider range of materials, such as alkali halides, alkaline earth fluorides and fused quartz.
Abstract: Excitonic mechanisms of defect formation and of sputtering from surfaces, induced as a consequence of exciton relaxation, are effective in a limited class of wide-gap materials, such as alkali halides, alkaline earth fluorides and fused quartz. In this paper, we point out that modification by electronic excitation can be achieved in a far wider range of materials. First, referring to STM observations of semiconductor surfaces irradiated by laser or intense-electron beams, we show that atomic emissions with electronic origin do take place even in materials in which the bandgap energy is smaller than the energy to remove an atom from the surface. The yield of this process is linear with the beam intensity for higher incident energies, but superlinear for lower incident energies. Secondly we analyse the phenomena of laser damage and laser ablation in a variety of wide-gap materials, emphasizing the role played by defect creation in the bulk, and atomic emissions from surfaces originating from excitation of existing defects. Finally, we discuss the amorphisation of less-ionic oxides and semiconductors by electron- and ion-beam irradiation in terms of nucleation followed by radiation-induced reactions at the interface between the crystalline and amorphous regions. The similarities and differences of the interface reactions induced by electronic excitation and by hot zones in collision cascades are discussed.
TL;DR: In this paper, the authors apply low temperature confocal optical microscopy to spatially resolve, and spectroscopically study, a single self-assembled quantum dot and compare the emission spectra obtained at various excitation levels to a theoretical many body model, showing that radiative recombination is very weak and sharp spectral lines are due to optical transitions between confined multiexcitonic states among which excitons thermalize within their lifetimes.
Abstract: We apply low temperature confocal optical microscopy to spatially resolve, and spectroscopically study, a single self-assembled quantum dot. By comparing the emission spectra obtained at various excitation levels to a theoretical many body model, we show that (a) single exciton radiative recombination is very weak, and (b) sharp spectral lines are due to optical transitions between confined multiexcitonic states among which excitons thermalize within their lifetimes. Once these few states are fully occupied, broadbands appear due to transitions between states which contain electrons in the continuum.
TL;DR: In this article, the authors investigated the emission mechanisms of a device-quality quantum well (QW) structure and bulk three dimensional (3D) InGaN materials grown on sapphire substrates without any epitaxial lateral overgrown GaN base layers.
Abstract: Emission mechanisms of a device-quality quantum well (QW) structure and bulk three dimensional (3D) InGaN materials grown on sapphire substrates without any epitaxial lateral overgrown GaN base layers were investigated. The InxGa1−xN layers showed various degrees of in-plane spatial potential (band gap) inhomogeneity, which is due to a compositional fluctuation or a few monolayers thickness fluctuation. The degree of fluctuation changed remarkably around a nominal InN molar fraction x=0.2, which changes to nearly 0.08–0.1 for the strained InxGa1−xN. This potential fluctuation induces localized energy states both in the QW and 3D InGaN, showing a large Stokes-like shift. The spontaneous emission from undoped InGaN single QW light-emitting diodes (LEDs), undoped 3D LEDs, and multiple QW (MQW) laser diode (LD) wafers was assigned as being due to the recombination of excitons localized at the potential minima, whose lateral size was determined by cathodoluminescence mapping to vary from less than 60 to 300 nm...
TL;DR: In this article, the optical properties and dynamics of charge carriers in self-organized arrays of type-II (staggered band lineup) GaSb/GaAs quantum dots are studied.
Abstract: The optical properties and dynamics of charge carriers in self-organized arrays of type-II (staggered band lineup) GaSb/GaAs quantum dots are studied. Interband absorption from type-II quantum dots is evidenced; the energetic positions of quantum dot absorption peaks coincide with those apparent in photoluminescence spectra. (Sb,As) intermixing with an antimony diffusion length of about 1 nm is found to make an important contribution to the observed transition energies. Dipole layer formation and quantum dot state filling contribute to the luminescence blueshift with increasing excitation density. The recombination rate of electrons with localized holes drastically depends on the average carrier density. When several carriers are localized at each dot, decay time constants around 5 ns, quite similar to type-I systems, are observed. Individual, spatially indirect excitons decay with much larger time constants close to 1 \ensuremath{\mu}s. The decay time of quantum dot luminescence is independent of the temperature in the measured range $Tl~65\mathrm{K}$ as expected for zero-dimensional excitons.
TL;DR: In this article, a unified microscopic theoretical framework for the calculation of optical excitations in molecular and semiconductor materials is presented, where the hierarchy of many-body density matrices for a pair-conserving many-electron model and the Frenkel exciton model is rigorously truncated to a given order in the radiation field.
Abstract: A unified microscopic theoretical framework for the calculation of optical excitations in molecular and semiconductor materials is presented. The hierarchy of many-body density matrices for a pair-conserving many-electron model and the Frenkel exciton model is rigorously truncated to a given order in the radiation field. Closed equations of motion are derived for five generating functions representing the dynamics up to third order in the laser field including phonon degrees of freedom as well as all direct and exchange-type contributions to the Coulomb interaction. By eliminating the phonons perturbatively the authors obtain equations that, in the case of the many-electron system, generalize the semiconductor Bloch equations, are particularly suited for the analysis of the interplay between coherent and incoherent dynamics including many-body correlations, and lead to thermalized exciton (rather than single-particle) distributions at long times. A complete structural equivalence with the Frenkel exciton model of molecular materials is established. [S0034-6861(98)00201-3]
TL;DR: In this paper, a numerically feasible, yet realistic, calculational method for incorporating electron-core-hole interactions in the Bethe-Salpeter equation in solids, without recourse to tight-binding or analogous basis-set approximations, is presented.
Abstract: This work demonstrates a numerically feasible, yet realistic, calculational method for incorporating electron-core-hole interactions (core-hole effects) in the Bethe-Salpeter equation in solids, without recourse to tight-binding or analogous basis-set approximations. The method includes ab initio treatments of separate electron and hole dynamics and electron-hole interactions. The method is used to treat x-ray absorption (XAS), resonant inelastic x-ray scattering (RIXS), and core-hold excitons. Materials studied include LiF, NaF, KF, graphite, diamond, and $h\ensuremath{-}\mathrm{BN}$.
TL;DR: In this article, coherent nonlinear optical spectroscopy of individual excitons in GaAs quantum dots is demonstrated and shows strong similarities with atoms in their coherent optical interaction, unlike higher dimensional heterostructures.
Abstract: : Coherent nonlinear optical spectroscopy of individual excitons in GaAs quantum dots is demonstrated and shows strong similarities with atoms in their coherent optical interaction, unlike higher dimensional heterostructures. The nonlinear response is dominated by an incoherent contribution (saturation) and a coherent contribution (population pulsations) of the single dot and compares well with present theory. The data shows that energy relaxation and dephasing rates are comparable, reflecting the absence of significant pure dephasing, and also demonstrate the presence of interdot energy transfer.
TL;DR: In this article, the relaxation dynamics of charge carriers in 4 nm CdS colloidal quantum dots are studied by means of picosecond time-resolved fluorescence and femtosecond transient absorption experiments.
Abstract: The relaxation dynamics of charge carriers in 4 nm CdS colloidal quantum dots are studied by means of picosecond time-resolved fluorescence and femtosecond transient absorption experiments. We also studied the effects of the adsorption of viologen derivatives as electron acceptors on the surface of these particles. From these experimental measurements, we reached a model of the electron-hole dynamics in these nanoparticles consistent with previous proposals. In particular, we have confirmed that the electron trapping in these particles is slower than the hole trapping (30 ps versus a few picoseconds). After excitation, rapid formation of an optical hole (bleach) within the lowest energy exciton (band gap) absorption region appears. The maximum of the bleaching band is red-shifted by 20 meV in 2.5 ps, and the bleach intensity recovers in 30 ps. Upon the adsorption of electron acceptors, the rate of the red shift of the optical hole is not affected while the bleach recovery time is reduced to a few picoseco...
TL;DR: In this article, the diamagnetic coefficient of an exciton in a semiconductor nanostructure has been calculated for quantum well, wire, and dot geometries with arbitrary size, shape, and dimensionality.
Abstract: A method is presented for calculating the diamagnetic coefficient of an exciton in a semiconductor nanostructure. The diamagnetic coefficient characterizes the response of a confined exciton to a weak magnetic field, and gives information about the roles of confinement and of the Coulomb interaction in determining the optical properties. A general formulation is presented for nanostructures of arbitrary size, shape, and dimensionality. We introduce a generalized gauge transformation that allows us to express the diamagnetic coefficient in terms of two characterizations of the size of an exciton, one involving confinement and the other involving the Coulomb interaction. Calculations of the diamagnetic coefficient are given for quantum well, wire, and dot geometries.
TL;DR: In this paper, a single-molecule measurement of the allophycocyanin trimer (APC), a light-harvesting protein complex from cyanobacteria, is presented.
Abstract: We report a study of the allophycocyanin trimer (APC), a light-harvesting protein complex from cyanobacteria, by room-temperature single-molecule measurements of fluorescence spectra, lifetimes, intensity trajectories, and polarization modulation. Emission spectra of individual APC trimers are found to be homogeneous on the time scale of seconds. In contrast, their emission lifetimes are found to be widely distributed because of generation of long-lived exciton traps during the course of measurements. The intensity trajectories and polarization modulation experiments indicate reversible exciton trap formation within the three quasiindependent pairs of strong interacting R84 and ‚84 chromophores in APC, as well as photobleaching of individual chromophores. Comparison experiments under continuous-wave and pulsed excitation reveal a two-photon mechanism for generating exciton traps and/or photobleaching, which involves exciton-exciton annihilation. These single-molecule experiments provide new insights into the spectroscopy, exciton dynamics, and photochemistry of light-harvesting complexes.
TL;DR: In this article, the spectroscopic properties of discotic hexa-alkylthiotriphenylenes are studied in solution and thin films and compared to those of hexa -alkyloxytripenylenes.
Abstract: The spectroscopic properties of discotic hexa-alkylthiotriphenylenes are studied in solution and thin films and compared to those of hexa-alkyloxytriphenylenes. The solution properties are analyzed in the light of CS-INDO-CIPSI quantum chemistry calculations. The absorption maximum is assigned to the degenerate S0 → S4 transition. The fluorescence of the neat phases is attributed to weakly bound excimers. The phase transition leading from ordered to disordered columnar stacks induces an increase in the oscillator strength of the S0 → S1 transition and favors excimer formation. The influence of structural disorder on the properties of the delocalized states is rationalized by using various approximations within the frame of the exciton theory; three models for the calculation of the exciton coupling (point dipole, extended dipole, atomic transition charge distribution) are tested, short and long range interactions are considered, and the introduction of a dielectric constant is discussed. The best agreemen...
TL;DR: In this paper, a detailed theoretical analysis is presented of the temperature dependent radiative decay in aggregates of pseudoisocyanine (PIC) using static disorder and second order exciton-phonon interactions.
Abstract: In this paper a detailed theoretical analysis is presented of the temperature dependent radiative decay in aggregates of pseudoisocyanine (PIC). Our approach extends the original linear exciton-phonon coupling model including static disorder and second order exciton-phonon interactions. It is shown that for a one-dimensional exciton model neither of these additional effects alone or in combination with linear electron–phonon coupling can explain the steep rise in radiative lifetime at 40 K observed in the J-aggregate of PIC. However, when the aggregate assembles into a two-dimensional bricklike structure its radiative dynamics can be simulated, with linear exciton-optical phonon coupling as the only source for exciton scattering. Exciton-phonon scattering transfers oscillator strength from the k=0 state to other band states and also generates a nonequilibrium population among the exciton states, which persists during the superradiant decay. These effects together explain the marked temperature dependence of the radiative lifetime of the PIC J aggregate. When disorder limits the coherence length at low temperatures to a few molecules, as seems the case in several light harvesting complexes, the exciton population can equilibrate on the time scale of the superradiance. This situation pertains to the strong collision limit of the master equation, where the radiative decay is insensitive to details of the electron–phonon coupling, but only senses change in the thermal population among the exciton states.
TL;DR: In this article, a theory comprising the exciton interaction with both adiabatic and Jahn-Teller phonons and also the external nonadiabaticity was developed to interpret the phonon-assisted optical transitions in semiconductor quantum dots.
Abstract: In order to interpret the phonon-assisted optical transitions in semiconductor quantum dots, a theory is developed comprising the exciton interaction with both adiabatic and Jahn-Teller phonons and also the external nonadiabaticity (pseudo-Jahn-Teller effect). The effects of nonadiabaticity of the exciton-phonon system are shown to lead to a significant enhancement of phonon-assisted transition probabilities and to multiphonon optical spectra that are considerably different from the Franck-Condon progression. The calculated relative intensity of the phonon satellites and its temperature dependence compare well with the experimental data on the photoluminescence of CdSe quantum dots, both colloidal and embedded in glass.
TL;DR: In this paper, an experimental study of transport and photoluminescence of indirect excitons in AlAs/GaAs CQW's at low temperatures and high magnetic fields is presented.
Abstract: Due to the long lifetime of indirect (interwell) excitons, exciton condensation (analogous to the Bose-Einstein condensation of bosons) is expected to occur in coupled quantum wells (CQW's). The critical conditions for the exciton condensation have been predicted to be strongly improved by high magnetic field perpendicular to the well plane. We present results of experimental study of transport and photoluminescence of indirect excitons in AlAs/GaAs CQW's at low temperatures $(Tg~350$ mK) and high magnetic fields $(Bl~14$ T). Strong anomalies in the transport and luminescence of indirect excitons have been observed at low temperatures and high magnetic fields: a large increase of the exciton diffusivity, a large increase of the exciton radiative decay rate and a huge noise in the integrated exciton PL intensity. An interpretation of the observed anomalies as evidence of the exciton condensation (i.e., in terms of the onset of exciton superfluidity, superradiance of the exciton condensate, and fluctuations near the phase transition) is analyzed. The parameter (temperature, exciton density, and magnetic field) dependences of the observed anomalous transport and photoluminescence of indirect excitons show that these effects are consistent with the exciton condensation in the presence of a random potential.
TL;DR: The photoluminescence emission peak energy of GaN quantum dots was observed to shift to higher energy with decreasing quantum dot size as discussed by the authors, which was found to be a combination of a blueshift from the confinement-induced shift of the electronic levels and a redshift from the increased Coulomb energy induced by a compression of the exciton Bohr radius.
Abstract: The photoluminescence emission peak energy of GaN quantum dots was observed to shift to higher energy with decreasing quantum dot size. This effect was found to be a combination of a blueshift from the confinement-induced shift of the electronic levels and a redshift from the increased Coulomb energy induced by a compression of the exciton Bohr radius. From this observation, absolute values of the exciton binding energy as a function of quantum dot size are determined.
TL;DR: In this article, a unified description of nonlinear optical spectroscopies of molecular aggregates and semiconductors is developed using the nonlinear exciton equations (NEE), which follow explicitly the complete set of one-, two-, and three-point dynamical exciton variables relevant for the third-order response.
Abstract: A unified description of nonlinear optical spectroscopies of molecular aggregates (starting with the Frenkel-Heitler-London Hamiltonian) and semiconductors (starting with the two-band model) is developed using the nonlinear exciton equations (NEE). The equations follow explicitly the complete set of one-, two-, and three-point dynamical exciton variables relevant for the third-order response. Effects of nuclear motions are incorporated through relaxation superoperators calculated perturbatively in exciton-phonon coupling. A Green's function expression for the third-order response is derived by solving the NEE using a new truncation scheme based on factorizing the three-point relaxation kernels. These results set the stage for designing multidimensional spectroscopies of excitons and analyzing them using coherence-transfer pathways.
TL;DR: In this paper, a splitting of the ground state into a linearly polarized doublet with an energy spacing up to 1.5 meV is found, varying from dot to dot in sign and magnitude.
Abstract: Zero-dimensional excitons (0DXs) in CdSe/ZnSe nanostructures have been studied by time- and spatially resolved photoluminescence spectroscopy. The three-dimensional confinement is confirmed by an exciton lifetime up to 550 ps, independent of temperature up to 130 K. By preparing mesa structures with diameters down to 50 nm as local probes, an extremely high spatial resolution is achieved, giving experimental access to single 0DXs. A splitting of the ground state into a linearly polarized doublet with an energy spacing up to 1.5 meV is found, varying from dot to dot in sign and magnitude. This indicates a noncircular shape with no preferential orientation of the dots. The dot density is estimated to increase from 5×1010 to 1.5×1011 cm−2, when changing the nominal CdSe layer thickness from 1 to 3 ML, i.e., close to the critical thickness.
TL;DR: In this paper, a peak near 3838 A appears in the low-temperature photoluminescence spectrum of 4H SiC homoepitaxial films.
Abstract: Two distinct boron-related centers are known in silicon carbide polytypes, one shallow (ionization energy ∼300 meV) and the other deep (∼650 meV). In this work, 4H SiC homoepitaxial films are intentionally doped with the shallow boron center by controlling the silicon to carbon source gas ratio during chemical vapor deposition, based on site competition epitaxy. The dominance of the shallow boron center for samples grown with a low Si/C ratio, favoring the incorporation of boron onto the silicon sublattice, is verified by the temperature dependent Hall effect, admittance spectroscopy and deep level transient spectroscopy. In these samples a peak near 3838 A appears in the low temperature photoluminescence spectrum. Further experiments support the identification of this peak with the recombination of a four particle (bound exciton) complex associated with the neutral shallow boron acceptor as follows: (1) The intensity of the 3838 A peak grows with added boron. (2) Momentum conserving phonon replicas are observed, with energies consistent with other four particle complexes in SiC. (3) With increasing temperature excited states are observed, as for the neutral aluminum and gallium acceptor four particle complexes. However, the intensity of the shallow boron spectrum is quenched at lower temperatures than the corresponding spectra for Al and Ga, and the lineshapes are strongly sample dependent. These results may be related to the unusual configurational and electronic structure of this center inferred from recent spin resonance experiments by other groups. When the Si/C ratio is high, the optical signatures of the deep boron center, nitrogen-boron donor-acceptor pairs and conduction band to neutral acceptor free-to-bound transitions, are observed in the photoluminescence. At T=2 K well resolved, detailed nitrogen-boron pair line spectra are observed in addition to the peak due to distant pairs. As the temperature is raised, the donor-acceptor pair spectrum decreases in intensity while the free-to-bound no-phonon peak appears. Extrapolation of the temperature dependence of the free-to-bound peak to T=0 K, after correction for the temperature dependence of the exciton energy gap, leads to the value EA(B)−EX=628±1 meV, where EA(B) is the ionization energy of the deep boron center and EX is the binding energy of the free exciton which, for 4H SiC, can only be estimated at this time.