Showing papers on "Spontaneous emission published in 1998"
TL;DR: In this paper, a spontaneous emission rate enhancement by a factor of up to 5 was selectively observed for the QB's which are on resonance with one-cavity mode. But the effect of the random spatial and spectral distributions of the QBs was not considered.
Abstract: Semiconductor quantum boxes (QB's) are well suited to cavity quantum electrodynamic experiments in the solid state because of their sharp emission. We study by time-resolved photoluminescence InAs QB's placed in the core of small-volume and high-finesse GaAs/AlAs pillar microresonators. A spontaneous emission rate enhancement by a factor of up to 5 is selectively observed for the QB's which are on resonance with one-cavity mode. We explain its magnitude by considering the Purcell figure of merit of the micropillars and the effect of the random spatial and spectral distributions of the QB's.
1,016 citations
TL;DR: Low-threshold lasing is observed at the edge of the stop band of a one-dimensional structure-a dye-doped cholesteric liquid-crystal film and the mode closest to the edge has the lowest lasing threshold.
Abstract: Low-threshold lasing is observed at the edge of the stop band of a one-dimensional structure—a dye-doped cholesteric liquid-crystal film. The mode closest to the edge has the lowest lasing threshold. The rates of spontaneous and stimulated emission are suppressed within the stop band and enhanced at the band edge. The ratio of right to left circularly polarized spontaneous emission is in good agreement with calculated density of photon states.
713 citations
TL;DR: In this article, temperature-dependent time-integrated and time-resolved photoluminescence (PL) studies of InGaN/GaN multiple quantum wells (MQWs) grown by metalorganic chemical vapor deposition were performed.
Abstract: We report temperature-dependent time-integrated and time-resolved photoluminescence (PL) studies of InGaN/GaN multiple quantum wells (MQWs) grown by metalorganic chemical vapor deposition. We observed anomalous emission behavior, specifically an S-shaped (decrease–increase–decrease) temperature dependence of the peak energy (Ep) for InGaN-related PL with increasing temperature: Ep redshifts in the temperature range of 10–70 K, blueshifts for 70–150 K, and redshifts again for 150–300 K with increasing temperature. In addition, when Ep redshifts, the spectral width is observed to narrow, while when Ep blueshifts, it broadens. From a study of the integrated PL intensity as a function of temperature, it is found that thermionic emission of photocarriers out of local potential minima into higher energy states within the wells is the dominant mechanism leading to the thermal quenching of the InGaN-related PL. We demonstrate that the temperature-induced S-shaped PL shift is caused by a change in the carrier dyna...
648 citations
TL;DR: In this article, the authors observed visible light emission from nanosize gold clusters and found that relatively intense photoluminescence occurs only when the size of the metal nanocluster is sufficiently small (<5 nm.
Abstract: We have observed visible light emission from nanosize gold clusters. Liquid chromatographic analysis of the metal clusters shows that relatively intense photoluminescence occurs only when the size of the metal nanocluster is sufficiently small (<5 nm). The emission is strongly Stokes shifted and is assigned to radiative recombination of Fermi level electrons and sp- or d-band holes. The electron and/or hole states are perturbed by surface states, as indicated by the dependence of the emission spectrum on the nature of the cluster surface. Finally, we found that large, nonemitting gold clusters can also be made luminescent by partial dissolution using KCN.
512 citations
TL;DR: In this paper, the phase difference of two laser pairs with equal frequencies was used for the control of spontaneous emission in a four-level system and effects such as extreme spectral narrowing and selective and total cancellation of fluorescence decay were shown as the relative phase is varied.
Abstract: We use the phase difference of two lasers with equal frequencies for the control of spontaneous emission in a four-level system. Effects such as extreme spectral narrowing and selective and total cancellation of fluorescence decay are shown as the relative phase is varied.
330 citations
TL;DR: The experiments demonstrate the importance of vacuum fluctuations in the environment for quantum dot devices and potential design constraints for their use for preparing long-lived quantum states.
Abstract: A double quantum dot device is a tunable two-level system for electronic energy states. A dc electron current was used to directly measure the rates for elastic and inelastic transitions between the two levels. For inelastic transitions, energy is exchanged with bosonic degrees of freedom in the environment. The inelastic transition rates are well described by the Einstein coefficients, relating absorption with stimulated and spontaneous emission. The most effectively coupled bosons in the specific environment of the semiconductor device used here were acoustic phonons. The experiments demonstrate the importance of vacuum fluctuations in the environment for quantum dot devices and potential design constraints for their use for preparing long-lived quantum states.
316 citations
TL;DR: By measuring the gain and loss in thin-film planar waveguides using a standard technique developed for inorganic laser materials, the authors showed that the narrow-line emission from photopumped waveguide of the conjugated polymer poly[2-butyl-5-${(2}^{\ensuremath{'}}$-ethyl-hexyl)-1,4-phenylenevinylene] results from amplification of spontaneous emission (ASE).
Abstract: By measuring the gain and loss in thin-film planar waveguides using a standard technique developed for inorganic laser materials, we show that the narrow-line emission from photopumped waveguides of the conjugated polymer poly[2-butyl-5-${(2}^{\ensuremath{'}}$-ethyl-hexyl)-1,4-phenylenevinylene] results from amplification of spontaneous emission (ASE). The narrowed linewidth of the ASE spectrum is determined by gain saturation. The ASE emitted from the edge of the waveguide is polarized because the waveguide is birefringent and only allows TE modes to propogate and be amplified. We compare our results and conclusions to other reports that claim that spectral narrowing in conjugated polymers is a result of superfluorescence or excitonic interactions. The techniques presented in the paper offer a simple and useful way for evaluating conjugated polymers as materials for solid state lasers.
306 citations
TL;DR: In this paper, a theoretical expression for the radiation pattern of light-emitting devices based on thin-film technology is provided, discriminating for polarization, emission angle, absorption, and transmission; and numerical calculation of discrete modes, narrow modes, and evanescent waves near absorbing media is discussed.
Abstract: In light-emitting devices based on thin-film technology, light waves that are partially or totally reflected at interfaces between different materials interfere and influence the angular distribution of the emitted light. For an electrical dipole transition, the radiation pattern is equivalent to that of an electrical dipole antenna. New theoretical expressions are provided for the radiation, discriminating for polarization, emission angle, absorption, and transmission; and the numerical calculation of discrete modes, narrow modes, and evanescent waves near absorbing media is discussed.
299 citations
TL;DR: In this paper, a modification of dipole emission that is due to its optical environment is calculated for planar layered structures, and the dipole is included by using additive source terms for the electric field that depend on dipole orientation and wave polarization.
Abstract: Modification of dipole emission that is due to its optical environment is calculated for planar layered structures. The layers are optically described by standard matrix techniques, and the dipole is included by using additive source terms for the electric field that depend on dipole orientation and wave polarization. These source terms also allow coupling through evanescent waves. We emphasize the applicability of this method to cases in which the power distribution into various modes is affected: dipole emission into guided modes and emission distribution into the various modes of structures that contain multilayer reflectors, such as microcavities.
259 citations
TL;DR: In this article, the spontaneous emission of dye molecules embedded in a threedimensional solid-state photonic crystal exhibiting a stop band in the visible range was investigated and the results were interpreted in terms of redistribution of the photon density of states in the photonic lattice.
Abstract: We report on modification of the spontaneous emission of dye molecules embedded in a threedimensional solid-state photonic crystal exhibiting a stop band in the visible range. Molecules embedded in artificial opal filled with a polymer show a dip in the fluorescence spectrum and nonexponential spontaneous decay kinetics containing both accelerated and inhibited components compared to the dye fluorescence in a reference polymer matrix. Results are interpreted in terms of redistribution of the photon density of states in the photonic crystal. [S0031-9007(98)06494-1] PACS numbers: 42.50. ‐ p
255 citations
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: Spontaneous, stimulated and laser emission spectra of ZnO epitaxial layers, grown by plasma-assisted molecular beam epitaxy, are presented in this paper, where samples are found to exhibit high-intensity near band-edge emissions at room temperature.
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, a model for the luminescence spectrum of silicon nanoclusters is presented, which is based on radiative recombination of confined excitons (quantum confinement).
Abstract: We present a model for the luminescence spectrum of silicon nanoclusters. We propose that the major contribution to luminescence is from radiative recombination of confined excitons (quantum confinement). Utilizing the effective mass approximation we consider the variation in oscillator strength with cluster size and the associated change in the number of available free carriers. By varying both the mean cluster size and size distribution of silicon nanoclusters, the luminescence spectra are modeled to a good fit. We compare our model with experimental photoluminescence and electroluminescence data from this group and from others.
01 Dec 1998
TL;DR: In photonic crystals, electromagnetic waves can be confined in all three dimensions leading to very small mode volumes as mentioned in this paper, which can lead to significant enhancement of spontaneous emission rates in semiconductor nano-cavities.
Abstract: In photonic crystals, electromagnetic waves can be confined in all three dimensions leading to very small mode volumes. Computational search has been for smallest electromagnetic mode which occurs in two-dimensional dielectric photonic crystals slabs. The smallest mode volume found was V/spl ap/2(/spl lambda//2n)/sup 3/, where n is the refractive index. This small mode volume can lead to significant enhancement of spontaneous emission rates in semiconductor nano-cavities due to the Purcell effect.
TL;DR: In this article, spontaneous emission from dye molecules embedded in periodic dielectric structures was investigated and the alteration in the emission spectra was explained in terms of the partial suppression of radiation modes due to the periodic Dielectric structure.
Abstract: Spontaneous emission from dye molecules embedded in periodic dielectric structures was investigated. The structures consist of close-packed arrays of dye-doped polystyrene microspheres. Periodic arrays of polystyrene spheres with a submicrometer diameter were shown to provide a nonoverlapping gap or pseudogap in the visible spectral region. The modification of the spontaneous emission from inside the pseudogap structure was observed as a deep dip in the photoluminescence spectra. The alteration in the emission spectra is explained in terms of the partial suppression of radiation modes due to the periodic dielectric structures.
TL;DR: In this paper, the spontaneous emission from a thin layer of a dilute solution of fluorescent dye molecules within an optical microcavity has been studied, and the results give fundamental information on the kinetics of dye molecules and allow one to speculate about the prospect of using single molecules as sources of single photons for quantum optics.
Abstract: The spontaneous emission from a thin layer of a dilute solution of fluorescent dye molecules within an optical microcavity has been studied. Strong fluctuations in fluorescence intensity are observed when the average number of molecules in the measurement volume is small. We have performed experiments that simultaneously characterize these fluctuations over nine orders of magnitude of time, from ns to s. These measurements have identified photon antibunching characteristic of single-molecule emission, along with triplet-state shelving and diffusion driven number fluctuations. The results give fundamental information on the kinetics of dye molecules and allow one to speculate about the prospect of using single molecules as sources of single photons for quantum optics.
TL;DR: In this paper, the inhibited spontaneous emission of organic dye rhodamine 6G infiltrated in a polymer replica of synthetic opal as a photonic crystal was observed and the morphology-dependent resonances, superimposed on the broadband emission of rhodamines due to spherical wavelength-sized microcavity enhancement of dye emission, were also observed.
Abstract: We report the observation of inhibited spontaneous emission of organic dye rhodamine 6G infiltrated in a polymer replica of synthetic opal as a photonic crystal. The morphology-dependent resonances, superimposed on the broadband emission of rhodamine 6G due to spherical wavelength-sized microcavity enhancement of dye emission, have also been observed.
TL;DR: In this article, the authors reported room-temperature measurements of optical gain and gain spectra of ultraviolet emission from ZnO thin film and attributed the large gain to the modification of the spontaneous emission rate by the dielectric photonic structure of these films.
TL;DR: In this paper, the authors reported very large output intensities corresponding to a gain larger than 10sup 5 for a single pass free-electron laser operating in the self-amplified spontaneous emission (SASE) mode at 12 µm.
Abstract: We report measurements of very large output intensities corresponding to a gain larger than 10{sup 5} for a single pass free-electron laser operating in the self-amplified spontaneous emission (SASE) mode at 12 {mu}m . We also report the observation and analysis of intensity fluctuations of the SASE radiation intensity in the high-gain regime. The results are compared with theoretical predictions and simulations. {copyright} {ital 1998} {ital The American Physical Society }
TL;DR: In this article, the authors observed ultraviolet laser emission from ZnO nanocrystal thin films at room temperature at 3.2 eV, increasing with a power of eight as increasing pumping intensity.
Abstract: We have observed ultraviolet laser emission from ZnO nanocrystal thin films at room temperature. ZnO films were epitaxially grown on sapphire substrates by laser molecular beam epitaxy. Closely packed and hexagonally shaped nanocrystals were formed in a spiral growth mode presumably due to a large lattice mismatch (∼18%) between ZnO and sapphire. Photoluminescence of these films clearly showed a sharp emission due to free excitons at room temperature. Above a threshold intensity as small as 24 kW cm −2 of pumping laser pulses (355 nm, 15 ps), we observed stimulated emission at 3.2 eV, increasing with a power of eight as increasing pumping intensity. This emission is shown to be due to an exciton–exciton collision process. Excitons are confined in these nanocrystals to show the giant oscillator strength effect and resulting in an excitonic stimulated emission even at room temperature.
TL;DR: In this article, the effects of quantum interference from spontaneous emission in the creation of atomic coherence in a closed V-type system were studied and it was shown that the absorption and dispersion properties of this atom can be significantly modified if this interference is optimized.
TL;DR: In this article, the spontaneous emission from a coherently prepared and microwave-driven doublet of potentially closely spaced excited states is discussed at a common ground level, and multiple interference mechanisms are identified that may lead to fluorescence inhibition in well-separated regions of the spectrum or act jointly in canceling the spontaneous emissions.
Abstract: We discuss the spontaneous emission from a coherently prepared and microwave-driven doublet of potentially closely spaced excited states to a common ground level. Multiple interference mechanisms are identified that may lead to fluorescence inhibition in well-separated regions of the spectrum or act jointly in canceling the spontaneous emission. In addition to phase-independent quantum interferences due to combined absorptions and emissions of driving field photons, we distinguish two competing phase-dependent interference mechanisms as means of controlling the fluorescence. The indistinguishable quantum paths may involve the spontaneous emission from the same state of the doublet, originating from the two different components of the initial coherent superposition. Alternatively the paths involve a different spontaneous photon from each of two decaying states, necessarily with the same polarization. This makes these photons indistinguishable in principle within the uncertainty of the two decay rates. The phase dependence arises for both mechanisms because the interfering paths differ by an unequal number of stimulated absorptions and emissions of the microwave field photons. @S1050-2947~98!03011-X#
TL;DR: In this paper, a systematic study of exciton dynamics is presented in quantum boxes formed naturally along the axis of a V-shaped quantum wire, by means of time and spatially resolved resonant photoluminescence.
Abstract: A systematic study of exciton dynamics is presented in quantum boxes formed naturally along the axis of a V-shaped quantum wire, by means of time and spatially resolved resonant photoluminescence. The dependence of radiative lifetimes and relaxation mechanisms of excitons is determined versus the size of the boxes. The radiative recombination rate varies linearly with the length of the box, showing that the exciton has a coherence volume equal to the volume of the box. In a low excitation regime, emission from excited states has not been observed, which would be a consequence of relaxation bottleneck, but there is clear evidence that relaxation via emission of LA phonons depends strongly on the energy separation between the different quantum box levels.
TL;DR: In this paper, the authors studied the pressure and temperature behavior of radiative recombination in InxGa1−xN/GaN quantum wells with x=0.06, 0.10, and 0.15.
Abstract: The energies of photo- and electroluminescence transitions in InxGa1−xN quantum wells exhibit a characteristic “blueshift” with increasing pumping power. This effect has been attributed either to band-tail filling, or to screening of piezoelectric fields. We have studied the pressure and temperature behavior of radiative recombination in InxGa1−xN/GaN quantum wells with x=0.06, 0.10, and 0.15. We find that, although the recombination has primarily a band-to-band character, the excitation-power induced blueshift can be attributed uniquely to piezoelectric screening. Calculations of the piezoelectric field in pseudomorphic InxGa1−xN layers agree very well with the observed Stokes redshift of the photoluminescence. The observed pressure coefficients of the photoluminescence (25–37 meV/GPa) are surprisingly low, and, so far, their magnitude can only be partially explained.
TL;DR: In this article, the electromagnetic field in a dielectric slab bounded by two half spaces with arbitrary refractive indices is quantized by computing the complete set of orthonormal electromagnetic modes.
Abstract: The electromagnetic field in a dielectric slab bounded by two dielectric half spaces with arbitrary refractive indices is quantized by computing the complete set of orthonormal electromagnetic modes. The zero-point fluctuations of the electromagnetic field are determined as a function of position. The dependence of the rate of spontaneous emission of thin dielectric films on the thicknesses of the films and the refractive index of the substrate is studied and compared with experimental results.
TL;DR: In this paper, the authors used a quantum box array placed inside a GaAs/AlAs pillar microcavity as an internal broadband light source to split the twofold polarization degenerate fundamental mode of circular micropillars into a pair of orthogonal linearly polarized modes.
Abstract: GaAs/AlAs pillar microcavities with elliptical cross section have been fabricated by molecular beam epitaxy, electron-beam lithography, and reactive ion etching. We study their lowest energy confined photonic modes by photoluminescence, using a quantum box array placed inside the cavity as an internal broadband light source. Such an anisotropic cross section allows to split the twofold polarization degenerate fundamental mode of circular micropillars into a pair of orthogonal linearly polarized modes. Their energy splitting, which is well accounted for by a simple perturbative model, is studied experimentally and theoretically as a function of the eccentricity and average radius of the pillars. Splittings as large as 15 meV are observed, which is very encouraging for applications ranging from the improvement of the polarization locking in vertical cavity lasers to the fabrication of light emitting diodes with a better control of the spontaneous emission.
TL;DR: In this paper, the spontaneous emission probabilities calculated by the use of the Judd-Ofelt theory and lifetime data were used to measure the upconversion spectra of Er3+ in Ga2S3-GeS2-La 2S3 glasses.
Abstract: Spectroscopic properties of Er3+ in Ga2S3–GeS2–La2S3 glasses are investigated on the basis of the spontaneous emission probabilities calculated by the use of the Judd–Ofelt theory and lifetime data Frequency upconversion spectra are also measured with excitation at 800 and 980 nm The spontaneous emission probabilities are much larger than those of the other glass systems such as oxides and fluorides This is mainly attributed to the large refractive indices of the sulfide glasses The quantum efficiencies of 4I11/2 and 4F9/2 levels are comparable or larger than the typical values of Er3+ in a fluoride glass This is a consequence of the large spontaneous emission probabilities and low phonon energies of the sulfide glasses However, the quantum efficiency of the 4S3/2 level is less than one-half of the value in the fluoride glass The nonradiative transition probability of the 4S3/2 level is much larger than the value which is evaluated from the multiphonon relaxation theory This suggests the presence
TL;DR: In this article, the spin relaxation of excitons in zero-dimensional semiconductor nanostructures was measured by using a polarization dependent time-resolved photoluminescence method.
Abstract: We report the observation of spin relaxation of excitons in zero-dimensional semiconductor nanostructures. The spin relaxation is measured in InGaAs quantum disks by using a polarization dependent time-resolved photoluminescence method. The spin relaxation time in a zero-dimensional quantum disk is as long as 0.9 ns at 4 K, which is almost twice as long as the radiative recombination lifetime and is considerably longer than that in quantum wells. The temperature dependence of the spin relaxation time suggests the importance of exciton–acoustic phonon interaction.
TL;DR: In this paper, the authors investigated the effect of light scattering by impurity atoms inside dielectric cubic lattices in the point-dipole limit and showed that the substitutional case occurs prevalently.
Abstract: Resonant classical light scattering by impurity atoms inside dielectric cubic lattices is investigated in the point-dipole limit. Modifications to resonance frequencies and linewidths are shown to be different for substitutional and interstitial impurities. Spontaneous emission rates inside dielectrics exhibit the well-known empty-cavity and Lorentz local-field factors for substitutional and interstitial atoms, respectively. The results are generalized to disordered dielectrics, indicating that the substitutional case occurs prevalently for impurity atoms. [S0031-9007(98)06814-8]