Search

Chaperones, as modulators of protein conformational states, are key cellular actors to prevent the accumulation of fibrillar aggregates. Here, we integrated kinetic investigations with structural studies to elucidate how the ubiquitous co-chaperonin prefoldin (PFD) inhibits diabetes associated islet amyloid polypeptide (IAPP) fibril formation. We demonstrated that both human and archaeal PFD interfere similarly with the IAPP fibril elongation and secondary nucleation pathways. Using archaeal prefoldin model, we combined NMR spectroscopy with EM to establish that the inhibition of fibril formation is mediated by the binding of prefoldin’s coiled-coil helices to the flexible IAPP N-terminal segment accessible on the fibril surface and fibril ends. AFM demonstrates that binding of prefoldin to IAPP leads to the formation of lower amounts of aggregates, composed of shorter fibrils, clustered together. Linking structural models with observed fibrillation inhibition processes opens new perspectives for understanding the interference between natural chaperones and formation of disease-associated amyloids.

Resume Objectif Comparer la pression intraoculaire (PIO) dans le mois suivant une chirurgie de cataracte non compliquee chez des patients presentant un glaucome primitif a angle ouvert (GPAO) et chez des sujets normaux. Materiel et methodes Cette etude observationnelle longitudinale a inclus prospectivement 19 patients glaucomateux et 18 patients sains devant beneficier de phakoemulsification avec mise en place d’un implant intraoculaire entre decembre 2013 et avril 2014. La PIO etait mesuree en preoperatoire, le jour meme (j0), le lendemain (j1), a une semaine (j7) et a un mois (M1). Les collyres hypotonisants etaient poursuivis sans modification par rapport au traitement preoperatoire. Un traitement par inhibiteur de l’anhydrase carbonique orale etait instaure en cas d’hypertonie oculaire (HTO) superieure a 30 mmHg. Resultats La PIO chez les patients glaucomateux etait a 18,42 et 21,16 mmHg respectivement a j0 et j1 contre 12,94 et 13,78 mmHg chez les patients normaux. La difference etait significative avec p = 0,05. Il y avait egalement plus d’ HTO significative dans le groupe GPAO (31,5 % contre aucune dans le groupe non glaucomateux, p Conclusion Les patients glaucomateux, meme equilibres sous traitement local, sont plus susceptibles de faire des pics pressionnels au decours immediat d’une chirurgie de cataracte non compliquee. Les patients a risque devraient beneficier d’une surveillance avec mesure de la PIO rapprochee le jour ou le lendemain de la chirurgie. D’autres etudes semblent necessaires pour evaluer l’efficacite d’un traitement hypotonisant prophylactique.

Purpose Diabetic retinopathy (DR) is the major cause of blindness in the working-age population. With an increasing number of diabetic patients worldwide, automated screening tools become indispensable. Recent progress in machine learning and image analysis enables efficient automated screening. Methods DreamUp Vision uses state-of the art technology based on deep-learning. Our algorithm was trained on over 70,000 labeled retinal images. Images were graded by ophthalmologists as follows: 0 (no retinopathy), 1 (mild non proliferative DR), 2 (moderate non proliferative DR), 3 (severe non proliferative DR) and 4 (proliferative retinopathy). Each patient in the dataset is represented by two images of left and right eyes. Grading is done for each eye image separately. Our algorithm performs quick and reliable detection of anomalies in retinal images, diagnoses their stage of diabetic retinopathy and provides the location of the anomalies detected in the pictures. We consider a patient as referable if the DR stage is between 2 and 4, otherwise we consider the patient as non-referable. We evaluate our model on over 10,000 fundus images from 5,000 patients taken from the Kaggle DR Detection Challenge dataset, provided by California Healthcare Foundation. Results Our algorithm achieves an area under the receiver operating characteristic curve AUROC of 0.946 with 96.2% sensitivity (95% CI: 95.8–96.5) and 66.6% specificity (95% CI: 65.7–67.5) for identifying referable DR on the Kaggle dataset. Conclusions The performances we have obtained enable a reliable automated DR screening. As the amount of available labeled data grows and given our technology's ability to learn from labeled images, we believe that significant performance improvement can be achieved. The same process can be applied to the detection of other eye diseases as well.

Long-time exposure on historical buildings can enhance significant differences in stone durability according to their intrinsic properties, structure and composition. This study aims to identify stone parameters inducing macroscopical behavior i.e. hygric dilation. The content, nature and properties of the clay coating of several Triassic siliceous sandstones were assessed. A special emphasis was put on cation exchange capacity and specific surface area measurements using different methods (BET, EGME and MBA). Hygric free swelling and mercury intrusion porosimetry measurements enabled the assessment of macroscale behavior and pore structure. Results showed very slight differences in clay coating properties and clay content. However, hygric free swelling coefficient values were well correlated to clay minerals when the clay content was taken into account together with the total specific surface area of the clay coating. Clay intracrystalline swelling had a relevant impact on the macroscale behavior of the stones submitted to relative humidity cycles.

In this prospective study 244 consecutive patients presenting with typical and chronic signs of gastroesophageal reflux were included. Conventional 24-hr esophageal pH monitoring was carried out to establish the symptom association probability, the concordance index, and the symptom sensitivity index. The symptom association probability could be calculated in 110 patients (45%). Two groups were identified: group 1 had normal duration of esophageal acid exposure; subgroup 1a (nonsignificant symptom association probability) included 39 patients (35.5%) and subgroup 1b (significant symptom association probability) included 24 patients (21.8%); group 2 had abnormal duration of esophageal acid exposure; subgroup 2a (nonsignificant symptom association probability) included 21 patients (19.1%) and subgroup 2b (significant symptom association probability) included 26 patients (23.6%). In all, 56.6% of the patients presented typical symptoms of reflux not directly determined by one or repeated acid reflux episodes. The correlation between symptom association probability and the symptom sensitivity index allows for more accurate determination of esophageal acid sensitivity (subgroups 1b and 2b).

Reference LPN-ARTICLE-1996-005doi:10.1016/0038-1101(95)00368-1View record in Web of Science Record created on 2008-02-29, modified on 2017-05-12

Self-ordering of GaAs/AlGaAs quantum wires grown by organometallic chemical vapor deposition on grooved substrates was studied. The evolution of the surface profile at the corner between two quasi-{111}A planes was evaluated using cross-sectional transmission electron microscopy. The radius of curvature at the corner exhibits a reproducible, self-limiting value of 7.7 ± 0.7 nm, which increases linearly during subsequent growth of GaAs layers and decreases exponentially to its self-limiting value during further growth of AlGaAs layers. This provides the basis for the self-ordering of periodic, vertically stacked arrays of quantum wires with virtually identical shape, size and composition.

Purpose To evaluate the reproducibility and safety of capsulorhexis femtosecond laser assisted. Methods Admitted patients for cataract surgery have been received into the “PERCY Military Hospital” refractive surgery unity.We have treated 11 eyes of 11 patients, on the Victus femtosecond laser platform.A control group (11 eyes of 11 patients) has been operated manually in the same center.The rhexis diameter measurement has been realized at one week postoperative,to the angiograph,as iris angiography,using the HRA OCT Spectralis.On the pictures,we measured the vertical diameter and the horizontal diameter.For the rhexis centration,three circles were drawn following the contour of the rhexis, the pupillary border,and the margin of limbus.We measured the position of rhexis center to pupillary center. Results In the laser assisted group,the mean horizontal rhexis diameter was 5.35 mm +/- 0.22,and the mean vertical rhexis diameter was 5.37 mm +/- 0.22. In the control group,the mean horizontal and vertical rhexis diameters were respectively 5.3 +/- 0.28,and 5.6 +/- 0.21.We found that 85% of femtosecond laser assisted rhexis were within 0.35 mm from the programmed diameter. In the control group,only 15% have obtained the same level of accuracy.In the laser group,the three circles have the same center,and then are concentric,tell-tale of a rhexis good centration,unlike the manual rhexis.The femtosecond laser rhexis was circular,regular and covers the IOL’s periphery including the haptics. Conclusion The femtosecond laser has multiple benefits for cataract surgery, as the regular and circular capsule, the reduced variability in the effective lens position. The technique is reproducible and very safe.

Purpose Every IOL formula has its limits in estimating Effective lens position (ELP). The main one is the variability in capsulotomy size and centration. Our purpose was to compare the variability and predictability in ELP based on capsulotomy created by a femtosecond laser with those using manual continual curvilinear capsulorhexis. Methods 32 eyes from 24 patients undergoing cataract surgery had either femtosecond created 5.2 mm capsulotomy (VICTUS™ Femtosecond Laser Platform - Technolas Perfect vision) (n=16) or manual continuous curvilinear capsulorhexis performed with an attempted diameter of 5.2 mm (n=16). One surgeon performed all the surgeries during a period of one month in April 2012, at Percy military hospital, Clamart. Three type of monofocal IOL were implanted in the bag. At one month after surgery, ELP was measured by Pentacam HR and compared to preoperative estimated lens position. Accuracy to target refraction was also calculated, being the difference between target refraction with Iolmaster v5 (Haigis, SRK/T and Holladay formula) and postoperative objective refraction Results Precision of IOL position was improved, as shown by statistically significant reduced variability of ELP (0,27 vs 0,45, p=0,03) in the laser group at one month. Better accuracy to target refraction using SRK/T formula (0,16 vs 0,5, p=0.02) and Holladay formula (0,26 vs 0,50, p=0,02) was also observed in the laser-treated eyes compared to the manual group. Conclusion Femtosecond laser system provides a perfectly round, well-centered, and precise diameter capsulotomy. In this study, this translated to more accurate and predictable ELP and IOL power calculation in femtosecond laser group compared to manual group.

We investigated the selective area growth of γ = 1.3 μm InGaAsP by low pressure OMVPE, and compared it with the selective growth behavior of InP, InGaAs, and InGaP. We found that, for all three alloys, the In content increased with decreasing stripe opening width. The growth rate enhancement is similar for InGaAsP, InGaAs, and InGaP, while it is somewhat higher for InP. Growth spikes at the stripe opening edges could be seen in the case of InP and InGaAsP, while InGaAs and InGaP did not exhibit such spikes.

A model is developed for predicting morphologies resulting from crystal growth on nonplanar surfaces with step and terrace configurations engineered prior to growth. Novel aspects of crystal growth dynamics are revealed by the interplay between diffusion across terraces and over convex corners, which leads to multiple step formation and unique morphologies on (100) and vicinal (100) surfaces. These effects are utilized to generate a variety of quantum wire structure arrays.

We provide a perspective on current capabilities for optically monitoring and controlling epitaxial growth, and discuss examples taken from recent work at Bellcore.

Some of the earliest theoretical speculation, stimulated by the growth of semiconductor superlattices, focused on novel devices based on vertical transport through engineered band structures; Esaki and Tsu promised Bloch oscillators in narrow mini-band systems and Kazarinov and Suris contemplated electrically stimulated intersubband transitions as sources of infrared radiation. Nearly twenty years later these material systems have been perfected, characterized and understood and experiments are emerging that test some of these original concepts for novel submillimetre wave electronics. The authors describe recent experiments on intersubband emission in quantum wells stimulated by resonant tunnelling currents. A critical issue at this time is devising a way to achieve population inversion. Other experiments explore 'saturation' effects in narrow miniband transport. Thermal saturation may be viewed as a precursor to Bloch oscillation if the same effects can be induced with an applied electric field.

Quantum wire (QWR) heterostructures suitable for optoelectronic applications should meet a number of requirements, including defect free interfaces, large subband separation, long carrier lifetime, efficient carrier capture. The structural and opticl properties of GaAs/AlGaAs and InGaAs/GaAs quantum wire (QWR) heterostructures grown by organometallic chemical vapor deposition on nonplanr substrates, which satisfy many of these criteria, are described. These crescent-shaped QWRs are formed in situ during epitaxial growth resulting in virtually defect free interfaces. Effective wire widths as small as ∼10nm have been achieved, corresponding to electron subband separations greater than KBT at room temperature. The enhanced density of states at the QWR subbands manifests itself in higher optical absorption and emission as visualized in photoluminescence (PL), PL excitation, amplified spontaneous emission and lasing spectra of these structures. Effective carrier capture into the wires via connected quantum well regions, which is important for enhancing the otherwise extremely small capture cross section of these wires, has also been observed. Room temperature operation of GaAs/AlGaAs and strained InGaAs/GaAs QWR lasers with threshold currents as low as 0.6mA has been demonstrated.

We report the first direct observation of reconstructions of semiconductor surfaces in atmospheric pressure (AP) environments. We use reflectance difference spectroscopy (RDS), a surface-sensitive optical probe, to bridge the gap between ultrahigh vacuum (UHV) and AP, and show that the primary surface reconstructions that occur on (001)GaAs surfaces in UHV also occur in the AP environments used with organometallic chemical vapor deposition (OMCVD). Our results justify the applicability of the results of UHV surface science to understanding surfaces under non-UHV environments. Our results also show that during OMCVD growth conditions the surface is terminated with multilayers of As, contrary to generally accepted models. We also apply our newly developed approach, multi-transient spectroscopy (MTS), to the study of atomic layer epitaxy (ALE). Using MTS, we observe surface spectra within a time resolution of 100 ms during actual ALE growth cycles, thus allowing the dynamics of surface reactions to be investigated.

Crescent-shaped multiple quantum wires (QWR's) having a center thickness of 5.5 nm and an effective width of ∼ 10 nm were grown in situ by organometallic chemical vapor deposition (OMCVD) on V-grooved substrates. This fabrication technique yields damage-free QWR interfaces and luminescence efficiencies comparable to that of conventional quantum wells. Nonradiative interface recombination, usually dominating the luminescence transient in etched QWR's is strongly reduced enabling for the first time a detailed investigation of the kinetics of carrier collection, relaxation and recombination in the QWR structure. Results of spatially- and time-resolved cathodoluminescence investigations of these structures are presented and discussed.

We present a far-infrared-absorption study of electrons in lightly doped GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As superlattices. A grating coupler enables us to observe transitions that require an electric-field polarization along the superlattice axis. We investigate weakly and strongly coupled superlattices, and demonstrate the difference between intersubband transitions and transitions between extended minibands. The line shape of the interminiband absorption deviates considerably from the predictions of a simple single-particle model. We discuss possible reasons in terms of fluctuations and localization. At low temperatures, the absorption spectra are dominated by donor transitions. The transition from the donor ground state to the pure 2${\mathit{p}}_{\mathit{z}}$ state, which is associated with the second subband, is observed. In a variational calculation, we compute the four-lowest donor states for a variety of superlattice parameters. Excellent agreement between experiment and theory is achieved.

Reflectance difference spectroscopy (RDS) is a surface analysis technique that was invented in 1985 by Aspnes. Here, we give a summary of its application to crystal growth techniques which gave new and valuable real time information about the growth process. Also, this information was obtained in-situ, in molecular beam epitaxy (MBE) and organometallic chemical vapor deposition (OMCVD) crystal growing setups which are routinely used to produce high quality device structures. The application of RDS to OMCVD allowed us to develop a “textbook” like model of growth kinetics, which includes two independent microscopic mechanisms, i.e. adsorption (at -26 kcal/mol) of the reacting molecule (trimethylgallium (TMG) in the case of GaAs), followed by its decomposition (at 39 kcal/mol) on the growing GaAs surface. Our model includes an effect called steric hindrance, associated with the large size of the TMG molecule. This study represents the first direct quantitative evaluation of the catalytic effect of the GaAs surface for the decomposition of TMG. We discuss implications of the model both for growth in the ALE mode as well as for conventional OMCVD growth and comment on the relative importance of surface and gas phase reactions. The application of RDS to MBE revealed remarkable details about the complex intermediate steps that surfaces undergo during growth and enabled to extract directly surface dielectric functions. Finally, applications of the technique as well as results obtained in a number of laboratories where RDS is currently being developed are discussed.

A novel technique for lateral bandgap patterning on non-planar GaAs substrates is introduced. When a grating is lithographically etched on a vicinal (100) GaAs substrate prior to organometallic chemical vapor deposition (OMCVD), growth results in a regular saw-tooth-type surface morphology consisting of macroscopic steps. These steps expose two distinct crystallographic orientations to the gas phase, i.e. (100) and (111)A. In this paper, we demonstrate that it is possible to take advantage of the selective deposition of reactants from the gas phase onto the two crystallographic facets to pattern the thickness and the corresponding effective bandgap of a quantum well. Spatially and spectrally resolved cathodoluminescence was employed to image directly the bandgap patterning, which was found to be in good agreement with transmission electron microscopy data on the structures. The ability to control the bandgap patterning with growth conditions is discussed. Finally, this study, which was first performed with 3.5 μm period gratings, was extended to submicron gratings where the step height becomes small enough to be in the range where confinement effects in two dimensions can be anticipated. This demonstrates that non-planar vicinal substrates can be used to grow lateral arrays of quantum wires by OMCVD.

In this paper, we describe the progress made in the fabrication of quantum wire lasers using growth on nonplanar substrates as a lateral patterning technique. GaAs/AlGaAs quantum wire injection lasers with up to three crescent-spaped quantum wire active regions have been fabricated. The lowest threshold current of 2.4 mA was obtained for lasers with 2 quantum wires. We also report on two techniques for the fabrication of quantum wire arrays in GaAs/AlGaAs. Finally, we present our results on an attempt to fabricate quantum wire lasers based on InP, and propose possible solutions to the problems encountered.

Two methods which achieve both lateral and longitudinal patterning of semiconductor properties by organometallic chemical vapor deposition (OMCVD) are presented. Both approaches utilize diffusion of reactant species from a nongrowth surface to an adjacent growth surface, presumably via the gas phase. In the first method, the nongrowth surface is the (111)B facet which develops during deposition on a GaAs substrate where mesas in the (011) orientation have been etched prior to growth. Low-loss single mode rib-type waveguides (0.6 dB/cm at 1.52 μm wavelength) were fabricated with this approach. In the second method, the nongrowth surface is a dielectric mask deposited on the GaAs surface prior to growth. Growth rate enhancements can be controlled, and can be as high as 280% with this second approach, which appears to be more practical than the first one. This new capability for OMCVD will offer a wide range of applications.

The spatial mode characteristics of index-guided ridge-waveguide broad-area (BA) quantum-well lasers grown by organometallic chemical vapor deposition were investigated experimentally. The index-guided BA lasers lase in a high-order lateral mode, and thus emit a double-lobed far-field pattern. This is significantly different from their gain-guided counterparts, which lase in the fundamental mode. For BA lasers with the same width and made on the same or similar material, the index-guided lasers have lower threshold currents, higher quantum efficiencies, and better linearity in the light versus current characteristics. It is observed that the order of the dominant high-order lateral-mode increased with increasing laser width or effective index step of the laser waveguide. In addition, it is found that the mechanism for degradation in the spatial coherence at high pumping levels is the onset of higher order lateral modes. >

Wet chemical etchants suitable for making GaAs/AlGaAs single-mode optical waveguide bends with radii as low as 300 mu m for excess bend losses of 1 dB/90 degrees and propagation losses as low as 1 dB/cm at lambda =1.5 mu m are described. Additional bend losses due to modal mismatch resulting from etchant anisotropy are investigated, and it is shown that in many cases such anisotropy does not limit bend performance. >

This paper summarizes results of our investigations of growth on (001) and (110) GaAs by atmospheric pressure organometallic chemical vapor deposition (OMCVD). We follow evolutions of surface species to a sensitivity of 0.01 monolayer (ML) on a time scale of 0.1 s under alternating flows of trimethylgallium (TMG) and arsine (AsH3) as functions of partial pressure, sample temperature, and surface orientation. The reaction of TMG with an AsH3-saturated (001) surface is rate-limited by competition between desorption and decomposition of TMG molecules chemisorbed to surface lattice sites via an exclude-volume mechanism, while the reaction of AsH3 with the TMG-saturated (001) surface is essentially instantaneous. In contrast, TMG reacts essentially instantaneously with the AsH3-saturated (110) surface while the AsH3 reaction with the TMG-saturated (110) surface is the rate-limiting step. However, the latter rate is not intrinsic to the AsH3-surface reaction but appears to be determined by desorption of adsorbed species that block active sites.

In this prospective study 244 consecutive patients presenting with typical and chronic signs of gastroesophageal reflux were included. Conventional 24-hr esophageal pH monitoring was carried out to establish the symptom association probability, the concordance index, and the symptom sensitivity index. The symptom association probability could be calculated in 110 patients (45%). Two groups were identified: group 1 had normal duration of esophageal acid exposure; subgroup la (nonsignificant symptom association probability) included 39 patients (35.5%) and subgroup lb (significant symptom association probability) included 24 patients (21.8%); group 2 had abnormal duration of esophageal acid exposure; subgroup 2a (nonsignificant symptom association probability) included 21 patients (19.1%) and subgroup 2b (significant symptom association probability) included 26 patients (23.6%). In all, 56.6% of the patients presented typical symptoms of reflux not directly determined by one or repeated acid reflux episodes. The correlation between symptom association probability and the symptom sensitivity index allows for more accurate determination of esophageal acid sensitivity (subgroups lb and 2b).

We demonstrate low‐loss optical mode‐size tapers in InGaAsP/InP optical waveguides fabricated by selective organometallic chemical vapor deposition. The tapers increase the vertical mode size by ≊300%. Taper losses as low as 0.4 dB (on‐chip, excluding coupling) were achieved, with 2.6 dB coupling loss to a flat‐end, single‐mode fiber at 1540 nm wavelength.

The recombination kinetics of the quasi‐one‐dimensional (1D) carriers confined in quantum wires grown by organometallic chemical vapor deposition on nonplanar, patterned substrates is reported for the first time. Due to the small (240 nm) wire spacing, excited carriers are efficiently captured into the wires on a sub‐ps time scale. The radiative recombination lifetime of the captured 1D carriers is longer than 310 ps. Time‐resolved cathodoluminescence spectra directly monitor thermalization of the carriers within the wire.

Arsenic dimers and multilayers are shown to exist on (001)GaAs surfaces under atmospheric pressure (AP) organometallic chemical vapor deposition (OMCVD) conditions. We obtained reflectance‐difference spectra from surfaces in AP H2 that are equivalent to those obtained from the (2×4) and disordered‐c(4×4) reconstructions prepared in ultrahigh vacuum by molecular beam epitaxy. Implications for models of OMCVD growth.

Luminescence properties of GaAs/AlGaAs quantum wire (QWR) heterostructures grown by organometallic chemical vapor deposition on V‐grooved substrates are reported. A model of the crescent‐shaped wires yields parabolic QWR potential wells with subbands separated by 21.7, 3.9, and 16.7 meV for electrons, heavy holes, and light holes and effective width of 16 nm for the ground electron state. Spectrally and spatially resolved cathodoluminescence images reveal highly uniform emission from the QWR regions. Photoluminescence excitation spectra exhibit enhanced absorption at the QWR subbands, with subband separations in good agreement with the model.

We determine types and concentrations of free carriers in GaAs layers under organometallic chemical vapor deposition growth conditions from the linear electro‐optic structure observed near 3 eV in reflectance‐difference spectroscopy. The sensitivity is about 1017 cm−3 at 400 °C and 1018 cm−3 at 600 °C, sufficient to measure common doping levels at or near growth temperatures. We observed the transition between n‐ and p‐type doping during atomic layer epitaxy of a carbon‐doped p‐type layer on an n‐type substrate at 470 °C.

We report the observation of two photon absorption which is strongly dependent on the applied electric field and the optical polarization. At 1.55 μm wavelength, the two‐photon absorption coefficient of the GaAs/AlGaAs multiquantum well (MQW) waveguides for transverse‐magnetic light is about seven times lower than for transverse‐electric polarized light and changes by a factor of approximately 4 for a change in applied direct‐current electric field of ∼140 kV/cm. Ultrafast nonlinear refraction causing phase changes of over π radians without appreciable excess loss is observed. These measurements demonstrate that GaAs/AlGaAs MQW waveguides could be successfully used for subpicosecond all‐optical switching near half‐band gap, at wavelengths corresponding to the 1.55 μm optical communications band.

We report the structure and lasing characteristics of GaAs/AlGaAs vertically stacked multiple‐quantum‐wire (QWR) semiconductor lasers grown by organometallic chemical vapor deposition on V‐grooved substrates. The active region in these lasers consists of three crescent‐shaped wires, placed at the center of a single‐mode optical waveguide. The higher optical confinement factor, compared to single‐QWR structures, leads to reduced threshold currents, as low as 0.6 mA for high‐reflection coated devices at room temperature. The lower threshold carrier density results in oscillation at a lower QWR subband as compared to single‐QWR laser structures.

Selective area growth (etching) by low‐pressure organometallic chemical vapor deposition (LP‐OMCVD) is utilized to intentionally modulate the local growth (etch) rate by choosing the pattern of dielectric‐masked areas, thereby defining III‐V semiconductor structures in situ. This technique is applied to tune the emission wavelength of a GaAs/AlGaAs quantum well structure, and to obtain InP/InGaAs superlattice structures tapered in thickness with growth rate increases as high as 800%, suitable for integrated optics applications. In contrast, selective deposition by organometallic molecular beam epitaxy (OMMBE) does not produce growth rate enhancements, thereby preventing similar in situ definition schemes but allowing to integrate structures with optimized nominal thicknesses.

We demonstrate a novel waveguide structure for realization of integrated optics with compact waveguide bends (radii≊1 mm), low propagation loss (0.45 dB/cm), and large guide dimensions (5 μm width) to facilitate input coupling. Experimental results using single‐mode GaAs/AlGaAs heterostructure guides at 1.52 μm wavelength are presented.

We report the study of organometallic chemical vapor deposition (OMCVD) on the stepped surfaces obtained by crystal growth on nonplanar vicinal (100) GaAs substrates. Scanning and transmission electron microscopy investigations were combined to identify two distinct diffusion mechanisms in OMCVD: one mechanism involves the gas phase and allows us to selectively distribute the incoming flux of volatile reactants between two adjacent facets; the other one involves surface diffusion of nonvolatile species over distances in the hundred nm range. Spectrally and spatially resolved cathodoluminescence imaging of the light emissions from a single quantum well structure gives direct evidence that the quantum well thickness variations, which can be controlled over a wide range (up to 1:5) by growth parameters, produce effective band‐gap variations.

We report on the growth of dense lateral arrays of GaAs quantum wire structures, obtained by organometallic chemical vapor deposition (OMCVD) on GaAs substrates where a submicron grating has been lithographically defined and etched prior to deposition. The experiments were performed simultaneously on (100) oriented substrates, where the wires are virtually ‘‘isolated’’ from each other, and on substrates that are vicinal with respect to the (100) orientation, where the wires are ‘‘smoothly connected’’ by quantum wells. Transmission electron microscopy investigations allowed the study of the morphology of the resulting structures, which was related to the microscopic step nature of the starting surfaces and revealed important basic aspects of growth dynamics.

Single (SQW) and double (DQW) quantum well composite cavity GaAs/AlGaAs lasers with an integrated passive section were fabricated and compared. Si implantation was used for partial quantum well disordering in the passive section. Implanted DQW lasers with a 2.6‐mm‐long cavity had threshold currents of 26.3 mA compared to 33 mA for implanted SQW lasers. The measured resonant absorption in the passive section showed an exponential roll‐off in agreement with Urbach’s law. The characteristic energy E0 associated with Urbach’s law was 6 meV for untreated SQW and DQW lasers and ≊11 meV for implanted SQW and DQW lasers.

Photoluminescence, electroluminescence, and current measurements are used to probe the internal field in an n+‐n‐n+ GaAs/AlGaAs superlattice. At low bias voltages, the photoluminescence spectrum shows several peaks arising from electric field domains in the superlattice. Their positions correlate with features in the current‐voltage characteristic and are consistent with a simple calculation of the Stark shift. Above the threshold voltage for impact ionization we observe spectrally narrow electroluminescence (width 3.5 meV). The energy of this emission coincides with the zero‐field photoluminescence signal, indicating that electrons and holes screen the applied electric field in the quantum wells.

We report measurements of the two‐photon absorption coefficient of GaAs optical waveguide structures at 1.06 μm. We show that for pulse lengths longer than ∼1 ps, light‐induced index changes sufficient to induce all‐optical switching will be predominantly due to carriers generated by two‐photon absorption. These results allow us to predict limitations for ultrafast all‐optical GaAs devices.

We report on the observation of intersubband emission from superlattices, excited by injecting electrons perpendicular to the layers into high lying subbands.

Rib waveguides have been fabricated without any post‐crystal growth processing steps. The ribs are defined by the two nongrowth (111)B surfaces that develop at each edge of (011) mesas on a patterned GaAs substrate during organometallic chemical vapor deposition (OMCVD) of GaAs/AlGaAs structures. Propagation losses as low as 0.6 dB/cm at 1.52 μm wavelength have been obtained, which is attributed to the smoothness of the (111)B facets defining the GaAs guiding layer. This study revealed the importance of surface diffusion‐enhanced crystal growth when a growth surface is adjacent to a nongrowth surface such as a (111)B facet. This effect was quantified here and its magnitude suggests that the OMCVD technique would be well suited for the growth of structures tapered in three dimensions, of interest for integrated optics applications.

The authors describe the observation of infrared emission stemming from intersub-band transitions in quantum wells and superlattices. Two methods of electron excitation into higher energy levels are employed: in-plane carrier heating and vertical injection. For both methods, considerations for the design of efficient emitters are given.

The realization of low-dimensional quantum structures, particularly quantum wires (QWRs) and quantum dots (QDs), has attracted considerable attention due to their interesting physics and their potential role in novel device applications. However, the formation of well resolved low-dimensional electronic subbands typically requires wire and dot structures with lateral dimensions in the l0nm range and with relative width fluctuations of not more than a few percent. Furthermore, these nanostructures should have defect free, abrupt interfaces in order to minimize the number of unwanted interface scattering and nonradiative recombination centers. These requirements present considerable challenges to current nanofabrication technologies.

Molecular beam epitaxy (MBE) has enabled the realization of two-dimensional (2D) structures with sharp interfaces and high electron mobilities producing much novel and unexpected physics. Recently, interest has turned to reducing the dimensionality still further in order to discover new or enhanced optical and transport phenomena.