Showing papers on "Spontaneous emission published in 1997"
TL;DR: In this paper, the authors measured microcavity resonances in two-and three-dimensional photonic-bandgap (PBG) structures integrated directly into a sub-micrometre-scale silicon waveguide.
Abstract: Confinement of light to small volumes has important implications for optical emission properties: it changes the probability of spontaneous emission from atoms, allowing both enhancement and inhibition. In photonic-bandgap (PBG) materials1,2,3,4 (also known as photonic crystals), light can be confined within a volume of the order of (λ/2n)3, where λ is the emission wavelength and n the refractive index of the material, by scattering from a periodic array of scattering centres. Until recently5,6, the properties of two- and three-dimensional PBG structures have been measured only at microwave frequencies. Because the optical bandgap scales with the period of the scattering centres, feature sizes of around 100 nm are needed for manipulation of light at the infrared wavelength (1.54 µm) used for optical communications. Fabricating features this small requires the use of electron-beam or X-ray lithography. Here we report measurements of microcavity resonances in PBG structures integrated directly into a sub-micrometre-scale silicon waveguide. The microcavity has a resonance at a wavelength of 1.56 µm, a quality factor of 265 and a modal volume of 0.055 µm3. This level of integration might lead to new photonic chip architectures and devices, such as zero-threshold microlasers, filters and signal routers.
979 citations
TL;DR: In this article, structural analysis was performed on a purple laser diode composed of In0.20Ga0.80N (3 nm)/In0.05Ga 0.95N (6 nm) multiple quantum wells.
Abstract: Structural analysis was performed on a purple laser diode composed of In0.20Ga0.80N (3 nm)/ In0.05Ga0.95N (6 nm) multiple quantum wells, by employing transmission electron microscopy and energy-dispersive x-ray microanalysis, both of which are assessed from the cross-sectional direction. It was found that the contrast of light and shade in the well layers corresponds to the difference in In composition. The main radiative recombination was attributed to excitons localized at deep traps which probably originate from the In-rich region in the wells acting as quantum dots. Photopumped lasing was observed at the high energy side of the main spontaneous emission bands.
850 citations
TL;DR: In this article, a thin slab of two-dimensional photonic crystal is shown to alter drastically the radiation pattern of spontaneous emission, and spontaneous emission can be coupled entirely to free space modes, resulting in a greatly enhanced extraction efficiency.
Abstract: A thin slab of two-dimensional photonic crystal is shown to alter drastically the radiation pattern of spontaneous emission. More specifically, by eliminating all guided modes at the transition frequencies, spontaneous emission can be coupled entirely to free space modes, resulting in a greatly enhanced extraction efficiency. Such structures might provide a solution to the long-standing problem of poor light extraction from high refractive-index semiconductors in light-emitting diodes.
637 citations
TL;DR: In this paper, the authors introduce a new way of band-gap engineering in which they expose a semiconductor quantum well of a direct gap material to a moving potential superlattice modulated in the plane of the well.
Abstract: The dynamics of photogenerated carriers in semiconductor structures with reduced dimensionality has been the subject of intensive investigations in recent years [1,2]. State-of-the-art band-gap engineering technologies enable us to tailor low-dimensional semiconductor systems with desirable optoelectronic properties and study the fundamental aspects of carrier dynamics. This has increased tremendously our fundamental understanding of the dynamic properties of artificial semiconductor structures and has also resulted in a wide range of novel devices such as quantum well lasers, modulators, and detectors, as well as all-optical switches. Nevertheless, the bulk band structure of semiconductors seems to dominate optoelectronic properties since the strength of interband transitions is largely governed by the atomiclike Bloch parts of the wave function [3]. Thus it appears at first glance unavoidable that strong interband optical transitions are linked to direct band-gap semiconductors with short radiative lifetimes such as GaAs, whereas long radiative lifetimes of photogenerated carriers imply utilization of semiconductors with indirect band gaps such as Si and correspondingly reduced interband absorption. Initial attempts to employ band-gap engineering in order to combine strong interband absorption with long radiative lifetimes have focused on so-called doping superlattices [4]. There, alternate n and p doping along the growth direction is utilized to combine a direct gap in momentum space with an indirect gap in real space which causes a spatial separation of photogenerated electron-hole se-hd pairs and hence considerably prolonged lifetimes. Here, we introduce a new way of band-gap engineering in which we expose a semiconductor quantum well of a direct gap material to a moving potential superlattice modulated in the plane of the well. We show that the confinement of photogenerated e-h pairs to two dimensions, together with the moving lateral superlattice, allows reversible charge separation [5]. We demonstrate that the combination of both the advantages of strong interband absorption and extremely long lifetimes of the optical excitations is achieved without affecting the superior optical quality of the quantum well material. The spatial separation of the electron-hole pairs is achieved via the piezoelectric potential of acoustic waves propagating along the surface of a semiconductor quantum well system. On a piezoelectric substrate, the elliptically polarized surface acoustic waves (SAWs) are accompanied by both lateral and vertical piezoelectric fields which propagate at the speed of sound. Those fields can be strong enough to field ionize optically generated excitons and to confine the resulting electrons and holes in the moving lateral potential wells separated by one-half wavelength of the SAW. The spatial separation dramatically reduces the recombination probability and increases the radiative lifetime by several orders of magnitude as compared to the unperturbed case. We further demonstrate that the dynamically trapped electron-hole pairs can be transported over macroscopic distances at the speed of sound and that deliberate screening of the lateral piezoelectric fields of the SAW leads to an induced radiative recombination after long storage times at a location remote from the one of e-h generation. This conversion of photons into a long lived e-h polarization which is efficiently reconverted into photons can serve as an optical delay line operating at sound velocities. The undoped quantum well samples used in our experiments are grown by molecular beam epitaxy on a (100)GaAs substrate. The quantum well consists of 10 nm pseudomorphic In0.15Ga0.85As grown on a 1 mm thick GaAs buffer and is covered by a 20 nm thick GaAs cap layer. The active area of the sample is etched into a 2.5 mm long and 0.3 mm wide mesa (see inset of Fig. 1) with two interdigital transducers (IDTs) at its ends. The IDTs are designed to operate at a center frequency fSAW › 840 MHz. They are partially impedance matched to the 50 V radio frequency (rf) circuitry using an on-chip matching network, thus reducing the insertion
258 citations
TL;DR: In this paper, the authors presented a preliminary guide to realize microcavity semiconductor lasers exhibiting spontaneous emission control effects, which includes theoretical consideration on the effects; processing techniques for semiconductor micro-cavities; and some demonstrations of photonic crystal and microdisk cavity.
Abstract: This paper presents a preliminary guide to realize microcavity semiconductor lasers exhibiting spontaneous emission control effects. It includes: 1) theoretical consideration on the effects; 2) processing techniques for semiconductor microcavities; and 3) some demonstrations of photonic crystal and microdisk cavity. It was shown that, even with a spectral broadening of electron transition, thresholdless lasing operation and alternation of spontaneous emission rate are expected in a cavity satisfying the single mode condition that only one mode is allowed in the transition spectrum. An ideal three-dimensional (3-D) photonic crystal has the potentiality for realizing this condition. In two-dimensional (2-D) crystals and microdisk cavities, thresholdless operation is also expected, but the alternation of spontaneous emission rate may be negligible due to the insufficient optical confinement. In the experiment, some processing techniques for GaInAsP-InP system were investigated and methane-based reactive ion beam etching was selected because of the smooth sidewalls and adaptability to arbitrary structures. A GaInAsP-InP 2-D photonic crystal constructed by submicron columns was fabricated using this method. Owing to the slow surface recombination of this material, a polarized photoluminescence and peculiar transmission spectra were observed at room temperature (RT), which can be explained by a photonic band calculation. However, some technical improvement is necessary for clear demonstration of photonic bandgap, which is minimally required for device applications. In contrast to this, a GaInAsP-InP microdisk cavity of 2 /spl mu/m in diameter, which corresponds to the cavity volume 2.5 times the single-mode condition, has achieved RT lasing with threshold current as low as 0.2 mA. Further reduction of diameter and realization of continuous-wave (CW) operation will provide a significant regime for the observation of spontaneous emission control effects.
239 citations
TL;DR: In this article, the authors investigate the possibility of all optical trapping and cooling a single atom at the antinodes of a high Q optical cavity mode to which the atom is strongly coupled.
Abstract: We investigate the possibility of all optical trapping and cooling a single atom at the antinodes of a high Q optical cavity mode to which the atom is strongly coupled. For properly chosen parameters the dynamics of the cavity field introduces a novel Sisyphus type cooling mechanism yielding final temperatures much below the Doppler limit and allowing for long trapping times, avoiding the problems induced by spontaneous emission. {copyright} {ital 1997} {ital The American Physical Society}
225 citations
TL;DR: In this paper, the authors present quantitative measurements of the interaction between a guided optical wave and a two-dimensional photonic crystal using spontaneous emission of the material as an internal point source.
Abstract: We present quantitative measurements of the interaction between a guided optical wave and a two-dimensional photonic crystal using spontaneous emission of the material as an internal point source. This is the first analysis at near-infrared wavelengths where transmission, reflection, and inplane diffraction are quantified at the same time. Low transmission coincides with high reflection or in-plane diffraction, indicating that the light remains guided upon interaction. Also, good qualitative agreement is found with a two-dimensional simulation based on the transfer matrix method. [S0031-9007(97)04591-2]
202 citations
TL;DR: In this paper, the spontaneous emission rate of Eu{sup 3+} ions depends upon both the distance from, and thickness of, a silver film, and it is shown that as the silver film thickness is reduced, the well established influence of the mirror on the spontaneous emissions is further modified.
Abstract: We have experimentally determined how the spontaneous emission rate of Eu{sup 3+} ions depends upon both the distance from, and thickness of, a silver film. We show that as the silver film thickness is reduced ({lt} 100 nm) the well established influence of the mirror on the spontaneous emission rate is further modified. By comparing our data to theory we show that this is due to the availability of an additional decay channel, the surface-plasmon polariton on the far side of the metal film. {copyright} {ital 1997} {ital The American Physical Society}
196 citations
TL;DR: In this article, coherent control of spontaneous emission from a three-level atom with one resonant frequency near the edge of a photonic band gap was demonstrated, where spontaneous emission can be totally suppressed or strongly enhanced depending on the relative phase between the control and pump laser fields.
Abstract: We demonstrate coherent control of spontaneous emission from a three-level atom with one resonant frequency near the edge of a photonic band gap. As a result of quantum interference and photon localization, spontaneous emission can be totally suppressed or strongly enhanced depending on the relative phase between the control and pump laser fields. The fractionalized steady state inversion of the atom depends sensitively on the initial conditions, suggesting the possibility of a phase-sensitive, optical memory device on the atomic scale.
181 citations
TL;DR: In this paper, the spontaneous emission from a three-level atom embedded in a photonic band gap structure is studied, where interference between two transitions leads to quasiperiodic oscillations of population between the two upper levels with large amplitudes.
Abstract: The spontaneous emission from a three-level atom embedded in a photonic band gap structure is studied. Interference between two transitions leads to quasiperiodic oscillations of population between the two upper levels with large amplitudes. The spontaneous emission of the atom is characterized by three components in the radiated field: a localized part, a traveling pulse, and a $(1/\sqrt{t}{)}^{3}$ decaying part. An analytical expression for the localization distance of the localized field is obtained. The energy velocity for the traveling pulse could be close to zero. By selecting an appropriate initial superposition state, a large amount of population trapping can be achieved.
171 citations
TL;DR: In this paper, the impact of selective quantum-well intermixing in the GaAs-AlGaAs system is studied, and it is shown that the leakage loss contributed by the GAAs cap layer is significant and increases with wavelength.
Abstract: Impurity-free vacancy disordering (IFVD) using SiO/sub 2/ and SrF/sub 2/ dielectric caps to induce selective quantum-well (QW) intermixing in the GaAs-AlGaAs system is studied. The intermixing rate of IFVD was found to be higher in n-i-p and intrinsic than in p-i-n structures, which suggests that the diffusion of the Group III vacancy is not supported in p-type material. Single-mode waveguides have been fabricated from both as-grown and bandgap-tuned double-quantum-well (DQW) laser samples. Propagation losses as low as 8.5 dB cm/sup -1/ were measured from the bandgap-tuned waveguides at the lasing wavelength of the undisordered material, i.e., 860 nm. Simulation was also carried out to study the contribution of free-carrier absorption from the cladding layers, and the leakage loss induced by the heavily p-doped GaAs contact layer. It was found that the leakage loss contributed by the GaAs cap layer is significant and increases with wavelength. Based on IFVD, we also demonstrate the fabrication of multiple-wavelength lasers and multichannel wavelength division multiplexers using the one-step "selective intermixing in selected area" technique. This technique enables one to control the degree of intermixing across a wafer. Lasers with bandgaps tuned to five different positions have been fabricated on a single chip. These lasers showed only small changes in transparency current, internal quantum efficiency, or internal propagation loss, which indicates that the quality of the material remains high after being intermixed. Four-channel wavelength demultiplexers based on a waveguide photodetector design have also been fabricated. Photocurrent and spontaneous emission spectra from individual diodes showed that the absorption edge was shifted by different degrees due to the selective degree of QW intermixing. The results obtained also imply that the technique can be used in the fabrication of broad-wavelength emission superluminescent diodes.
TL;DR: In this paper, a planar microcavity light emitting diodes with different device diameters were used to achieve a record external quantum efficiency of 20% for a 1.5 mm light emitting diode and a good correspondence was obtained when spectral broadening and photon recycling were taken into account.
Abstract: Results are presented on planar microcavity light emitting diodes with different device diameters. A record external quantum efficiency of 20% is achieved for a 1.5 mm light emitting diode. The strong dependence of the quantum efficiency on current density and device size are compared with theoretical results. A good correspondence is obtained when spectral broadening and photon recycling are taken into account.
TL;DR: In this paper, the authors obtained pulsed lasing operation in 2-5/spl mu/m diameter microdisk injection lasers using GaInAsP-InP compressively-strained multiple-quantum-well (MQW) wafers around room temperature.
Abstract: We have obtained pulsed lasing operation in 2-5-/spl mu/m diameter microdisk injection lasers using GaInAsP-InP compressively-strained multiple-quantum-well (MQW) wafers around room temperature. The effective cavity volume of the 2-/spl mu/m-diameter device is the smallest among those for any type of electrically-pumped lasers. The threshold current of this device was as low as 0.2 mA. Cavity modes in emission spectra observed under CW conditions coincide well with theoretically predicted whispering gallery modes. Further reduction of diameter to less than 1.5 /spl mu/m will realize the condition for spontaneous emission almost coupling into a single mode, which results in thresholdless lasing operation.
TL;DR: In this paper, the population inversion on some specific atomic transitions can be created using very weak incoherent pumping, which may be useful in principle for generation of high-frequency and/or high power laser light.
Abstract: A four-level atom, driven by a coherent field, is considered. We show that under certain conditions complete quenching of spontaneous emission is possible. Hence the population inversion on some specific atomic transitions can be created using a very weak incoherent pumping. We investigate the physics of the effect using bare and dressed states. The proposed scheme may be useful, in principle, for generation of high-frequency and/or high power laser light.
TL;DR: In this paper, the preparation and purification of GeGa-S glasses were studied and the effects of Ga and S contents on glass transition and crystallization temperatures, density, refractive index and spontaneous emission probabilities from the 4 F 3 2 level of Nd 3+ were examined.
Abstract: The preparation and purification of GeGaS glasses were studied. A purified glass doped with 1000 ppm Pr 3+ exhibits a quantum efficiency of 50% in the 1 G 4 → 3 H 5 transition at 1.3 μm based on fluorescence lifetime measurement and Judd-Ofelt analysis. The effects of Ga and S contents on glass transition and crystallization temperatures, density, refractive index and spontaneous emission probabilities from the 4 F 3 2 level of Nd 3+ were examined. Compositional dependences of properties are discussed in terms of bond types.
TL;DR: In this paper, the authors analyzed the internal differential efficiency of quantum-well lasers in terms of current spreading, carrier injection into the well, and the radiative efficiency within the well.
Abstract: We have analyzed the internal differential efficiency of quantum-well lasers in terms of current spreading, carrier injection into the well, and the radiative efficiency within the well. We quantify the first two of these processes by extracting information from spontaneous emission measurements as a function of device length, current, and temperature. We show that the carrier injection efficiency is responsible for the temperature dependence of the external differential efficiency of GaInP quantum-well (QW) lasers by comparing values from the slope of the laser power output versus current characteristic with our experimental values for current spreading and injection efficiency.
TL;DR: In this paper, the mean interaction strength between ultracold atoms using an appropriately detuned laser was investigated and it was shown that useful changes of the mean field interaction require sufficiently high laser intensities that the rate of laser-induced stimulated emission exceeds the natural spontaneous emission rate.
Abstract: We explore a recent proposal [Fedichev et al., Phys. Rev. Lett. 77, 2913 (1996)] for altering the mean interaction strength between ultracold atoms using an appropriately detuned laser. Although care must be taken to minimize laser-driven loss processes, we find large ranges of intensities and detunings where useful changes might be affected. Accordingly, we present simple formulas for the effects of laser light that should prove useful in designing specific experiments. We demonstrate the validity of these formulas by comparison with exact close-coupling models. In particular, we find that useful changes of the mean-field interaction require sufficiently high laser intensities that the rate of laser-induced stimulated emission exceeds the natural spontaneous emission rate.
TL;DR: In this paper, the phase change on reflection of incident light is dependent on the wavelength, the angle of incidence, the type of metal, and the metal thickness, which can be exploited to improve the performance of planar metal mirror microcavities.
Abstract: In thin metal films the phase change on reflection of incident light is dependent on the wavelength, the angle of incidence, the type of metal, and the metal thickness. These properties have been exploited to improve the performance of planar metal mirror microcavities. We model substantial alteration of peak emission wavelength and linewidth with mirror thickness. This allows the tuning of the cavity resonance wavelength by variation of metal mirror thickness. The dependence of the phase change on wavelength and angle of incidence can also be used to suppress the angular dependence of the cavity resonance wavelength. These effects are observed in silver-mirrored cavities containing the polymers poly(p-phenylene vinylene), (PPV), and a cyano-substituted derivative of PPV, MEH-CN-PPV.
TL;DR: In this paper, the authors report on the band-edge stimulated emission in InGaN-GaN multiple quantum well light-emitting diodes with varying widths and barrier thicknesses of the quantum wells.
Abstract: We report on the band-edge stimulated emission in InGaN–GaN multiple quantum well light-emitting diodes with varying widths and barrier thicknesses of the quantum wells. In these devices, we observe that the stimulated emission peak wavelength shifts to shorter values with decreasing well thickness. From the comparison of the results of the quantum mechanical calculations of the subbands energies with the measured data, we estimate the effective conduction- and valence-band discontinuities at the GaN–In0.13Ga0.87N heterointerface to be approximately 130–155 and 245–220 meV, respectively. We also discuss the effect of stress on the estimated values of band discontinuities.
TL;DR: In this article, the authors studied the ultrafast properties of secondary radiation of semiconductor quantum wells under resonant excitation and showed that the exciton density dependence allows one to identify the origin of the secondary radiation.
Abstract: We study the ultrafast properties of secondary radiation of semiconductor quantum wells under resonant excitation. We show that the exciton density dependence allows one to identify the origin of secondary radiation. At high exciton densities, the emission is due to incoherent luminescence with a rise time determined by exciton-exciton scattering. For low densities, when the distance between excitons is much larger than their diameter, the temporal shape is independent of density and rises quadratically, in excellent agreement with recent theories for resonant Rayleigh scattering.
TL;DR: In this paper, the authors measured the electroluminescence (EL) spectra of GaAs/InGaAs and AlGaAs/GaAs single quantum well (QW) p-i-n photodiodes at temperatures between 200 and 300 K and forward biases close to the open circuit voltage.
Abstract: We have measured electroluminescence (EL) spectra of GaAs/InGaAs and AlGaAs/GaAs single quantum well (QW) p-i-n photodiodes at temperatures between 200 and 300 K and forward biases close to the open circuit voltage. Integrated EL spectra vary like eqV/nkT with an ideality factor n=1.05±0.05 over five decades, indicating purely radiative processes. The spectra are calibrated into absolute units enabling comparison to be made with the predictions of a theoretical model. For each temperature and bias we calculate the EL spectrum and radiative current expected in the detailed balance limit, integrating the theoretical emission spectrum over the surface of the device, in order to establish the quasi-Fermi potential separation, Δφf, in the QW and, where possible, in the host material. For the GaAs/InGaAs cell we are able to model emission from the QW and the host material simultaneously. We find that, in all cases, the QW emission is overestimated by theory if it is assumed that Δφf=V. QW emission corresponds instead to a value of Δφf which a few tens of mV less than V. In contrast, emission from the host material, where visible, is well fitted by the model with Δφf=V at all biases and temperatures. We attribute the variation in Δφf to irreversible thermally assisted escape from the QWs.
TL;DR: In this paper, the authors report spatially resolved photoluminescence images of excitons confined to an isolated gallium arsenide quantum wire, showing that at sufficiently low temperatures, the quantum wire acts like a sparse set of quantum dots.
Abstract: Bound states of electron–hole pairs (excitons) in semiconductors possess desirable properties — such as an enhanced oscillator strength for radiative recombination — that hold promise for the next generation of optical devices. However, at typical device operating conditions (room temperature and moderate charge densities), excitons dissociate to form an electron–hole plasma. Dissociation may be prevented by confining excitons to lower dimensions, where their binding energy is expected to increase significantly1. But such confinement may in turn influence the dynamical properties of the excitons. Here we report spatially resolved photoluminescence images of excitons confined to an isolated gallium arsenide quantum wire. As the temperature of the structure is lowered, we observe a striking transition from broad and fairly continuous photoluminescence to an intense set of emission peaks which are both energetically sharp and spatially localized. Such behaviour indicates that, at sufficiently low temperatures, the quantum wire acts like a sparse set of quantum dots. Furthermore, at the site of an isolated quantum dot, we observe an unusual decrease in the relaxation rate of excitons, such that they radiate (via recombination) from higher energy states before relaxing to their ground state. We argue that this is the manifestation of an exciton relaxation ‘bottleneck’, the existence of which could pose problems for the development of optical devices based on quantum dots.
TL;DR: In this paper, the optical properties of two-dimensional photonic crystals combined with a waveguide geometry, and etched into a light-emitting (GaAs/InGaAs) semiconductor are assessed.
Abstract: We describe an experimental setup, which allows assessing the optical properties of two-dimensional photonic crystals combined with a waveguide geometry, and etched into a light-emitting (GaAs/InGaAs) semiconductor By means of a guiding layer, the spontaneous emission of the material is used as a built-in source to probe the properties of the etched microstructure, conveniently compared to the usual measurement schemes We show polarized transmission and coefficients largely depending on the photonic crystal orientation
TL;DR: In this paper, the formation of a 7×7×7 nm3 size GaAs quantum dot (QD) at the intersection of three quantum wells is demonstrated for the first time.
Abstract: The formation of a 7×7×7 nm3 size GaAs quantum dot (QD) at the intersection of three quantum wells is demonstrated for the first time. Intense radiative recombination between zero-dimensional states in the QDs is clearly identified by microscopic photoluminescence ( μPL). In contrast to the inhomogeneously broadened quantum well and quantum wire signals originating from the complex twofold cleaved edge overgrowth structure, the strongly spatially localized QD response is characterized by spectrally sharp lines in μPL excitation spectra with a linewidth below 70 μeV.
TL;DR: In this article, optically pumped GaN quantum dots in an AlxGa1−xN separate confinement heterostructure fabricated on 6H-SiC(0001) substrate by metal organic chemical vapor deposition were observed.
Abstract: Stimulated emission was observed from optically pumped GaN quantum dots in an AlxGa1−xN separate confinement heterostructure fabricated on 6H-SiC(0001) substrate by metal organic chemical vapor deposition. Nanostructural GaN quantum dots, with an average size of ∼10 nm width, ∼1–2 nm height, and density of ∼1011 cm−2, were self-assembled on the AlxGa1−xN cladding layer surface. The stimulated emission peak was observed at ∼3.48 eV, which is ∼50 meV lower than that of spontaneous emission. The excitation power dependence on the emission intensity clearly indicates threshold pump power density of 0.75 MW/cm2 for the onset of stimulated emission.
TL;DR: In this paper, the effect of a static electric field on the photoluminescence (PL) of ladder-type poly(paraphenylene) (m-LPPP) in devices of polymer-light-emitting diodes was investigated.
Abstract: We report on the effect of a static electric field on the photoluminescence (PL) of laddertype poly(paraphenylene) (m-LPPP) in devices of polymer-light-emitting diodes. External electric fields of 4.5 MV/cm reduce the integral PL intensity down to 24% of the zero-field value and alter the shape of the PL spectra. The field-induced PL quenching in the m-LPPP polymer is ascribed to field dissociation of the emitting species. Time-resolved spectroscopy showed that the emission is composed of contributions from the radiative recombination of free singlet excitons and of a second emitting species, which we describe as self-trapped excitons. The observed field-induced changes of the shape of the PL emission can be attributed to the different field response of these two emitting species. Thus the PL emission color can be changed with the electric field in the blue-green spectral range. The magnitude of the binding energy of free singulet excitons is discussed.
TL;DR: In this article, the electrical and optical properties of blue light-emitting diodes (LEDs) based on III-V nitrides were measured by means of the relation I=I0 exp(qV/14 KT) in this equation α is temperature independent, suggesting a process of conduction by tunneling.
Abstract: We have measured the electrical and optical properties of blue light-emitting diodes (LEDs) based on III–V nitrides The current–voltage characteristic is described by means of the relation I=I0 exp(αV) In this equation α is temperature independent, suggesting a process of conduction by tunneling, as was recently reported also for blue-green LEDs based on III–V nitrides [Appl Phys Lett 68, 2867 (1996)] We explain the differences between blue and blue-green devices taking into account the tunneling process across semiconductor interfaces, in which a great number of defects is present The light output intensity of the LED as a function of junction–voltage data reveals a dependence on the junction–voltage of the type L=L0 exp(qV/14 KT), indicating that the radiative recombination path is via deep levels located at the forbidden gap Furthermore, we find that the light output–current characteristic follows a power law like L∝Ip From the analysis of data it appears that, contrary to expectations, the n
TL;DR: In this paper, a detailed analysis of radiative recombination processes in CuInSe2 thin films grown by multisource physical vapor deposition is performed as a function of stoichiometry, temperature and excitation intensity.
Abstract: We perform a detailed analysis of the radiative recombination processes in CuInSe2 thin films grown by multisource physical vapor deposition The photoluminescence and photoluminescence excitation spectra are investigated as a function of stoichiometry, temperature and excitation intensity Using samples with a large composition gradient, we are able to obtain a coherent picture of the optical transitions in the films The broad–band photoluminescence spectrum typical for In–rich films breaks into a number of well–defined emission lines in Cu–rich CuInSe2 At low temperatures, emission peaks due to free–exciton, bound–exciton, and free–to–bound recombination are identified in Cu–rich films The spectra of In–rich films tend to be dominated by donor–acceptor transitions From the optical spectra, exciton ionization energies and the temperature dependence of the band gap are determined The observed optical transitions are related to intrinsic defectsWe perform a detailed analysis of the radiative recombination processes in CuInSe2 thin films grown by multisource physical vapor deposition The photoluminescence and photoluminescence excitation spectra are investigated as a function of stoichiometry, temperature and excitation intensity Using samples with a large composition gradient, we are able to obtain a coherent picture of the optical transitions in the films The broad–band photoluminescence spectrum typical for In–rich films breaks into a number of well–defined emission lines in Cu–rich CuInSe2 At low temperatures, emission peaks due to free–exciton, bound–exciton, and free–to–bound recombination are identified in Cu–rich films The spectra of In–rich films tend to be dominated by donor–acceptor transitions From the optical spectra, exciton ionization energies and the temperature dependence of the band gap are determined The observed optical transitions are related to intrinsic defects
TL;DR: In this paper, a continuous-wave operation of bluish-purple InGaN multiquantum-well (MQW)-structure laser diodes (LDs) was achieved at room temperature with a lifetime of 35 h.
Abstract: High-power InGaN single-quantum-well (SQW) structure blue/green light-emitting diodes (LEDs) with an output power of 3-5 mW were fabricated. The continuous-wave operation of bluish-purple InGaN multiquantum-well (MQW)-structure laser diodes (LDs) was achieved at room temperature with a lifetime of 35 h. The threshold current and the voltage of the LD were 80 mA and 5.5 V, respectively. Photocurrent spectra of the InGaN SQW LEDs and MQW LDs were measured at room temperature. The Stokes shifts of the energy difference between the absorption and the emission energy of the blue/green InGaN SQW LED's and MQW LDs were 290, 570, and 190 meV. Both spontaneous and stimulated emission of the LDs originated from this deep localized energy state which is equivalent to a quantum dot-like state. When the temperature or the operating current of the LDs was varied, large mode hopping of the emission wavelength was observed. The carrier lifetime and the threshold carrier density were estimated to be 4.7 ns and 1/spl times/10/sup 20//cm/sup 3/, respectively. The optical confinement factor (/spl Gamma/) of the InGaN MQW LDs was estimated to be 0.025 from the measurement of the near-field radiation patterns.
TL;DR: The strong mid-IR emission and spectral properties of Pr 3+ doped BaInGaGeSe chalcogenide glasses make this glass a strong candidate for lasers, amplifiers, and high brightness sources in the mid- IR.
Abstract: The spectral properties of Pr3+ doped BaInGaGeSe chalcogenide glasses are reported. Absorption spectra, emission spectra, and lifetimes of the lower lying manifolds have been measured. Radiative transition rates are calculated and compared with measured experimental lifetimes. The strong mid-IR emission and spectral properties of this glass make this glass a strong candidate for lasers, amplifiers, and high brightness sources in the mid-IR.