High-power quantum-dot superluminescent LED with broadband drive current insensitive emission spectra using a tapered active region

High-power and broadband quantum-dot (QD) superluminescent light-emitting diodes are realized by using a combination of self-assembled QDs with a high density, large inhomogeneous broadening, a tapered angled pump region, and etched V groove structure. This broad-area device exhibits greater than 70-nm 3-dB bandwidth and drive current insensitive emission spectra with 100-mW output power under continuous-wave operation. For pulsed operation, greater than 200-mW output power is obtained.

Single-photon emission from a single InAs quantum dot

Excitation power-dependent micro-photoluminescence spectra and photon-correlation measurement are used to study the optical properties and photon statistics of single InAs quantum dots. Exciton and biexciton emissions, whose photoluminescence intensities have linear and quadratic excitation power dependences, respectively, are identified. Under pulsed laser excitation, the zero time delay peak of second order correlation function corresponding to exciton emission is well suppressed, which is a clear evidence of single photon emission.

Fabry-Perot microcavity modes observed in the micro-photoluminescence spectra of the single nanowire with InGaAs/GaAs heterostructure

We report on the fabrication of the nanowires with InGaAs/GaAs heterostructures on the GaAs(111) B substrate using selective-area metal organic vapor phase epitaxy. Fabry-Perot microcavity modes were observed in the nanowires with perfect end facets dispersed onto the silicon substrate and not observed in the free-standing nanowires. We find that the calculated group refractive indices only considering the material dispersion do not agree with the experimentally determined values although this method was used by some researchers. The calculated group refractive indices considering both the material dispersion and the waveguide dispersion agree with the experimentally determined values well. We also find that Fabry-Perot microcavity modes are not observable in the nanowires with the width less than about 180 nm, which is mainly caused by their poor reflectivity at the end facets due to their weak confinement to the optical field. (C) 2009 Optical Society of America

Two-photon excited fluorescence from CdSe quantum dots on SiN photonic crystals

Two-photon excited fluorescence from CdSe quantum dots on a two-dimensional SiN photonic crystal surface is investigated by using a femtosecond laser. By using a photonic crystal, a 90-fold enhancement in the two-photon excited fluorescence in the vertical direction is achieved. This is the highest enhancement achieved so far in the two-photon excited fluorescence in the vertical direction. The mechanism of the enhancement for two-photon excited fluorescence from quantum dots on photonic crystals is analyzed.

Relationship between the electric performance and the photoluminescence spectra of resonant tunnelling diodes

Resonant tunnelling diodes with different structures were grown. Their photoluminescence spectra were investigated. By contrast, the luminescence in the quantum well is separated from that of other epilayers. The result is obtained that the exciton of the luminescence in the quantum well is partly come from the cap layer in the experiment. So the photoluminescence spectrum is closely related to the electron transport in the resonant tunnelling diode structure. This offers a method by which the important performance of resonant tunnelling diode could be forecast by analysing the integrated photoluminescence intensities.

Fourier series expansion method for gain measurement from amplified spontaneous emission spectra of Fabry-Perot semiconductor lasers

A gain measurement technique, based on Fourier series expansion of periodically extended single fringe of the amplified spontaneous emission spectrum, is proposed for Fabry-Perot semiconductor lasers. The underestimation of gain due to the limited resolution of the measurement system is corrected by a factor related to the system response function. The standard deviations of the gain-reflectivity product under low noise conditions are analyzed for the Fourier series expansion method and compared with those of the Hakki-Paoli method and Cassidy's method. The results show that the Fourier series expansion method is the least sensitive to noise among the three methods. The experiment results obtained by the three methods are also presented and compared.

Photoluminescence and transmission spectra of nanocrystalline GaAs1-xSbx embedded in silica films

The composite films of the nanocrystalline GaAs1-xSbx-SiO2 have been successfully deposited on glass and GaSb substrates by radio frequency magnetron co-sputtering. The 10K photoluminescence (PL) properties of the nanocrystalline GaAs1-xSbx indicated that the PL peaks of the GaAs1-xSbx nanocrystals follow the quantum confinement model very closely. Optical transmittance spectra showed that there is a large blue shift of optical absorption edge in nanocrystalline GaAs1-xSbx-SiO2 composite films, as compared with that of the corresponding bulk semiconductor, which is due to the quantum confinement effect.

A comparison between contactless electroreflectance and photoreflectance spectra from n-doped GaAs on a semi-insulating GaAs substrate

Contactless electroreflectance (CER) and photoreflectance (PR) measurements have been performed on samples with the structure of an n-doped GaAs epitaxial layer on a semi- insulating GaAs substrate. Modulated reflectance signals from the n-GaAs surface and those from the n-GaAs/SI-GaAs interface are superposed in PR spectra. For the case of CER measurement, however, Franz-Keldysh oscillations (FKOs) from the interface, which are observed in PR spectra, cannot be detected. This discrepancy is attributed to different modulation mechanisms of CER and PR. In CER experiments, the electric field modulation cannot be added to the interfacial electric field because of the effective screening by the fast response of carriers across the interface. FKOs from the interface without any perturbation by the surface signals are extracted by subtracting CER spectra from PR spectra.

Estimation of frequency response of directly modulated lasers from optical spectra

A simple method for estimating the frequency responses of directly modulated lasers from optical spectra is presented. The frequency-modulation index and intensity-modulation index of a distributed feedback laser can be obtained through the optical spectrum analyses. The main advantage is that the measurement setup is very simple. Only a microwave source and an optical spectrum analyser are needed and there is no need to use a calibrated broadband photodetector. Experiment shows that the proposed method is as accurate as the swept frequency method using a network analyzer and is applicable to a wide range of modulation powers.

Study on Raman spectra of GaMnAs

Raman spectra of diluted magnetic semiconductor GaMnAs alloy were reported. The coupled plamon-LO-phonon ( CPLP) mode has LO-like polarization properties. With increasing Mn concentration, the CPLP mode shifts to low frequency. The hole density in the alloy was determined from the ratio of the intensity of the CPLP mode to that of the unscreened LO mode in the depletion layer. The hole density increases with the increase of the Mn composition. The Raman spectra of GaMnAs alloy were measured at different temperature. It is confirmed that the hole density in the alloy increases with the increase of the temperature.

Time-resolved photoluminescence spectra of self-assembled InAs/GaAs quantum dots

Two types of InAs self-assembled Quantum dots (QDs) were prepared by Molecular beam epitaxy. Atomic force microscopy (AFM) measurements showed that, compared to QDs grown on GaAs substrate, QDs grown on InGaAs layer has a significantly enhanced density. The short spacing (several nanometer) among QDs stimulates strong coupling and leads to a large red-shift of the 1.3 mu m photoluminescence (PL) peak. We study systematically the dependence of PL lifetime on the QDs size, density and temperature (1). We found that, below 50 K, the PL lifetime is insensitive to temperature, which is interpreted from the localization effects. As T increases, the PL lifetime increases, which can be explained from the competition between the carrier redistribution and thermal emission at higher temperature. The increase of carriers in QDs migrated from barriers and wetting layer (WL), and the redistribution of carriers among QDs enhance the PL lifetime as T increases. The thermal emission and non-radiative recombination have effects to reduce the PL lifetime at higher T. As a result, the radiative recombination lifetime is determined by the wave function overlapping of electrons and holes in QDs, and QDs with different densities have different PL lifetime dependence on the QDs size. (c) 2005 Elsevier B.V. All rights reserved.

Energy spectra of two-electron two-dimensional quantum dots confined by elliptical and bowl-like potentials

The laterally confining potential of quantum dots (QDs) fabricated in semiconductor heterostructures is approximated by an elliptical two-dimensional harmonic-oscillator well or a bowl-like circular well. The energy spectrum of two interacting electrons in these potentials is calculated in the effective-mass approximation as a function of dot size and characteristic frequency of the confining potential by the exact diagonalization method. Energy level crossover is displayed according to the ratio of the characteristic frequencies of the elliptical confinement potential along the y axis and that along the x axis. Investigating the rovibrational spectrum with pair-correlation function and conditional probability distribution, we could see the violation of circular symmetry. However, there are still some symmetries left in the elliptical QDs. When the QDs are confined by a "bowl-like" potential, the removal of the degeneracy in the energy levels of QDs is found. The distribution of energy levels is different for the different heights of the barriers. (C) 2003 American Institute of Physics.

Optical spectra and exciton state in vertically stacked self-assembled quantum discs

We study the oscillator strengths of the optical transitions of the vertically stacked self-assembled InAs quantum discs. The oscillator strengths change evidently when the two quantum discs are far apart from each other. A vertically applied electric held affects the oscillator strengths severely, while the oscillator strengths change slowly as the radius of one disc increases. We also studied the excitonic energy of the system, including the Coulomb interaction. The excitonic energy increases with the increasing radius of one disc, but decreases as a vertically applied electric field increases.

Study on the Raman spectra of GaP nanorods synthesized within carbon nanotube templates

The Raman spectra of GaP nanorods grown in carbon nanotube templates have been reported. The red shifts of the TO and LO modes were observed due to phonon confinement effect in GaP nanorods. The measured red shifts range from 2 to 10cm(-1) depending on the size of the measured nanorods. It has been found that the polarization properties, which cannot be well explained by the selection rules of single nanorod, result from the direction disorder of nanorods in the measured area. The more the disorder is, the weaker the directionality of polarization properties is. The decrease of the Raman frequency of the TO and LO mode of the nanorods with the increasing power of the exciting laser suggests that the heating effect of the nanorods is far stronger than the bulk material. In addition, the saturation and then decrease of the Raman intensity with the increasing laser power indicate the rapid increase of the defects in the nanorods exposed to a strong exciting laser.

Mode splitting in photoluminescence spectra of a quantum-dot-embedded microcavity

A microcavity structure, containing self-assembled InGaAs quantum dots, is studied by angle-resolved photoluminescence (PL) spectroscopy. A doublet with the splitting energy of 0.5-1.5 nm appears when the detection angle is larger than 35degrees. This doublet is identified as mode splitting (not the Rabi splitting) by polarization measurements. We find that it is the considerable deviation of the cavity-mode frequency from the central frequency of the stop band that makes the TE and TM cavity modes split more discernibly. The inhomogeneous broadening of quantum dots gives the TE and TM cavity modes a chance to show up simultaneously in the PL spectra. (C) 2003 American Institute of Physics.