Showing papers on "Photoluminescence excitation published in 1996"
TL;DR: In this paper, the authors used photoluminescence excitation and fluorescence line narrowing spectroscopies to examine structure observed in the band-edge absorption feature of CdSe quantum dots.
Abstract: We use photoluminescence excitation and fluorescence line narrowing spectroscopies to examine structure observed in the band-edge absorption feature of CdSe quantum dots. We study eight samples ranging from \ensuremath{\sim}15 to \ensuremath{\sim}50 \AA{} in radius to probe the size dependence of this structure. We compare our results with recent theories, which predict band-edge exciton splittings in CdSe dots due to their internal crystal structure, nonspherical shape, and the exchange interaction between the electron and hole. We find reasonable agreement between our data and theory, supporting the observation of exciton fine structure. \textcopyright{} 1996 The American Physical Society.
492 citations
TL;DR: The homogeneous linewidths in the photoluminescence excitation spectrum of a single, naturally formed gallium arsenide (GaAs) quantum dot have been measured with high spatial and spectral resolution and provide a new perspective on the dephasing dynamics of the exciton in a quantum-confined, solid-state system.
Abstract: The homogeneous linewidths in the photoluminescence excitation spectrum of a single, naturally formed gallium arsenide (GaAs) quantum dot have been measured with high spatial and spectral resolution. The energies and linewidths of the homogeneous spectrum provide a new perspective on the dephasing dynamics of the exciton in a quantum-confined, solid-state system. The origins of the linewidths are discussed in terms of the dynamics of the exciton in zero dimensions, in particular, in terms of lifetime broadening through the emission or absorption of phonons and photons.
437 citations
TL;DR: Three-photon excitation fluorescence axial images are shown of polystyrene beads stained with the fluorophore 2,5-bis(4-biphenyl)oxazole (BBO) using a mode-locked titanium sapphire laser, proving the feasibility of scanning fluorescence microscopy by three-ph photon excitation.
Abstract: We show experiments proving the feasibility of scanning fluorescence microscopy by three-photon excitation. Three-photon excitation fluorescence axial images are shown of polystyrene beads stained with the fluorophore 2,5-bis(4-biphenyl)oxazole (BBO). Three-photon excitation is performed at an excitation wavelength of 900 nm and with pulses of 130 fs duration provided by a mode-locked titanium sapphire laser. Fluorescence is collected between 350 and 450 nm. The fluorescence image signal features a third-order dependence on the excitation power, also providing intrinsic 3-D imaging. The resolution of a three-photon excitation microscope is increased over that of a comparable two-photon excitation microscope.
208 citations
TL;DR: In this article, optical studies of relaxation processes in self-organized InAs/GaAs quantum dots (QDs) reveal a series of sharp lines, and the shape of the PLE spectra is explained by multiphonon relaxation processes involving LO phonons of the QD.
Abstract: We report on optical studies of relaxation processes in self‐organized InAs/GaAs quantum dots (QDs). Near resonant photoluminescence excitation spectra reveal a series of sharp lines. Their energy with respect to the detection energy does not depend on QD size and their energy separations are close to the InAs LO phonon energy of 32.1 meV estimated for strained pyramidal InAs QDs. The shape of the PLE spectra is explained by multiphonon relaxation processes involving LO phonons of the QD as well as of the wetting layer, an interface mode, and low frequency acoustical phonons.
197 citations
TL;DR: The combined evidence of all of the experimental results suggests that emission between surface or interface states is a more likely mechanism for emission for which the decay times are very sensitive to surface and/or interface states.
Abstract: Thin films of Si nanoclusters passivated with oxygen or hydrogen, with an average size of a few nanometers, have been synthesized by thermal vaporization of Si in an Ar buffer gas, followed by subsequent exposure to oxygen or atomic hydrogen. High-resolution transmission electron microscopy and x-ray diffraction revealed that these nanoclusters were crystalline. However, during synthesis, if oxygen was the buffer gas, a network of amorphous Si oxide nanostructures (an-${\mathrm{SiO}}_{\mathit{x}}$) with occasional embedded Si dots was formed. All samples showed strong infrared and/or visible photoluminescence (PL) with varying decay times from nanoseconds to microseconds depending on synthesis conditions. Absorption in the Si cores for surface passivated Si nano- crystals (nc-Si), but mainly in oxygen related defect centers for an-${\mathrm{SiO}}_{\mathit{x}}$, was observed by photoluminescence excitation spectroscopy. The visible components of PL spectra were noted to blueshift and broaden as the size of the nc-Si was reduced. There were differences in PL spectra for hydrogen and oxygen passivated nc-Si. Many common PL properties between oxygen passivated nc-Si and an-${\mathrm{SiO}}_{\mathit{x}}$ were observed. Our data can be explained by a model involving absorption between quantum confined states in the Si cores and emission for which the decay times are very sensitive to surface and/or interface states. The emission could involve a simple band-to-band recombination mechanism within the Si cores. The combined evidence of all of our experimental results suggests, however, that emission between surface or interface states is a more likely mechanism. \textcopyright{} 1996 The American Physical Society.
150 citations
TL;DR: A spectroscopic investigation of the electronic energy levels and carrier-relaxation mechanisms in self-organized InAs/GaAs quantum dots reveals two mechanisms for the relaxation of carriers from the excited states to the ground state: a nonresonant mechanism dominant in the upper state, and a resonant mechanism involving the emission of one or more LO phonons of well-defined energy.
Abstract: We report a spectroscopic investigation of the electronic energy levels and carrier-relaxation mechanisms in self-organized InAs/GaAs quantum dots. Power-dependent photoluminescence (PL) and photoluminescence excitation (PLE) are used to study the energy-level structure. Two excited states, 74 and 120 meV above the luminescent ground state, are identified. As expected for a zero-dimensional system, it is not possible to observe PL from the ground state of the dots when exciting between the energies of the ground and first excited state due to the discrete, atomiclike nature of the electronic states. Selectively excited PL and PLE reveal two mechanisms for the relaxation of carriers from the excited states to the ground state: a nonresonant mechanism dominant in the upper state, and a resonant mechanism, involving the emission of one or more LO phonons of well-defined energy, which is dominant in the lower excited state. The resonant mechanism is shown to be a consequence of the distribution of energy-level spacings in the inhomogeneous ensemble of dots; preferentially selecting dots with an energy-level spacing close to an integer multiple of the LO phonon energy.
149 citations
TL;DR: The exciton kinetic energy distribution is found to be a Maxwellian one having the temperature of the lattice, which suggests an enhanced rate of free exciton scattering on the acoustic phonons, in contrast to other wurzite-type semiconductors.
Abstract: We present a detailed study of the free exciton emission in GaN. Photoluminescence and photoluminescence excitation techniques are employed to show the dominant role of exciton-phonon interaction in the creation of the free exciton states and the free exciton emission cascade in GaN. Up to six longitudinal optical (LO) phonon replicas are observed in both photoluminescence and photoluminescence excitation spectra. From an analysis of the free exciton line shape we are able to conclude that free exciton emission and the interaction of free excitons with LO phonons have to be described within the framework of the momentum conservation law. The exciton kinetic energy distribution is found to be a Maxwellian one having the temperature of the lattice. This suggests an enhanced rate of free exciton scattering on the acoustic phonons, in contrast to other wurzite-type semiconductors. \textcopyright{} 1996 The American Physical Society.
123 citations
TL;DR: In this paper, the photoluminescence excitation spectra of thermally oxidized porous silicon (OPS) and SiO2 powder (340, 350, and 370 nm) have been observed.
Abstract: Ultraviolet (UV) light emission with almost the same peak wavelengths from thermally oxidized porous silicon (OPS) (340, 355, and 370 nm) and SiO2 powder (340, 350, and 370 nm) has been observed. Photoluminescence excitation spectra of OPS without Si nanoscale particles (SNP) and those of SiO2 powder are very similar, however, very different from those of the OPS with SNP. Three types of luminescence centers with luminescence wavelengths around 350 nm are responsible for UV light emission, and photoexcitation in OPS with SNP occurs in SNP as well as in Si oxide layers covering SNP.
116 citations
TL;DR: The photoluminescence from individual InP quantum dots embedded in a matrix of GaInP has been studied in this article, where the ground state emission consists of several peaks, and the excited states of the dot are observed either via state filling or with photolumeinescence excitation spectroscopy.
Abstract: The photoluminescence from individual InP quantum dots embedded in a matrix of GaInP has been studied. In addition to the ground state emission that consists of several peaks, we observe excited states of the dot. These states are observed either via state filling or with photoluminescence excitation spectroscopy. We observe a fast relaxation to the set of states with lowest energy but no relaxation between these states.
92 citations
TL;DR: In this paper, optical properties of CdS semiconductor nanocrystals doped with manganese ions were studied by means of absorption, continuous and time resolved luminescence as well as photoluminescence excitation spectra at room temperature and at 10 K.
82 citations
TL;DR: An ABA triblock conjugated copolymer, poly(2,5-benzoxazole)-block-poly(benzobisthiazole-2,6-diyl-1,4-phenylene), was synthesized, characterized, and used to demonstrate spatial confinement effects on the electronic structure and the optical and optoelectronic properties of block copolymers as mentioned in this paper.
Abstract: An ABA triblock conjugated copolymer, poly(2,5-benzoxazole)-block-poly(benzobisthiazole-2,6-diyl-1,4-phenylene)-block-poly(2,5-benzoxazole), 1, was synthesized, characterized, and used to demonstrate spatial confinement effects on the electronic structure and the optical and optoelectronic properties of block copolymers. Optical absorption, photoluminescence, and photoluminescence excitation spectra of thin films of the triblock copolymer evidence spatial confinement of excitons as well as microphase separation. Efficient energy transfer via interchain and intrachain mechanisms was observed in the triblock conjugated copolymer. The results suggest that microphase separation in bulk block conjugated copolymers can significantly modify the spatial modulation and confinement dimensionalities expected in isolated block copolymer chains.
TL;DR: With increasing magnetic field the enhancement of the exciton binding energy is stronger for the direct than for the indirect exciton, which leads to an enhanced splitting of indirect and direct excitons as well as to a strong redistribution of oscillator strength from the indirect to the direct exciton.
Abstract: Heavy-hole and light-hole excitons in symmetric, coupled GaAs/${\mathrm{Al}}_{0.30}$${\mathrm{Ga}}_{0.70}$As double quantum wells have been investigated by photoluminescence and photoluminescence excitation spectroscopy in magnetic fields B\ensuremath{\leqslant}13 T. The excitonic states in these structures can be controlled by varying the width of the AlAs barrier, which separates the GaAs quantum wells. The ground-state exciton binding energy shows a highly nonlinear dependence on the AlAs barrier width which originates from the redistribution of the electron and hole wave functions by the barrier. The barrier width also controls the spatial character of the ground-state excitons. In the case of relatively wide barriers (five monolayers of AlAs) the Coulomb interaction mixes symmetric and antisymmetric single-particle states and generates exciton states, which have predominantly direct or indirect character. With increasing magnetic field the enhancement of the exciton binding energy is stronger for the direct than for the indirect exciton, which leads to an enhanced splitting of indirect and direct excitons as well as to a strong redistribution of oscillator strength from the indirect to the direct exciton. For narrow barriers (one monolayer), on the other hand, spatially direct and indirect character are strongly mixed in all exciton states, leading to a very similar magnetic-field dependence of the energies and oscillator strengths of equivalent exciton states. \textcopyright{} 1996 The American Physical Society.
TL;DR: It is found that Indo-1 could be readily excited with the femtosecond pulses from a mode-locked Ti:sapphire laser at 885 nm, too long for two-photon excitation, and the emission intensity was found to depend on the cube of the laser power, as expected for simultaneous interaction with three photons.
TL;DR: In this paper, the experimental temperature-dependent absorption, phololuminescence and photoluminecence excitation spectra of poly (p -phenylenevinylene) (PPV) are presented and a molecular theory is used to analyze the results.
TL;DR: In this article, a correlation between the co-ordination numbers of the first, second and third Si neighbor shells from Fourier transform fitting of EXAFS and both emission peak energies and optical band gaps estimated by PLE (photoluminescence excitation dependence) suggests that the nanostructures of the newly produced red, yellow and green emitting porous Si specimens are nanowires, rather than nanocrystalline.
Abstract: Freshly produced red, yellow and green emitting porous Si specimens have been studied by NEXAFS and EXAFS (near edge and extended x‐ray absorption fine structure). The emission peaks are at 690, 580, and 520 nm, which almost covers the full visible range that direct anodization can achieve. The correlation between the co‐ordination numbers of the first, second and third Si neighbor shells from Fourier transform fitting of EXAFS and both emission peak energies and optical band gaps estimated by PLE (photoluminescence excitation dependence) suggests that the nanostructures of the PS are nanowires, rather than nanocrystalline. Two types of quantum nanowire with one and one‐plus‐a‐fraction dimensionality are proposed to interpret the correlation. The order factors of the theoretical fits suggest the nanowires of the freshly produced PS have crystalline cores.
TL;DR: In this paper, the photoluminescence of a single quantum wire is easily distinguished from that of the surrounding quantum well, and a factor of 3 absorption enhancement for these wires compared to the wells is concluded from the photoline excitation data.
Abstract: Low temperature near‐field scanning optical microscopy is used for spectroscopic studies of single, nanometer dimension, cleaved edge overgrown quantum wires. A direct experimental comparison between a two dimensional system and a single genuinely one dimensional quantum wire system, inaccessible to conventional far field optical spectroscopy, is enabled by the enhanced spatial resolution. We show that the photoluminescence of a single quantum wire is easily distinguished from that of the surrounding quantum well. Emission from localized centers is shown to dominate the photoluminescence from both wires and wells at low temperatures. A factor of 3 absorption enhancement for these wires compared to the wells is concluded from the photoluminescence excitation data.
TL;DR: In this article, the first results of spectroscopic properties of X1-Y2SiO5:Eu3+ were reported, which showed that the sample under UV or dye laser excitation at room temperature exhibits a strong orange-red fluorescence.
TL;DR: In this article, the dielectric enhancement of excitons is demonstrated by measuring the splitting of the $2s$ and $1s$ excitions and the diamagnetic shift of the exciton state.
Abstract: The excitons in near-surface ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}/\mathrm{GaAs}$ QW's have been investigated by photoluminescence excitation and magnetophotoluminescence spectroscopy. The dielectric enhancement of excitons is demonstrated by measuring the splitting of the $2s$ and $1s$ excitons and the diamagnetic shift of the $1s$ exciton state. In agreement with theoretical calculations the exciton binding energy is found to be enhanced 1.5 times by the dielectric confinement for 5-nm-wide quantum wells with cap layer thicknesses below 3 nm.
TL;DR: In this paper, the optical properties of spontaneously ordered GaInP 2 grown by MOVPE are studied by means of photoluminescence, time-resolved photoluminance, and polarization dependent excitation spectroscopy, and a density of states model is proposed which allows to explain the results obtained in the last few years.
Abstract: The optical properties of spontaneously ordered GaInP 2 grown by MOVPE are studied by means of photoluminescence, time-resolved photoluminescence, and polarization dependent photoluminescence excitation spectroscopy. Ordered samples grown on a substrate which is misoriented 6° toward [111]B show clear excitonic features both in luminescence and absorption. In excitation spectra both valence band edges are clearly resolved and are shown to be polarization dependent as predicted by band structure calculations. In these samples an additional non-excitonic low-energy photoluminescence emission is observed the peak energy of which depends strongly on excitation density and temperature and which shows extremely long luminescence lifetimes. A density of states model for ordered GaInP 2 is proposed which allows to explain the results obtained in the last few years. The density of states model is discussed with respect to the microstructure of the ordered alloy as known from TEM investigations.
TL;DR: In this paper, the optical transition energies for the lowest electron-hole pair states of CdSe quantum dots embedded in glass have been determined based on theoretical calculations, and the dominant features in the spectra have been assigned to different transitions from the ground state to the one-pair states.
TL;DR: In this article, a double exponential function with lifetimes of 0.41 ms and 2.12 ms was found for photoluminescence excitation spectroscopy between 700 and 860 nm, suggesting that the transfer of energy from the pump source to the erbium ions is mainly via the recombination of electron-hole pairs.
TL;DR: In this paper, spatially resolved photoluminescence and Raman experiments on the substrate interface region of wurtzite GaN layers were conducted to find the source of photoluminance.
Abstract: We present results from spatially resolved photoluminescence and Raman experiments on the substrate interface region of wurtzite GaN layers. We show that the broad photoluminescence band with an intensity maximum at 2.4 eV is not an intrinsic property of GaN. We found that this photoluminescence band is strong only near the interface. Our investigations reveal that both the substrate interface and a region of structural reorientation of the layer near the interface act as a source of the photoluminescence.
TL;DR: In this paper, a broad, below-gap photoluminescence excitation (PLE) spectroscopy of Dy2S3-doped As12Ge33Se55 glasses is presented.
Abstract: Photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy of Dy2S3‐doped As12Ge33Se55 glasses demonstrate that the broad, below‐gap PLE mechanism observed previously for rare‐earth emissions in Er‐ and Pr‐doped chalcogenide glasses is a general or universal feature of rare‐earth‐doped chalcogenide glasses, provided the transition energies of the rare earths are in the correct energy range. The PL spectrum excited in the 815 nm Dy3+ absorption band shows the 1150, 1340, and 1700 nm sequence of Dy3+ transitions expected for Dy‐doped chalcogenide glasses. The PLE spectra of the 1340 (6F11/2,6H9/2→6H15/2) and 1700 nm (6H11/2→6H15/2) Dy3+ emissions exhibit broad excitation bands from ∼500 to 1000 nm, upon which the sharp intra F‐band absorptions of Dy3+ are superimposed. These broad PLE bands are characterized by an exponentially decreasing slope with decreasing energy in the spectral range below the Urbach edge which is associated with the below‐gap, defect‐ and impurity‐induced exponential t...
TL;DR: The conditions of validity of two different models relating the Stokes shift to the linewidth broadening have been established in terms of different regimes of disorder and temperature, and a continuous transition between those regimes has been demonstrated.
Abstract: Low temperature photoluminescence and photoluminescence excitation measurements have been performed in a set of ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs samples with a different indium molar fraction, well width, growth conditions, and post-growth treatment. This has allowed to change in a controlled way the degree and source of disorder in the samples, thus resulting in an excitonic absorption linewidth varying between 1 and 18 meV, and an ensuing Stokes shift changing between zero and 8 meV. The conditions of validity of two different models relating the Stokes shift to the linewidth broadening have been established in terms of different regimes of disorder and temperature. A continuous transition between those regimes has been demonstrated. \textcopyright{} 1996 The American Physical Society.
TL;DR: In this article, a spectroscopic study of GaAs:Er,O samples grown by metalorganic chemical vapor deposition, with excitation in the nearband-edge region, 1.22 −1.62 eV, was performed.
Abstract: We report a spectroscopic study of GaAs:Er,O samples grown by metalorganic chemical vapor deposition, with excitation in the near‐band‐edge region, 1.22–1.62 eV. Photoluminescence under host‐excitation is dominated by luminescence due to the Er‐2O center (an erbium atom at a gallium site coupled with two adjacent oxygen atoms) in these samples. The characterization of different Er centers is demonstrated by the observation of 4I15/2→4I11/2 intra‐4f‐shell transitions by photoluminescence excitation (PLE) spectroscopy. The transitions between these crystal‐field‐split 4f‐shell levels are observed at ∼1.26 eV (∼980 nm) which is at an energy attainable by PLE spectroscopy with a Ti:Sapphire laser. Two lines are assigned to the Er‐2O center. This allows a semi‐quantitative measurement of the relative concentrations of different centers to be made. PLE spectroscopy was also employed to study the trap levels related to the Er‐2O center. However, we find no evidence for such a trap level in this energy range. The...
TL;DR: In this paper, the efficiency and time dependence of photoluminescence in a high electron affinity cyano-substituted derivative of poly (p-phenylenevinylene) were investigated.
TL;DR: In this article, a photoluminescence excitation (PLE) study was performed of Er•implanted porous Si with two different porosities, and the results provided the first direct experimental evidence that infrared photolUMinescence at 1.54 μm arises from Er3+ ions in porous Si and that ions are excited through the recombination of excess carriers spatially confined in Si nanograms.
Abstract: A photoluminescence excitation (PLE) study was performed of Er‐implanted porous Si with two different porosities. Erbium was implanted at a dose of 1×1015 cm−2 at 380 keV and the samples were annealed for 30 min at temperatures from 650 to 850 °C. We observed that PLE spectra from Er3+ at 1.54 μm are nearly identical to those from the visible‐emitting porous Si layers. Our results provide the first direct experimental evidence that infrared photoluminescence at 1.54 μm arises from Er3+ ions in porous Si and that ions are excited through the recombination of excess carriers spatially confined in Si nanograms.
TL;DR: In this paper, the conduction band offset in GaAs/InGaAs quantum wells has been determined to a value between 0.62 and 0.64, for a range of indium fractions.
Abstract: We report on characterization studies of high quality metal‐organic vapor phase epitaxy and molecular beam epitaxy grown GaAs/InGaAs quantum wells, set within p‐i‐n diodes, to determine the well widths, indium mole fractions, and conduction band offset. We present photocurrent spectra containing a larger number of transitions than revealed in photoluminescence or photoluminescence excitation experiments. The energies of these transitions have been modeled using a theoretical characterization tool known as ‘‘contouring,’’ which is used in this strained system for the first time. This has enabled determination of the conduction band offset in GaAs/InGaAs quantum wells, to a value between 0.62 and 0.64, for a range of indium fractions between 0.155 and 0.23. As a final, additional check on our results, we compare the field dependence of the e1‐hh1 exciton transition energy with our theoretical calculations and find good agreement.
TL;DR: In this article, the authors used photoluminescence to study residual transition metal contaminants in GaN layers, which are grown by the sandwich technique either on 6H•SiC substrate or on sapphire substrate.
Abstract: We use photoluminescence to study residual transition metal contaminants in GaN layers, which are grown by the sandwich technique either on 6H‐SiC substrate or on sapphire substrate. We observe three no‐phonon lines in the near infrared optical region at 1.3 eV, 1.19 eV, and 1.047 eV caused by 3d transition metals. The appearance of GaN related host modes in the phonon sideband of these emissions proves that the luminescence centers are incorporated in the hexagonal GaN layers. In this paper we especially focus on the luminescence band with the no‐phonon line at 1.047 eV. Temperature dependent photoluminescence measurements reveal an excited state splitting of 8 meV. In photoluminescence excitation spectroscopy we observe a further excited state at 1.6 eV with a fine structure splitting. The appearance of this excited state in the n‐type samples gives evidence that the defect must already exist in its luminescent charge state without illumination. The experimental results on the 1.047 eV emission fit to a...
TL;DR: The high cross section, photostability, and clear cos6θ photoselection of p-QT make it an ideal three-photon standard for spectroscopy and microscopy.
Abstract: We observed emission fromp-quaterphenyl (p-QT) at 360 nm when exposed to the focused light from a femtosecond (fs) Ti:sapphire laser at 850 nm. This wavelength is too long to allow two-photon excitation of p-QT. The emission intensity of p-QT was found to depend on the cube of the laser power at 850 nm, suggesting that excitation occurs due to a three-photon process. The same emission spectrum and single exponential decay times were observed for three-photon excitation at 850 nm as for two-photon excitation at 586 nm and for one-photon excitation at 283 nm. The same rotational correlation times were observed for one-, two-, and three-photon excitation, but higher time-zero anisotropies were observed for two- and three-photon excitation. The steady-state anisotropies for one-, two-, and three-photon excitation are precisely consistent with cos(2)θ, cos(4)θ, and cos(6)θ excitation photoselection, where θ is the angle between the electric field of the incident light and the absorption dipole. These experiments were performed with 3×10(-5) M solutions of p-QT. Use of such low concentrations was possible because p-QT displays one of the highest apparent cross sections we have observed to date for three-photon excitation. The spatial distribution of the excited fluorescence was less for three-photon excitation than for two-photon excitation of Coumarin 102 at the same 850-nm excitation wavelength. The high cross section, photostability, and clear cos(6)θ photoselection of p-QT make it an ideal three-photon standard for spectroscopy and microscopy.