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Funding Information: This work was supported by Swiss National Science Foundation and Academy of Finland (Grant No. 308394). Publisher Copyright: © 2022 authors. Published by the American Physical Society. Radiation properties of a pointlike source of light, such as a molecule or a semiconductor quantum dot, can be tailored by modifying its photonic environment. This phenomenon lies at the core of cavity quantum electrodynamics (CQED). Quantum dots in photonic crystal microcavities have served as a model system for exploring the CQED effects and for the realization of efficient single-photon quantum emitters. Recently, it has been suggested that quantum interference of the exciton recombination paths through the cavity and free-space modes can significantly modify the radiation. In this work, we report an unambiguous experimental observation of this fundamental effect in the emission spectra of site-controlled quantum dots positioned at prescribed locations within a photonic crystal cavity. The observed asymmetry in the polarization-resolved emission spectra strongly depends on the quantum dot position, which is confirmed by both analytical and numerical calculations. We perform quantum interferometry in the near-field zone of the radiation, retrieving the overlap and the position-dependent relative phase between the interfering free-space and cavity-mode-mediated radiative decays. The observed phenomenon is of importance for realization of photonic-crystal light emitters with near unity quantum efficiency. Our results suggest that the full description of light-matter interaction in the framework of CQED requires a modification of the conventional quantum master equation by also considering the radiation mode interference.

Sagliker syndrome characterized with uglifying the appearance of the face due to secondary hyperparathyroidism in patients with chronic renal failure receiving dialysis long time ago. The aim of the study was to identify any patients with Sagliker syndrome among the 78 patients receiving dialysis at the Dialysis ward, University Hospital "St. George", Plovdiv, Bulgaria.Four patients metthe criteria for Sagliker syndrome: chronic renal insufficiency, secondary hyperparathyroidism, short stature, severe changes in the skull and jaw that lead to festoon-like uglifying face, dental abnormalities, "brown" tumors, deformations of the phalanges of the fingers, psychological diseases and depression.The lack of control of the level of calcium and phosphorus in the blood in patients with chronic kidney disease leads to severe bone changes and psychological changes in patients with prolonged dialysis.

This chapter contains sections titled: Introduction Theory of Conductivity in 1D Wires Fabrication of Quantum Wires in V-Grooves Experimental Results and Analysis for Short V-Groove Quantum Wires Resistance Dependence on T and Length of Longer V-Groove Quantum Wires Conclusions ]]>

The results of a group theoretical analysis of the excitonic fine structure are presented and compared with spectroscopic data on single quantum dots. The spectral features reveal the signatures of a symmetry higher than the crystal symmetry (C-3v). A consistent picture of the fine structure patterns for various exciton complexes is obtained with group theory and the concepts of symmetry elevation and symmetry breaking.

A review is presented of recent progress in low-loss III-V semiconductor integrated optics for the 1-1.6 mu m wavelength range, including propagation, bending, coupling, and modulator losses. Tradeoffs between loss and other performance issues are discussed in detail. The authors show that significant advances have resulted in a research shift from work on the general issues of improved material transparency and straight guide propagation loss to more application-specific areas. In particular, size-loss tradeoffs in bend and modulator structures remain key issues for widespread use of III-V integrated optics. >

We demonstrate polarization stable wafer fused VCSELs operating at 1320 nm wavelength showing record high single mode power of 5.4 and 3.1 mW, at 25deg and 75degC, respectively, and open eye diagrams with 40 ps fall time at 10 Gb/s modulation.

Epitaxial growth on nonplanar substrates provides a powerful approach for producing high quality laterally-confined semiconductor nanostructures. In this technique, nonplanar surface features prepared using lithography serve as seeds for the formation of self-limiting nonplanar profiles, whose details depend only on the growth parameters. Lower bandgap layers deposited on these nonplanar, self-limiting surfaces can then yield a variety of laterally-confined structures such as quantum wires, quantum dots, vertical quantum wells and vertical quantum wires. This talk reviews recent progress achieved with such nanostructures grown by organometallic chemical vapor deposition, including the understanding of the self-ordering process involved, their electronic and optical properties, and their applications in optoelectronic devices.

This paper reviews the application of growth on nonplanar, patterned substrates for producing high quality semiconductor quantum dots (QDs) with good control over their position on the substrate as well as their energy spectrum. The approach consists of organometallic chemical vapor deposition (OMCVD) of (In)GaAs/(Al)GaAs heterostructures on (111)B GaAs substrates patterned with pyramidal pits. The perfect site-control of the pyramidal quantum dots are useful for incorporation into optically active photonic crystal devices. The incorporation of pyramidal QDs into Bragg optical cavities as well as their use in efficient QD lasers are also discussed.

Coupled arrays of VCSELs are employed as a model for two-dimensional photonic crystals, making possible the investigation of optical disorder, strain and photonic heterostructure confinement experimentally and theoretically.

V-grooved quantum wire (QWR) lasers were studied by near-field optical microscopy with 0.1 /spl mu/m spatial resolution. We mapped the electroluminescence from the quantum structures and measured the lasing mode evolution. Localized photocurrent measurements show spectral features consistent with the QWR lateral confinement.

Operation of a new triple fused long wavelength VCSEL structure is demonstrated for the first time. In this structure both highly resistive p-AlGaAs/GaAs DBR and strained p-GaAs/p-InP fused junction are replaced with a composite mirror comprising a p-InGaAsP/InP partial DBR fused to an undoped AlGaAs/GaAs DBR and an isoperiodic, low barrier p-InP/p-InGaAsP fused junction.

Summary form only given.Arrays of coupled vertical cavity surface emitting lasers (VCSELs) were previously shown to have a coherent photonic mode with high immunity against structural imperfections. In such periodic arrays the photonic mode stability depends on disorder in the position or size of the pixels, injection current, and boundary conditions. In the present work, we show that nonuniform VCSEL arrays with a properly designed variation in the lattice parameters can exhibit a new type of photonic confinement, namely confinement of the envelope function of the lattice modes. The resulting novel structures, that we term photonic heterostructures, confine photon modes much like semiconductor heterostructures that confine the envelope functions of charge carriers. The photonic heterostructures are fabricated in a similar way as other types of coupled arrays of VCSELs. They are based on GaAs/AlGaAs/InGaAs structures emitting through the substrate at 940-nm wavelength. Each photonic heterostructure consists of several rows of a lattice with larger, square pixels surrounded by a lattice of smaller pixels. The pixels are defined by covering their surface with gold acting as a reflection enhancing layer. The region between the pixels is covered with chromium.

The lateral quantum confinement imposed on electrons and holes in semiconductor quantum wires (QWRs) and quantum dots (QDs) has been predicted to bring about significant advantages for optoelectronic device applications. Early on, the increasingly sharp density of states (DOS) achieved with more degrees of confinement was expected to dramatically enhance optical absorption and emission due to the spectral confinement of the reduced DOS. Furthermore, it was anticipated that the lateral confinement would increase the exciton binding energy beyond what is obtained with two-dimensional (2D) quantum well (QW) structures, giving rise to enhanced linear and nonlinear optical excitonic effects. In addition, the extremely small number of charge carriers residing in short wire segments or small arrays of dots makes these nano-optoelectronic structures suitable for integration with novel electronic devices inherently designed for low power consumption, such as conducting nanowires and single electron transistors.

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 cathodoluminescence (CL) imaging and transmission electron microscopy (TEM) studies of patterned GaAs/AlAs quantum well (QW) heterostructures grown by molecular beam epitaxy on periodically corrugated substrates. Faceting and surface diffusion during crystal growth result in significant lateral variations in QW thickness which translate quantum size effects into lateral band‐gap patterning. The CL images directly display lateral periodic modulation in quantum well thickness and indicate the existence of two lateral potential wells formed at the bottom and apex of corrugations. Emission wavelengths are in agreement with TEM data on the patterned structures. Such periodic patterned QW heterostructures should be useful for the realization of arrayed one‐ and zero‐dimensional semiconductor quantum structures, particularly lateral superlattice heterostructures.

Carrier capture in quantum wells (QWs) has been intensively studied in the past years, especially concerning the dependence of the capture time on QW and barrier dimensions [1]. On the other hand, little is known about the carrier transfer and capture times into one-dimensional (ID) quantum wires (QWRs). In particular, in structures such as crescent-shaped QWRs grown on V-groove substrates, in which the carriers are transferred via both 3D barrier and 2D QW regions surrounding the wires, the carrier capture into the QWRs can be quite complex.

Quantum wire (QWR) semiconductor lasers, grown by organometallic chemical vapor deposition OMCVD) on nonplanar substrates, exhibit two dimensional (2D) quantum confinement and sub-mA threshold currents. The in situ formation of the wires in these lasers eliminates excessive nonradiative recombination at their interfaces, which is essential for the efficient operation of these devices. One of the expected advantages of QWR heterostructures is the enhanced optical gain and absorption resulting from the increased density of states at the quasi-1D subbands. This feature would make QWR heterostructures useful for applications in low power consumption integrated optoelectronics. Here, we report the first measured gain and absorption spectra of QWR lasers. The multi-QWR lasers discussed here were grown by OMCVD on V- grooved substrates. Their active regions consist of 4 crescent-shaped GaAs wires, placed at the center of a 2D, graded index AlGaAs optical waveguide. A band structure model of these wires yields electron-heavy hole QWR subband transitions separated by 19meV, with an effective wire width of 15nm for the ground electron state, The subband structure was evident in the amplified spontaneous emission and lasing spectra of the devices, with observed transition energies in good agreement with the calculated values.

Phase-locked arrays of vertical-cavity surface emitting lasers (VCSELs) offer a wide range of applications, including optical computing, optical interconnects, printing, free-space communications and laser radar. The unique cavity geometry of VCSELs allows the construction of large arrays with both one- and two-dimensional (2D) configurations. For a given number of array elements, closely-packed 2D arrays should exhibit better spatial coherence since (a) their smaller diameter reduces adverse effects of wafer nonuniformities; and (b) the larger number of nearest neighbors and higher degree of connectivity in 2D arrays should yield more effective coupling. Here, we show that 2D arrays indeed exhibit improved coherence compared to their 1D counterparts because of their higher dimensionality, and demonstrate efficient phase locking of large (up to 25×25) arrays of VCSELs.

Realization of uniform, dense arrays of semiconductor quantum wires (QWRs) with subband separation >kBT is important for the investigation and application of one-dimensional structures. We have demonstrated uniform arrays of GaAs/AlGaAs QWRs grown by organometallic chemical vapor deposition on V-grooved substrates showing record-large subband separation. The crescent-shaped wires are formed in situ at the comer of two intersecting {111} A planes due to diffusion of the Ga and Al species1. The smaller diffusion length of Al results in rapid recovery of the radius of curvature at the bottom of the GaAs crescents during growth of subsequent AlGaAs barriers. This recovery is characterized quantitatively using transmission electron microscopy (TEM) data. With sufficiently thick barriers, quantum well layers grown on the nonplanar substrate self-order to form vertical arrays of QWRs having identical size and shape. Modeling of the QWR subbads based on TEM cross sections of the structure yields subband separations of 45, 11 and 30meV for electrons, heavy holes and light holes, with effective wire width of 9.5nm for electrons. Photoluminescence excitation spectra show enhanced absorption at the QWR subbands at temperatures as high as 200K (kBT=17.2meV) with observed electron-heavy hole subband separation of ~50meV, in good agreement with the model.

Recently, several groups reported the observation of phase-locking in vertical cavity surface-emitting laser (VCSEL) arrays.1,2 The most efficient phase-locking was obtained using internal diffractive coupling, brought about by a tailored spatial mirror reflectivity modulation.1 Following a critical review of the various coupling methods available for VCSEL arrays, the reflectivity modulation method and its compatibility with the special features of VCSELs are analyzed. Exploiting the high efficiency of this coupling method, we were able to design and fabricate a number of array geometries tailored for various uses. High peak power (>300 mW) coherent beams were obtained from rectangular multielement arrays with optimized laser size, element spacing, and reflectivity modulation depth. Beams with a prescribed number of equal-intensity lobes (e.g., twenty lobes) were obtained using ring shaped arrays for the implementation of fanned-out laser sources. In both of these examples the two-dimensionality of the arrays played a major role: stabilizing the coherent output mode and adding modal design flexibility. The importance of both the array geometry and its dimensionality are discussed in detail. Theoretical analysis of the modal behavior using coupledmode theory for the weak coupling cases, and beam propagation methods for the strong coupling regime agrees well with measured array characteristics. Finally, the possibility of dynamically shaping the coherent modes by changing the current injection pattern to the array elements are discussed.

We have recently demonstrated room temperature operation of quantum wire (QWR) lasers with threshold currents as low as 0.6 Ma.1,2 A potential problem in using QWRs in laser structures is the very small cross-section of the wire, which can reduce the capture efficiency of injected carriers. We investigated carrier-capture mechanisms in multi-QWR lasers using photoluminescence (PL) and cathodoluminescence (CL) spectroscopy. In our lasers, the crescent-shaped QWRs are formed in situ by OMCVD on V-grooved substrates and are laterally bounded by thinner quantum well (QWL) layers formed on the slopes of the groove. At low temperatures (T < 100 K) we observe two luminescence lines, emitted from the QWR and QWL regions, respectively, as identified by CL imaging. At higher temperatures, the QWR feature becomes dominant, indicating efficient carrier capture from the QWLs to the lower energy QWR potential well. This is confirmed by PL excitation spectra. Two-dimensional quantum confinement in the wires is evidenced by the appearance of the second QWR subband at T > 140 K, with an energy separation of 19 meV, consistent with a model of the QWR subbands. The efficient carrier capture from the QWL region to the QWRs observed in these lasers is important for their efficient operation.

We report experimental measurements of two-dimensional spatial lasing supermodes in phase-locked vertical-cavity semiconductor laser arrays. The phase locking was achieved by patterning the reflectivity of the back laser mirror. The dependence of the modal behavior on the array geometry for varius configurations is presented.

Directional couplers are important building blocks in the design of a variety of integrated-optics devices. A precise knowledge of the device loss is important, both for practical use in systems applications and for effective design through a thorough understanding of the device operation. To the best of our knowledge, all the loss measurement techniques for coupled waveguides in use today use indirect methods under the assumptions that both a calibration waveguide and the device have the same input coupling efficiency and the same propagation loss per unit length. These assumptions are invalid for many cases and are a major source of uncertainty in the measurement of losses of such devices.

The advent of vertical cavity surface emitting laser arrays (VC-SELAs) creates new possibilities of integration and miniaturization in a host of optical information processing and interconnection applications.1,2 Beams of individual lasers in a VC-SELA exit through the substrate while diverging due to diffraction causing beam overlap after short distances. Here, we present results of integrating an easy to fabricate binary phase Fresnel microlens array (BP-FMA) on the substrate of a VC-SELA to focus individual beams making them suitable for above applications. A VC-SELA with 10 m aperture laser on 100 m pitch was first fabricated detail of which is given elsewhere.2 By the selective ion beam milling of the substrate, the BP- FMA pattern was fabricated in the exit interface. The generated step height was 0.19 m (to induce 180 phase shift between neighboring zones of Fresnel microlenses). Each microlens in the array has an aperture of 80 m situated 100 m apart. In one case, we used such microlenses to reduce the beam divergence of 10 m aperture lasers (situated 200 m away) from 11 (diffraction without the lens) to 4 (with the lens). In this paper, we will present the quality of beams generated in addition to the diffraction and the throughput efficiencies of the fabricated microlenses. The effect of back reflection on the performance of laser resonators will also be examined.

A case of a 20-year-old woman who developed a hemolytic-uremic syndrome while using the contraceptive drug Rigevidon is reported. Clinically the disease was manifested by acute renal failure, thrombocytopenic purpura and hemolytic anemia. The treatment with antiaggregants, anticoagulants, hemodialysis and plasmapheresis was without effect. The diagnosis was proved by the postmortem examination. The risks of the use of oral contraceptives and the need of new and better methods for contraception are discussed.

1.52 µm double fused InGaAsP/AlGaAs vertical cavity surface emitting lasers with in-situ built-in lateral current confinement were fabricated using localised wafer fusion process. These devices operate at room temperature in quasi-CW regime at 10% duty cycle. Operation characteristics of these lasers are limited by the high resistivity of p-InP/p-GaAs fused junctions which is a result of using carbon-doped p-AlGaAs/GaAs distributed Bragg-mirrors.

Rate equations analysis of phase-locked semiconductor laser arrays has been carried out. It was found that for given (Laser) current densities, the photon density distribution in the array elements is that particular one which maximizes the total photon density. The results of this analysis were then combined with the waveguiding properties of the laser array waveguide, yielding a basic model of phase-locked diode laser arrays. This model explains the effects of the variation of the current combination through the array elements on its mode structure that were observed recently.

TE and TM coupling coefficients for double-heterostructure distributed feedback (DFB) Pb 1-x Sn x Te diode lasers are calculated. Results are presented for rectangular, sinusoidal, symmetric-triangular, and sawtooth gratings for both intermode and intramode coupling. Based on these results, the design considerations of PbSnTe DFB lasers are also discussed.

The supermode structure of index-guided, phase-locked arrays of diode lasers with nonuniform laser spacing is investigated theoretically. It is shown that in a variable spacing array, the super-mode field patterns can be significantly different from those of a uniform array. However, in a variable-spacing array with N elements, supermodes ν and N + 1 - \nu(\nu = 1, 2, ..., N) have similar near-field intensity patterns and modal gains, as in the case of uniform arrays. As a result, the discrimination between the fundamental (0° phase shift) and the highest order (180° phase shift) supermodes is small and depends sensitively on the amount of gain in the interchannel regions, as for uniform arrays. This may explain the inefficient supermode discrimination and the wide beams exhibited by variable-spacing diode laser arrays, in contrast to the strong discrimination and the narrow beams obtained with chirped (variable channel width) arrays structures.

Perturbation theory is used in order to calculate the effective-index distribution in dielectric waveguides that exhibit slow lateral variations. As an example, the method is employed to find the optical modes in diode lasers with lateral spatial variations in thickness. Very good agreement with experiment is obtained for the case of GaAlAs double heterostructure lasers with a crescent-shaped waveguide.

Spatial separation of modes is important in multiport switching and wavelength multiplexing for integrated optics, and in controlling the spatial modes in multimode semiconductor laser structures. It has been recently shown that the m normal modes of an m - mode channel waveguide can be separated by branching into m single-mode channels whose propagation constants are all different. By means of examples, we answer two questions about this mode separation scheme. 1) How widely separated must the channels be in order for each mode to be concentrated in a different one of the channels? 2) How small must the branching angle be in order for the power to remain in the launched mode? The effects of wavelength, dimensions, and refractive index step are discussed. Important conclusions are that the branching angle must be rather small, of the order of a few milliradians, and that it is helpful to have the largest possible difference in the widths of the guiding channels, thereby giving them the largest possible difference in uncoupled propagation constants.

The fabrication of photoresist gratings on reflecting surfaces is considered both theoretically and experimentally. We show that it is important to take into account the actual phase shift of the waves reflected from the surface. It is found that the intensity at the photoresist‐surface boundary may be significantly larger than that at the standing‐wave minimum. The theory is confirmed by the examination of the profiles of photoresist gratings made on PbSnTe, in comparison to GaAs. This phenomenon may have wider applications when the photolithographic fabrication of fine features on reflecting surfaces is considered.

We have studied the OMCVD growth of GaAs/AlGaAs quantum well heterostructures on non-planar substrates in the temperature range of 625 to 750°C. The lateral variation in layer thickness and other growth features were found to depend not only on the growth temperature but also on the aluminum content of the layer. An example of the application of this technique of producing lateral thickness variations in quantum well heterostructures to a quantum well semiconductor laser is given. A unique feature of this laser is the formation of a quantum-wire-like crescent shaped active region at the center of a two-dimensional optical waveguide.

The N Delta interaction in the intermediate energy range is treated in the framework of a scattering theory where the interaction is assumed to proceed via non-static multi-pion exchange. This interaction is studied in three models based on (i) the unitarisation of the Feynman one-pion exchange graph, (ii) the unitary model of Aaron and Amado (1968) and (iii) a realisation of the NV sector of the Lee model (1954). Ignoring spin and isospin effects, s-wave amplitudes and cross sections are calculated numerically within each model.

Distributed Bragg-reflector (DBR) diode lasers were designed and fabricated from lattice-matched Pb 1-x Sn x Te/PbSe y Te 1-y single heterostructures grown by liquid-phase epitaxy. These DBR lasers operated in a single longitudinal mode within a limited range of heat-sink temperatures, 8.5-38 K, with a threshold current density of ∼3 kA/cm2at 20 K. Single longitudinal mode operation was maintained up to more than three times the threshold current. Continuous tuning of the laser output frequency over a range of ∼6 cm-1, near 775 cm-1(12.9 μm), was acheived by varying the heat-sink temperature. The average tuning rate was 0.21 cm-1/K, and it was much smaller than the rate for corresponding Fabry-Perot lasers, which was 2.3 cm-1/K. The measured effective mode index of the DBR lasers agrees well with the calculated one.

The growth characteristics of GaAs AlGaAs superlattice heterostructures on nonplanar GaAs substrates were studied by using both molecular beam epitaxy and organometallic chemical vapor deposition. Substantial quantum well lateral thickness variations are obtained with both growth methods, but their exact nature depends on the growth method and on the orientation of the patterned features on the substrate. These lateral thickness variations should translate into lateral patterning of the physical properties which depend on the quantum well thickness, e.g., the refractive index and the energy band gap. Such patterned superlattices should be useful in optoelectronic device applications, paticularly those involving reduced carrier dimensionality.

The effect of complex coupling coefficients on the performance of distributed feedback (DFB) lasers is studied. The physical origins of a complex coupling coefficient are specified, and its relation with gain or loss mechanisms which occur inside the laser is discussed. Numerical results are presented for the oscillation frequencies, threshold gains, and intensity patterns of the longitudinal modes of the DFB laser.

We demonstrate a single contact tailored gain-guided array in which the gain profile across the array is made strongly asymmetric by varying the width of the contact stripes. A proton isolated array of six (GaAl)As lasers with 5-µm separations and widths varying linearly between 3 and 8 µm had a single lobed far field 2° wide, close to the diffraction limit for a single supermode. Fabrication of this device is simple, and suited to large-scale processing techniques. We also show that in such an asymmetric gain-guided array the fundamental mode is favored over higher order modes, and that higher order modes can have single lobed far-field patterns differing only slightly from that of the fundamental.

Low-loss semiconductor waveguide components are essential for integrating guided-wave optical components with other optoelectronic devices. Trade-offs between fabrication-related scattering loss and bend radiation loss will determine the smallest size practical for semiconductor integrated-optic devices. Rib waveguides with tight optical confinement (small mode size, large index differences) are required for compact bends but are more susceptible to fabrication-induced scattering loss from rough surfaces. An understanding of how rib geometry affects scattering would provide a valuable tool to optimize guide performance. Although scattering in 1-D planar guides has been extensively investigated theoretically,1,2 a complete model for 2-D rib guides is not available.

We report the fabrication and the characterization of low‐loss, single‐mode GaAs/AlGaAs ridge waveguides. The waveguides were grown by organometallic vapor phase epitaxy and were characterized at 1.52 μm wavelength. Their propagation losses were evaluated by measuring the finesse of the Fabry–Perot waveguide resonators formed by cleaving two opposite ends of these waveguides. Losses as low as 0.15 dB/cm were measured for single‐mode waveguides, which is the lowest value reported to date for single‐mode semiconductor waveguides. Such low‐loss waveguides should be useful in guided‐wave components for integrated optoelectronics.

A new monolithic phase-locked semiconductor laser array has been fabricated. Employing two-level metallization, each of the eight elements in the array has a separate contact, thus making it possible to compensate for device nonuniformities and control the near-field and far-field patterns. Threshold currents are approximately 60 mA for each 5-µm-wide laser in the array. Phase locking has been observed via the narrowing of the far-field pattern. Experimental results are compared to those obtained from the same arrays operated with all the lasers connected in parallel.

Inverted‐V chirped arrays of multiple quantum well GaAs/GaAlAs lasers were grown by molecular beam epitaxy. These arrays consisted of seven gain‐guided lasers whose stripe widths decreased, from the central laser to the outermost ones, symmetrically. This structure makes it possible to discriminate against the higher order array supermodes, which results in diffraction limited beams with a single lobe directed perpendicular to the laser facet. Single lobed far‐field patterns, 3°–4° wide, were obtained from inverted‐V chirped arrays operated up to 1.5Ith. The supermode structure of these arrays was identified by studying their spectrally resolved near fields.

A new type of buried heterostructure InGaAsP/InP lasers grown by a single-step liquid phase epitaxy on Cd diffused mesa substrate is described. These lasers exhibit excellent current and optical confinement. Threshold currents as low as 15 mA are achieved for a laser with a 2-µm-wide active region.

The spectrum of semiconductor laser arrays with separate contacts is investigated. It is demonstrated that the individual laser currents can be selected such that the array operates in a single longitudinal mode in contrast to the multimode nature of its individual constituents. Moreover, it is possible to tune the lasing frequency by varying the laser currents. Wavelength tuning range of ~50 Å, with tuning rate of ~5 Å/mA, is demonstrated. It is suggested that these spectral features, characteristic of lasers which are coupled in parallel, result from the strong frequency dependence of their spatial mode pattern near the phase-matching frequency of their coupled waveguides.

The spatial mode characteristics of gain‐guided broad‐area quantum well lasers grown by organometallic chemical vapor deposition were investigated experimentally. GaAs/AlGaAs quantum well lasers grown on 6°‐off (100) oriented substrates exhibit excellent material uniformity, which allows study of their modal behavior. Gain‐guided broad‐area lasers fabricated on such uniform material demonstrate nearly ideal gain‐guiding modal behavior. These lasers tend to lase in the fundamental mode near threshold and emit single‐lobed far‐field patterns. In these well‐behaved broad‐area lasers, we have identified the mechanism for degradation in the spatial coherence at high pumping levels as the onset of higher order lateral modes.

Single‐mode rib waveguides with low propagation loss (0.2 dB/cm) were fabricated on GaAs/AlGaAs single heterostructures by dry etching. The attenuation of these guides was measured to study the effect of rib structure on propagation loss at 1.52 μm. An increase in loss with increased rib height and decreased rib width was associated with scattering due to a rib surface roughness of approximately 50 nm for ion milled waveguides. Fabrication with reactive ion etching reduced the rib roughness and its associated scattering loss by a factor of 3 or more in waveguides with strong optical confinement (effective index difference of 0.012) of the fundamental guided mode.

In exploring new techniques to produce controllable lateral variations in the physical properties of semiconductor superiattices, substrates containing etched patterns were used for epitaxial crystal growth. High resolution TEM was used for the first time to study this kind of structures. It was found that the resultant superlattices exhibit wavy or zigzag layers which register the history of the topographic evolution of the crystal surfaces during growth and reveal useful information on the fundamental mechanism of the epitaxial process.Alternate layers of Al0.3Ga0.7As and GaAs were grown by MBE and OMCVD on (100) GaAs substrates patterned with grooves and mesas along the [011] and [011] directions. The morphology of the superlattices grown on the grooves and mesas was analyzed with TEM cross-sectional imaging. It was found that the evolution of non-planar crystal surfaces depends not only on the source flux and surface affinity, but also on the lateral diffusion of the adsorbed atoms or molecules on the crystal surfaces. Several growth features at the intersections of adjacent facets are illustrated in this work. Implications on the epitaxial process are discussed.