Molecular beam epitaxy (MBE) at high growth rate more than twice compared with the conventional rate was investigated for ZnSe layers and related laser structures by using polycrystalline compounds as source materials. A clearly-streaked (2 x 1) pattern indicating Se-stabilized surface was observed by reflection of high-energy electron diffraction during the growth of ZnSe:N layers at 260 - 360 degrees Celsius with around 2 micrometer/h. Full width at half-maximum of double crystal x-ray rocking curve was as narrow as 85 arc sec for a 6.5- micrometer-thick layer with mirror-like surface morphology. ZnCdSe/ZnSSe/ZnMgSSe separate confinement heterostructure lasers operated at room temperature were obtained with a time required to grow of 80 min. II-VI layers grown by the rapid compound-source (CS) MBE had high crystalline quality comparable to that of the layers grown at the conventional growth rate. CSMBE has the surface migration enhancement effect at the growth front due to high kinetic energy of source molecules. The CSMBE technique solved the problem of low productivity for the conventional II-VI MBE growth.

In order to grow ZnSe based optoelectronical devices on GaAs, the III/V-II/VI interface has to be controlled very precisely. To obtain lattice matching sulfur is added to reduce the lattice constant to the value of GaAs. This element, much more reactive than selenium, aggravates the growth of a perfect interface even more. For ZnSe grown on tellurium terminated GaAs, an improved layer quality and a much higher critical thickness was reported. It is the scope of this paper to explore the advantages of a Te terminated initial surface for the growth of ZnS_xSe_1-x. For this purpose we have investigated the Te and the competing Se and sulfur terminated surfaces with x-ray photo-electron spectroscopy (XPS) and reflection high energy electron diffraction (RHEED). For the use of Te/GaAs as a protection against the reaction of S and Se with GaAs its stability in a Se or sulfur flux was investigated. To use Te/GaAs as a starting surface for an optoelectronic device it is necessary to clarify what happens to the Te after the growth start. From temperature dependent growth experiments, which were interrupted to measure XPS, we could not detect tellurium incorporation. This was confirmed by photoluminescence measurement (PL). With high resolution x-ray diffraction (HRXRD) we detected pseudomorphic growth up to a sulfur content of 19% and a thickness of 240 nm. Both methods show exceptional good layer quality, which is possible due to the tellurium termination of GaAs prior to ZnSe growth.

The formation of contacts by means of indium, aluminum, tin and indium-tin-oxide (ITO) to n-type ZnSe layers grown by MBE has been investigated using x-ray-photoelectron- spectroscopy (XPS) and current-voltage (I-V) techniques. Quite different behavior was found for metal contacts compared to ITO. For the latter it is possible to form ohmic contacts for ZnSe doping levels in the range of n equals 2 multiplied by 10¹⁷ to 3.5 multiplied by 10¹⁸ cm^-3 with specific contact resistances as low as R_c equals 9 multiplied by 10^-3 (Omega) cm². Whereas metal contacts behave according to thermionic emission theory with nonlinear I-V-curves depending on the doping level of ZnSe epilayers.

We have grown p-type ZnSe_1-xTe_x:N (x equals 0.08 - 1.0) epilayers by molecular beam epitaxy on GaAs substrates, and characterized their electrical behavior. The Te fraction x was determined by energy-dispersive x-ray spectroscopy and by high-resolution x-ray diffraction. The free-hole concentrations and mobilities were determined by Hall-effect measurements, and the contact resistances of evaporated PdAu metal to the epilayers were measured using standard transmission-line techniques. The contact resistance decreases sharply with increasing Te content, falling from 0.6 (Omega) cm² for a film with 8% Te to 3.5 multiplied by 10^-7 (Omega) cm² for a pure ZnTe film. Under the growth and doping conditions used, the hole mobility shows a minimum of about 1 cm²/Vs at about 25% Te. It is expected that by optimizing these single-layer properties, the building blocks of an improved electrical contact to ZnSe can be obtained.

Molecular beam epitaxy (MBE) has been used to grow our II-VI green and blue light-emitting diodes (LEDs) on n-GaAs substrates. The main structure consists of a ZnCdSe/ZnSSe triple quantum-well active region, ZnSSe carrier confining layers, ZnMgSSe cladding layers, and a p-ZnTe/p-ZnSe multiple quantum-well contact region. The LED chips, 0.3 multiplied by 0.3 mm² in size, were mounted on LED leadframes and were encapsulated in epoxy. The devices produce light output powers of 3.5 mW (513 nm) and 1.5 mW (486 nm) for a direct current (dc) of 20 mA at room temperature. The corresponding external quantum efficiencies are 7.2% for the green, and 2.9% for the blue LEDs. In particular, the blue LED operated at a low applied voltage of 2.63 V for 20 mA. An aging test showed a half-intensity lifetime of 1000 hours for the candela-class blue LED under a constant dc drive current of 10 mA at an ambient temperature of 27 degrees Celsius.

InGaN single-quantum-well-structure (SQW) light-emitting diodes (LEDs) with an emission wavelength between violet and orange were fabricated. The maximum on-axis luminous intensity of green LEDs was 12 cd and the output power was as high as 3 mW at a forward current of 20 mA, while those of blue LEDs were 2 cd and 5 mW, respectively. The peak wavelength and the full width at half-maximum of the green LEDs were 525 nm and 30 nm, respectively, while those of blue LEDs were 450 nm and 20 nm, respectively. Successfully, InGaN multi-quantum-well (MQW) structure laser diodes were fabricated from III-V nitride materials for the first time. The laser consisted of a InGaN MQW, GaN optical guiding layers and AlGaN cladding layers. The observed stimulated emission was at a wavelength around 420 nm, with a threshold current of 610 mA (8.7 kA/cm²) and a threshold voltage of 21 V under pulsed current injection at room temperature. The stimulated emission also showed a strong transverse electric polarization. The beam full widths at half power for the parallel and perpendicular far-field radiation patterns were 5 and 17 degrees, respectively.

Room temperature hole concentrations of 5 multiplied by 10¹⁷ cm^-3 and mobilities of 8.4 cm²/V-s have been measured on heavily Mg doped GaN layers grown on SiC. Specific contact resistivities of 0.046 (Omega) -cm² have been obtained from TLM measurements on ohmic contacts to these layers. Double heterostructures (DH) of GaN/Al_xGa_1-xN with x equals 0.1 have been grown on n-type 6H-SiC substrates. High quality facets have been fabricated by cleaving these DH structures. Photopumped stimulated emission has been observed in undoped structures at 372 nm at a threshold power density of 72 kW/cm². An optical gain of 1000 cm^-1 was measured in the same samples at 200 kW/cm².

The reliability of devices fabricated in GaN and related alloys, especially under high current densities as would be found in lasers, has yet to be fully characterized. Our previous work investigated the degradation of GaN-based blue light emitting diodes (LEDs) under high pulsed current stress. This work indicated a possible correlation between the high crystal defect density and failures caused by metal migration along these defect tubes. To assess the impact of this data on devices under more normal conditions, several LEDs from both older and more recent production lots were placed in a controlled temperature and current environment for several thousand hours. The test started with a constant 20 mA current for the first 1000 hours and continued at a range of currents up to 70 mA for the remaining 1650 hours, all at a temperature of 23 degrees Celsius. One of the older generation LED's output degraded by more than 50% during the test. The I-V characteristics of this device indicated that there was an ohmic leakage path across the junction which was similar to, but much higher resistance, than was observed on failures from the previous high current tests. The similarity indicates that these LEDs may fail at room temperature and at moderate currents in the same manner as was observed at high currents. Subsequent failure analysis proved that this was not the case, since a crack was found on the degraded LED which isolated part of the active region from the p-contact.

Device modeling is used in the design and characterization of GaN/SiC heterostructure bipolar transistors (HBTs), operating at high power, high frequency, and high temperature. The differential current gain ((beta) ) simulated to be constant for an emitter current over several order of magnitudes and decreased significantly with increasing temperature. The current gain as a function of temperature was obtained from a maximum of 300,000 at room temperature to a value of about 200 at 300 degrees Celsius. Simulated results show maximum cutoff frequency of 8 MHz. The high temperature device modeling approach taken in this study is shown to be essential in the design and optimization of GaN/SiC HBTs for high power high frequency high temperature operation and compares well with the experimental data.

A variety of spectroscopic techniques has been used to study the optical properties of epitaxial GaN based materials grown by metalorganic chemical vapor deposition and molecular beam epitaxy. The emphasis was on the issues vital to device applications such as stimulated emission and laser action, as well as carrier relaxation dynamics. Sharp exciton structures were observed by optical absorption measurements above 300 K, providing direct evidence of the formation of excitons in GaN at temperatures higher than room temperature. Using a picosecond streak camera, the time decay of free and bound exciton emissions was studied. By optical pumping, stimulated emission and lasing were investigated over a wide temperature range up to 420 K. In addition, the optical nonlinearity of GaN was studied using wave mixing techniques.

GaN/In_0.05Ga_0.95N/Al_0.15Ga_0.85N double heterostructures doped with Zn and Si, used in Nichia LEDs, are investigated. Electrical, electroluminescent and photoluminescent properties are presented and discussed. Blue photoluminescence (PL) is analyzed to obtain optical transition parameters (phonon coupling strength and zero-phonon line position) involved in formation of the impurity-related emission band. With a minor modification of parameters for Zn centers in GaN, a satisfactory fit is achieved for PL spectra.

Calculations are presented aimed at optimizing the design of GaN/AlGaN double- heterostructure diode lasers emitting in the near-UV spectral region. Material parameters of GaN active medium are reviewed and specified for modeling of both edge- and surface- emitting laser devices. Comparison with published experimental results indicates that nonradiative recombination in the active region (most likely occurring at heterojunction interfaces) limits the carrier lifetime to approximately 1 ns. Theoretical minimum threshold current density is shown to depend weakly on the assumed band parameters.

We have measured the gain spectrum of an optically pumped 40 angstrom ZnCdSe-ZnSe multiple quantum well. Our calculation, which includes many body effects such as Coulomb enhancement and spectral broadening due to carrier scattering, gives excellent agreement with the experimental gain measurements. We then show the importance of the inclusion of the Coulomb enhancement for the calculation of optical gain when predicting laser threshold currents. This is emphasized by using our gain calculation as a basis to theoretically optimize a simple ZnCdSe-ZnSe quantum well laser structure incorporating the leakage current over the p-type cladding.

An internal loss of 4.6 cm^-1 is estimated from the cavity length dependence of photo-pumped blue-lasing at room temperature (RT) with ZnSe/ZnMgSSe double- heterostructure (DH) grown by metal-organic chemical vapor deposition (MOCVD). Operation of the first blue-green laser diode grown by MOCVD has been demonstrated at 77 K under pulsed current injection. The laser consists of a ZnCdSe single quantum-well ZnSe optical guiding layers, and ZnMgSSe/ZnSSe dually-stacked cladding layers grown on a (100) n-GaAs substrate. The observed stimulated emission is at a wavelength of 473 nm, with threshold current ranging from 90 to 180 mA (0.9 - 1.8 kA/cm²). The emission also shows a strong transverse electric polarization.

In the work presented here we study polarization-related dynamic and noise properties of vertical-cavity surface-emitting lasers (VCSELs) and analyze their performance in polarization-selective free-space optical data transmission links.

Recently, continuous wave (cw) operation of 1.54 micrometer vertical-cavity surface-emitting lasers (VCSELs) up to 33 degrees Celsius ambient temperature has been demonstrated for the first time. These devices employ strain-compensated InGaAsP multi-quantum wells and GaAs/AlGaAs distributed Bragg reflectors that are fused on both sides of the InP spacer. Lasing operation of those double-fused lasers is analyzed using a comprehensive numerical model including thermal finite element simulation, optical transfer matrix analysis, and a k(DOT)p band structure calculations. The simulation of pulsed laser performance measured at different temperatures delivers internal laser parameters. Intervalenceband absorption is found to be the dominating loss mechanism that prevents lasing at higher temperatures. The thermal conductivity of the multilayer mirror is only 33% of the value expected. Optimized lasers with reduced gain offset and with smaller pillar heat generation are simulated exhibiting lasing at higher temperatures. Improved heat sinking by top-down mounting shows the strongest impact leading to cw operation up to 62 degrees Celsius.

A single narrow-linewidth quantum-well absorption peak is coupled to the single-mode resonance of a moderately high reflectivity microcavity, resulting in an anticrossing curve as a function of relative detuning. For zero detuning, two cavity peaks and two photoluminescence peaks are seen. Two quantum wells with different resonant energies result in a system of three coupled oscillators. Nonlinear studies include determination of the nonlinearities of the quantum wells, observation of optical bistability, and saturation of the microcavity transmission.

We have realized a new class of high-Q resonant cavity using two-dimensional photonic bandgap (PBG) structures and showed that its Q-value can be as high as approximately 23,000 in the mm-wave regime. We further show that its modal properties, such as the resonant frequency, modal linewidth and number of modes, can be tuned by varying the cavity size. In addition, we present a new nano-fabrication technique for constructing PBG resonant cavities in the near infrared and visible spectral regime.

A theoretical analysis of the influence of carrier diffusion on the threshold characteristics of semi-conductor microdisc lasers is given. The cold-cavity modes (Whispering Gallery Modes) and their wavelengths are obtained from approximate analytical solutions of Maxwell's equations. In the steady-state, the parameters of these modes are modified in an active semiconductor medium by the presence of intensity dependent gain, refractive index and diffusion of the carriers. An increase of threshold current with decreasing carriers' lifetime is predicted. Also, the dependence of the mode pulling on the diffusion length is discussed. These effects can be explained by considering spatially varying carrier density and therefore the spatially dependent refractive index and gain.

A fully quantum electrodynamical model is presented for the spontaneous emission of light from a quantum well system in a vertical cavity device. The model solves the combined Maxwell and Schrodinger equations to describe the time evolution of both the carrier populations in the quantum well and the radiation field. Structures that present major challenges to other models can be handled by our model without any difficulty. Our model is validated against more standard approaches to the spontaneous emission problem for a simple case.

Vertical-cavity surface-emitting lasers (VCSELs) are a new type of semiconductor laser with intriguing properties. As the small cavity ensures single longitudinal mode operation, the VCSEL mode structure is fully determined by the transverse spatial profile combined with the polarization behavior. After discussing some general ideas behind mode formation in VCSELs, we present a number of experiments performed to obtain the relevant numbers for practical AlGaAs-GaAs devices. These include measurements of: the (polarization-resolved) light output-versus-current characteristic, far-field patterns, the wavefront curvature inside a VCSEL, a spectral analysis of spontaneous emission below and above threshold, and a study of the influence of an axial magnetic field. This article contains a relatively large number of equations and experimental and literature values to make it more useful for later reference. Of course it is difficult to tell how specific some of these numbers are related to the particular planar VCSELs we have investigated.

We extend previous analyses of a recently developed model describing polarization switching in VCSELs. We report on elliptically polarized states and their stability, as well as on the effect of small amplitude anisotropies on polarization state selection caused by birefringence and saturable dispersion. We also study polarization switching by optical injection.

This paper studies excitons and bi-excitons in ternary (Zn,Cd)Se/ZnSe quantum wells, widely used as active region in blue-green laser diodes. Localization on alloy disorder characteristically influences the electronic structure of these excitations and their dynamical behavior. The low-temperature lasing is controlled by bi-excitons. Gain as large as 2 (DOT) 10⁴ cm^-1 and optical threshold densities as low as 2 kW cm^-2 are observed. Due to their localization-enhanced binding energy, bi-exciton signatures are present up to 150 K.

We present a model for gain in a quasi zero-dimensional quantum confined semiconductor system. Due to a multitude of one-electron-hole pair and two-electron-hole pair transitions, the gain region is broad, quasi-continuous and stretches below the absorption edge. Femtosecond experiments in the gain region of strongly confined CdSe quantum dots confirm our theoretical predictions.

The dynamics of radiatively coupled quantum well excitons is investigated. It is shown that collective effects determine the optical response of semiconductor multiple quantum well stacks. In multiple quantum well Bragg structures, collective phenomena manifest themselves as a superradiant decay of coherent electronic excitations. In anti-Bragg structures, the optical coupling induces an interwell energy transport and a splitting of the excitonic resonance.

The optical gain of a quantum-well laser is studied taking into account the valence-band mixing, non-Markovian relaxation and the many-body effects. Conventional gain spectra calculated with the Lorentzian line shape function show two anomalous phenomena: unnatural absorption region below the band-gap energy and mismatch of the transparency point in the gain spectra with the Fermi-level separation, the latter suggesting that the carriers and the photons are not in thermal (or quasi) equilibrium. It is shown that the non-Markovian gain model with many-body effects removes the two anomalies associated with the Lorentzian line shape function with the proper choice of the correlation time.

We have performed ultrafast three-pulse experiments in order to investigate the relaxation of charge carriers from a non-equilibrium distribution in inverted GaAs multiple quantum wells. The first pulse, used to create optical gain, was followed by a conventional pump-probe study. The pump-induced spectral hole inside the optical gain was accompanied by sidebands which occurred about 39 meV above the original spectral hole. These sidebands can be explained in terms of LO-phonon scattering of the charge carriers into the vacancies of the spectral hole.

We investigate systems of one, two, and multiple quantum wells separated by thick barriers in such a way that the electronic states are localized in each well. However, the different quantum wells are coupled due to the electromagnetic field. It is shown that the loss of translational invariance in the growth direction affects the interaction between the optical field and the interband polarization. Multiple reflections of the optical field between the different quantum wells occur and significantly modify the interaction dynamics between the optical field and each well. Depending on the geometry and initial conditions the propagation and amplification dynamics of a short pulse in such a system are investigated. An energy transfer between different quantum wells mediated by the laser pulse is observed. Time resolved reflected and transmitted signals for different geometries and numbers of quantum wells are calculated. The influence of Coulomb effects on the reflected and transmitted pulses is studied. It is shown that the interaction dynamics between the laser pulse and the sample shows a strong dependence on the number of quantum wells and the separation between the different wells.

We present theory and experiments of four-wave mixing in bulk-semiconductor amplifiers. The theory includes bimolecular and auger recombinations. We show experimentally conversion efficiency larger than unit up to 2 THz frequency shift. We measure a signal-to- background ratio compatible with that required by practical applications as frequency converters. The high efficiency of the four-wave mixing process permits the investigation of the carrier dynamics down to an equivalent time resolution of the order of few tens of femtoseconds.

Explicit forms for the rates of energy loss by a hot carrier to optical phonons, plasmons, and quasiparticle excitations are obtained for use in laser modeling. These forms contain the explicit dependence upon all system parameters, and give insight into the origin of each mechanism. Self-consistent screening is included, but it is seen that if the phonon and plasma frequencies ((omega) ₀ and (omega) _p, respectively) are comparable, the usual case, the total rate due to both is well given with no screening of either interaction. The interaction producing quasiparticle excitations is found to be screened to a good approximation by a dielectric function 1 plus (h(omega) _p)²/(2(delta) (epsilon) )² with (delta) (epsilon) the energy exchanged in the collision, quite different from the static form frequently assumed. Losses to plasmons and quasiparticle excitations are seen to be usually comparable and to dominate the phonon rate if the number of carriers per atom exceeds the ratio of the carrier effective mass to the reduced mass of the atoms, a ratio understandable in terms of classical collisions between carriers and between carriers and atoms. Energy loss rates as a function of energy are compared with a detailed computation by Jalabert and Das Sarma.

We have studied exciton tunneling in (Zn,Cd)Se/ZnSe asymmetric double quantum wells using femtosecond time resolved transmission, photoluminescence and time-resolved photoluminescence measurements. The strong Coulomb correlation as well as Frohlich electron LO-phonon interaction in II-VI semiconductors make the tunneling process significantly different from that in III-VI structures. We observe fast (1 ps) exciton tunneling out of the narrow well, although LO-phonon scattering is forbidden for holes in a single- particle picture. However, our theoretical analysis shows that tunneling of the exciton as a whole entity with the emission of only one LO-phonon is very slow. Instead, the exciton tunnels via an indirect state in a two-step process whose efficiency is dramatically enhanced by Coulomb effects.

Results of microscopic modeling of semiconductor vertical-cavity surface-emitting lasers (VCSELs) are discussed. The treatment of the laser as a nonequilibrium many-body system provides a detailed understanding of the various processes that determine the laser output and the electron-hole-plasma excitation. It is shown that the transient gain dynamics are strongly influenced by nonequilibrium carrier effects. These gain dynamics together with the cavity design determine the delayed onset and the temporal and spectral shape of the laser output. The theory is evaluated to investigate how the laser output properties can be controlled in terms of (1) excitation conditions of the VCSEL, (2) the mirror design, which allows us to change the cavity quality and the resonance frequency, and (3) the number and position of semiconductor quantum wells as active material.

A microscopic approach for the computation of semiconductor quantum well laser power spectra is presented. The theory is based on nonequilibrium Greens functions techniques that allow for a consistent description of the coupled photon/carrier system fully quantum- mechanically. Many-body effects are included through vertex corrections beyond the random phase approximation. Bandstructure engineering effects are incorporated in the theory as dictated by the coupled band solutions of the Luttinger Hamiltonian. The influence of the detailed cavity-mode structure is accounted for by the photon Greens function. Numerical results are presented for III-V systems at quasi-equilibrium and active optical switching is demonstrated in specially designed structures.

A variational and diffusion quantum Monte Carlo study of excitons in the In_1-xGa_xAs/GaSb_1-yAs_y quantum wells is reported. The variational Monte Carlo technique estimates the variational energy of an assumed trail wave function, and diffusion quantum Monte Carlo gives the exact ground state binding energy by simulating a branching random walk. As the unique band-edge relationship between the two host materials leads to a semiconductor-semi-metal transition with composition, we present the ground exciton state in two cases of (Delta) E greater than 0 and less than 0, where (Delta) E is the energy gap. The calculated binding energy (E_B) is shown as a function of well thickness (L). At L equals 30 angstrom, much larger than an earlier calculation (approximately 0.9 meV) given by a simple variational method, but are close to the experiments based on far-infrared magnetospectroscopy. Our calculations suggest that a novel semiconductor-excitonic insulator transition should be observable in the spatially separated electron-hole system.

Carrier transport and carrier capture were reported to markedly influence the carrier and photon dynamics in quantum-well semiconductor lasers and to limit the modulation bandwidth. Recently, model calculations of various degrees of complexity have isolated special aspects of the problem. We given an extended overview and report on our theoretical and experimental results on 1.55 micrometer AlGaInAs/InP lasers with strongly asymmetric transversal waveguide structures. The self-consistent solution of the Poisson and continuity equations is based on measured carrier mobilities and not only limited to the confinement region. The confinement factor is pointed out to be important when comparing different asymmetric structures. The use of optimized asymmetric structures is demonstrated theoretically and experimentally to enable a distinct improvement in modulation bandwidth and to counteract the limiting physical processes such as carrier transport and carrier capture-escape. Finally, the influence of the shape of the longitudinal carrier and photon density profiles on the modulation behavior is studied. We found that a better homogenization of these profiles for transversally optimized structures may slightly increase the bandwidth. This is further confirmed experimentally by comparing lasers of different profiles applying chirped DFB gratings implemented by bent waveguides.

Dynamical behavior of a four-stripe array of semiconductor lasers is modeled in time and across the transverse spatial dimension for different values of the injection current, various current and carrier diffusion rates, and different spacings of the stripes, including the limit in which the lasing regions merge giving a single broad-stripe laser. Various stages of the dynamics are revealed as a function of the coupling strength, ranging from quasi-independent operation of the oscillators under weak coupling of the oscillators through chaotic modulation under moderate coupling and finally into the formation of stable or modulated broad area transverse modes under strong coupling.

Some key carrier transport models in the Minilase-II quantum well laser simulator are presented. They include the ballistic injection of bulk carriers into the quantum well, the scattering dynamics of quantum carriers, and quantum carrier temperature. The simulator is then used to investigate the nonlinear effects of certain carrier transport processes. Next the simulator is compared directly to experimental data. The comparison shows the importance of spectral hole burning and hot carrier effects at high output power, and it also introduces some new ideas involving the fast and slow pumping of the quantum lasing states. Finally, some preliminary simulations of multiple quantum well lasers are presented to show the highly nonuniform nature of multiple well active regions.

One of the interesting features in a semiconductor laser with two closely arranged waveguides is the crosscoupled-mode operation, where the light is coupled from one waveguide to the other resulting in an asymmetric output. In the operation, the laser has bistable characteristic which is similar to a set-reset flip-flop in electronics, and hence, useful for optical switching and logic operations. The characteristic has so far been observed with around 500-micrometer long twin-stripe lasers, which is rather long from the viewpoint of integration. Here, the means for shortening the laser is proposed, the theoretical limit of shortening the cavity of AlGaAs lasers is derived, and practical issues for shortening the laser cavity are discussed.

In this work we have studied the nonlinear behavior of direct semiconductor lasers, based in the rate equation description of the laser. We have dedicated special attention to the influence of the nonlinear gain compression, which has served in recent studies to include many new effects affecting carriers and photons. We have studied its effect in the onset of nonlinear behavior, in particular, in the first period doubling bifurcation. Also, the nonlinear route to chaos is studied, finding for the first time numerically a route to chaos previously observed experimentally.

We present an overview of the design considerations for ultra high frequency (UHF) direct small signal modulation of semiconductor lasers. The influences of the optical guide design and cavity design are examined.

Theoretical studies of aluminum-free RISAS and ARROW type lasers operating at 800 nm and 940 nm, respectively, are presented. At 800 nm, the electron leakage current over the hetero barriers leads to a sub-linear light-current characteristic. In order to obtain a high output power at moderate currents, either the losses must be kept as small as possible, or the barriers for the electrons must be increased, for example by higher p-doping. At 940 nm, the leakage current is not as problematic. In both RISAS and ARROW lasers, excess loss for the higher- order modes is needed to prevent them from lasing. The theoretical maximum single mode power of ARROW lasers obtained with a two-dimensional FEM-solution of the scalar wave equation is lower than found with the effective index method.

The features observed in luminescence and photoreflectance spectra are interpreted by detailed modeling of the electronic states, absorption and luminescence of the active region. The electronic states for the full active region (quantum wells together with separate confinement layers) are calculated using an eight band k (DOT) p model. The axial approximation, tested to be sufficiently accurate, is used to reduce the computational burden. The Poisson equation is included self consistently for the optically pumped case. Analysis of the photoreflectance spectra includes incorporation of an electric field across the active region. Good agreement for the positions of the features and their trends with compositional variables verifies the accuracy of the model. Higher lying transitions involve electron levels above the barrier energy which can be confined to the region of the wells by the self consistent field for pumped material.

We have examined the critical factors in calculations of optical gain and spontaneous recombination in GaAs, GaInP, and GaN semiconductor quantum well systems, in particular, the behavior of the transparency current and the effects of Coulomb enhancement. We have also compared the optical confinement factors, and the relative importance of spectral broadening by de-phasing of the polarization due to carrier-carrier interactions and by unintentional monolayer well width variations. These effects are quantified for the three material systems. We also present the results of full simulations of the potential distribution through GaInP laser structures and compare these with the common flat-band approach.

This paper reviews the use of the contactless electromodulation spectroscopy methods of photoreflectance and contactless electroreflectance for the nondestructive, room temperature investigation of wafer-scale single or multiquantum well laser structures including 0.98 micrometer InGaAs/GaAs/GaAlAs (graded index separate confinement heterostructure), 1.3 micrometer InGaAsP/InP, 0.98 micrometer InGaAs/GaAs/InGaP and 0.65 micrometer InGaP/AlInP/AlGaInP planar as well as InGaAs/GaAs/GaAlAs and GaAs/GaAlAs vertical cavity emitting samples.

This presentation consists of three parts: (1) A new mode solver for structures with high gain and/or losses is discussed. By using 'scattering matrices,' great numerical robustness is obtained. (2) In planar waveguides, the amplification for TE modes can be much larger than that for TM modes, even though the confinement to the active layer is comparable. The connection between confinement factors and gain is elucidated. We show that an often used approximation for the TM gain is not valid for practical configurations. An improved approximation is given. (3) We describe how a waveguide with an anisotropic active layer can be modeled using a scattering approach. This model should then tell how much anisotropy is needed to get a gain that is polarization-independent.

A theoretical and experimental study of passive colliding pulse mode-locked quantum well lasers is presented. The theoretical model for the gain dynamics is based on semi-classical density matrix equations. The gain dynamics are characterized experimentally by pump-probe experiments and compared with theory. The mode-locked pulse width dependence upon dispersion, gain current and absorber bias is presented and compared with experimental results.

This short tour through computer-aided engineering (CAE) for integrated optics reports on the following topics from a CAE point of view: eigenmode analysis beam propagation, coupled mode theories, computer-aided design and some software aspects. Status and trends are discussed in terms of the fully developed CAE scenario of microelectronics. BPM benchmark results are presented to provide a feeling for the predictive power of today's solvers.

We examine several of our recent findings regarding field evolution methods. We focus particularly on three-dimensional solution algorithms for wide-angle wave equations including Lanczos, hybrid finite-difference/fast Fourier transform and finite-difference/Lanczos procedures. A novel alternating directional implicit formalism for general paraxial vector wave equations is considered and its stability and applicability to problems of practical interest are discussed in some detail. We also examine methods for stabilizing wide-angle vector algorithms and examine the role of modified Pade approximants and of appropriate dissipative boundary conditions. Finally, we compare the effectiveness of absorbing and transparent boundary conditions and consider the relative advantage which may be realized by employing both conditions simultaneously.

Thin-film induced stress has been investigated on GaAs mesa structures integrated with ZnO films. ZnO films were sputter deposited on GaAs with a SiO₂ thin buffer layer. Stress on the cleaved facet of the GaAs mesa was imaged with a spatially-resolved and polarization- resolved photoluminescence technique. The result shows that the GaAs mesa is stressed up to 1 by 10⁹ dyn/cm² (10^-3 strain) due to a residual compressive stress from the deposited films. A finite element analysis was also carried out to calculate the stress distribution. The simulation result shows a good agreement with the experimental result. As an application of the thin-film induced stress, we propose a new waveguide structure, in which vertical confinement of light is obtained via a photoelastic effect while lateral confinement is made by a mesa structure. A numerical analysis shows that the proposed waveguide structure can support vertical modes with the amount of thin-film induced stress observed in this work. The proposed structure can be fabricated on bulk semiconductor substrates without requiring any separate cladding layers for vertical confinement of light. The proposed structure, therefore, is promising for low propagation loss, 2-dimensional waveguides that can be formed with a simple and economical method.

The results are reported on the study of nonlinear optical properties of poly(methyl methacrylate) waveguides doped with 2,9,16,23-Tetrakis(phenylthio)-29H,31H- phthalocyanine. Polymeric host provides acceptable waveguide quality that is hardly achievable for phthalocyanine monocrystaline films or glasses. In comparison with ordinary materials such as lead-phthalocyanine, 2,9,16,23-Tetrakis(phenylthio)-29H,31H- phthalocyanine shows better solubility and is less affected by polymeric matrix. It preserves strong absorption in visible region with single peak at 627 nm. Third order nonlinearity of phthalocyanine doped polymer was studied with Z-scan and degenerate four wave mixing techniques, and also with waveguide mode spectroscopy based on prism coupling. Slow thermal component of third order susceptibility was measured using cw Ar⁺ laser (514 nm line) in the spectral region out of the absorption band, and He-Ne laser (633 nm line) inside the absorption band. In the latter case nonlinear refractive index was minus 1.87 multiplied by 10^-2 cm²/W providing strong shift of waveguide modes to the region of low indices where light confinement might no longer be achieved. Transient and fast third order susceptibility is studied using nanosecond and picosecond pulses from frequency doubled Nd:YAG laser at pump power up to 100 MW/cm(superscript 2$.

Concept, basic physics and modeling results of a novel modulation technique, associated with a variable heating of the electron-hole plasma in the active region of a lasing device, are reviewed with respect to the ultimately high-speed performance of semiconductor lasers. It is shown, that independent control of the plasma concentration and temperature provides a way for generating the picosecond gain-switched optical pulses and multi-Gbit/s modulation with, optionally, no parasitic frequency chirp. To practically realize this method, three-terminal laser structures are suggested, in which two bias voltages are intended to drive the pumping rate and the energy, yielded in the active region plasma as a result of injecting a single electron.

Short pulse sources are finding a wide range of applications in high bit rate and soliton fiber optic communication systems. Dye lasers and solid state lasers can generate such short pulses; however, their size and energy consumption make it less desirable to include them in practical systems. A monolithic semiconductor source is the most attractive option, because it is both small and efficient. We present a theory for passive mode-locking in semiconductor lasers including the effects of self-phase modulation, dispersion and pulse collisions. Material parameters and different configurations, such as Fabry-Perot and ring lasers, are considered and the stability for mode-locked operation is quantified. A ring configuration is found to have the largest range of stable mode-locking operation. Finally we present results of colliding pulse mode-locking in lasers consisting of monolithically integrated gain, absorption and passive waveguide sections. The lasers, operating around 1.3 micrometer wavelength, consist of InGaAsP active and guiding regions grown on InP, with ridges defined by reactive ion etching. Pulses as short as 560 fs, measured by intensity autocorrelation, have been produced. It is shown that the length of the pulse is not limited by the absorber length, but by the spectral width of the active material, as predicted by the theoretical model.

We present a new method for the generation of short optical pulses in DFB-lasers with repetition rates from 10 to greater than 100 GHz. The method does not require any electronic HF-control and is based on cw-light injection into another cw operated DFB-laser. The pulse generation is studied by a transmission line laser model (TLLM) containing spatial hole burning and all relevant effects including spontaneous emission noise of complex coupled multisection DFB-lasers. A special pulse source has been optimized by the TLLM with respect to single-mode output power under cw-operation and yield. Modulation of pulse sequences by an external modulator has been simulated at bit-rates of 40 Gb/s. The effect of pulse broadening on long haul dispersion shifted single mode fiber (DSSF) transmission due to the spectral width is studied. Two by 20 Gb/s OTDM results are compared to 40 Gb/s direct external modulation.

We propose a new method for obtaining high-power short optical pulses without trailing pulses from an actively mode-locked laser diode. To quench the residual optical gain, a part of the output is reflected back into the laser diode with an orthogonal polarization and an appropriate time delay. The dynamics of the laser diode were numerically analyzed using traveling-wave rate equations. We show that the trailing pulses caused both by the excess gain and the residual reflection at the laser diode facet are drastically suppressed. We also report experimental results which demonstrate the effectiveness of the proposed method for the reduction of the trailing pulses caused by the excess gain using 1.55 micrometer InGaAsP laser diodes.

A mathematical formulation for the analysis of relaxation oscillation in mix-coupled distributed feedback laser is developed and presented. A set of generalized rate equations is established where the spatial hole burning, the nonlinear gain, the standing wave effect and the distributed noises have been taken into account. An analytical expression for the relaxation oscillation in such lasers is derived. As a result, the dependence of the relaxation oscillation on spatial hole burning, complex coupling and side mode are examined.

The temperature sensitivity of the threshold current of 1.3 micrometer semiconductor lasers, denoted by the characteristic temperature T₀, has remained low, with values ranging from 40 K up to a maximum of order 100 K. We report here on a combined theoretical and experimental analysis to identify the dominant factors contributing to this poor temperature sensitivity. We have determined directly the temperature dependence of the radiative current density, J_rad, by measuring the integrated spontaneous emission, L, from bulk and strained quantum well buried heterostructure devices. We find an effective T₀ for J_rad of around 200 K for the bulk device and around 300 K for the quantum well device, in good agrement with the theoretical prediction for ideal lasers. This radiative temperature dependence compares with the measured T₀ of around 50 - 60 K for the total threshold current density in both devices, from which we conclude that radiative recombination is not the dominant mechanism of the temperature sensitivity of the laser. We also find from the spontaneous emission data that just below threshold L varies with current I as I varies direct as L^3/2, which is expected in the Boltzmann approximation if auger recombination is the dominant current path. We have used these findings to estimate T₀ from as simple analytic expression we have derived and find values at room temperature of 40 - 100 K, in agreement with experiment. This poor T₀ results both from the temperature dependence of the differential gain and by the major contribution of auger recombination to the total threshold current.

Distributed feedback (DFB) lasers are used extensively as optical sources within fiber optic telecommunications systems. These semiconductor lasers are valued for their single optical mode operation which enables their use in high bit rate digital and broadband analog transmission systems. We discuss requirements imposed upon DFB lasers used in such applications. We briefly review and illustrate general techniques for modeling DFB laser operation by comparison of measured and modeled device characteristics below and above lasing threshold. We focus on the phenomenon of spatial hole burning which substantially affects operation of all DFB lasers, especially those tailored to produce high fiber coupled power from a single mirror facet. We illustrate insights gained by measurement and modeling of DFB lasers using as an example the particular case of analog transmission of multichannel video signals.

Leakage of electrons from the active region of InGaAs/InP laser heterostructures with different profiles of acceptor doping was measured by a purely electrical technique together with the device threshold current. Comparison of the obtained results with modeling data and SIMS analysis shows that carrier leakage of electrons over the heterobarrier depends strongly on the profile of p-doping and level of injection. In the case of a structure with an undoped p- cladding/waveguide interface the value of electron leakage current can reach 20% of the total pumping current at an injection current density of 10 kA/cm² at 50 C. It is shown that carrier leakage in InGaAsP/InP multi-quantum-well lasers can be minimized and the device performance improved by utilizing a p-doped SCH layer.

Numerical models enable novel devices to be designed without the need for costly prototypes. Furthermore, internal variables, such as carrier density, are easily monitored, allowing a greater understanding of device operation to be gained. Recent advances in numerical techniques for the design and study of photonic devices, circuits systems are discussed. A comprehensive computer-aided design package for photonics is presented, and examples of photonic device, circuit and system simulation are shown.

Injection current dependence of side mode suppression ratio of HR-AR facet coated, loss coupled DFB lasers is examined through numerical simulation and experiments.

Spectral study is presented of strained InGaAs/GaAs/AlGaAs quantum-well laser structure (emission peak near 980 nm) in wide temperature and injection current density ranges. Special measures are undertaken to reduce the distortion of the spectral distribution of spontaneous emission by stimulated optical processes. Dynamic degeneration level over 10 kT at low temperature is achieved. The band filling process and spectral broadening are investigated, and characteristics of both processes are determined.

We present an asymptotic analysis of the equations describing a semi-conductor laser with optical injection. This analysis focuses on the phase drift regime where the slave laser is not locked to the injected signal. In addition to the basic four wave mixing solution, we find subharmonic bifurcations and coexisting isolated periodic solutions (dynamic isolas), which occur close to integer detunings. We also present experimental data that confirm some of these predictions.

We present an all-optical scheme based on continuous delayed optical feedback for controlling delay-induced chaotic behavior of high-speed semiconductor lasers. The successful control of continuous wave and unstable periodic or quasiperiodic dynamic states with vanishing controlling force term is demonstrated by detailed numerical analysis.

A new, broadly applicable technique for obtaining optical rate equations is derived for laser cavities having a variety of possible exterior couplings, including field injection and feedback. The method, based on analytical continuation of the roundtrip condition, retains essential resonator physics over the whole spectral range and all magnitudes of coupling, in contrast to existing rate equations. Injection locking, external feedback, and phase conjugative interaction are discussed as applications.

Most of the previous treatments of semiconductor lasers subject to optical feedback from a phase-conjugate mirror (PCM) have assumed the PCM responds instantaneously. Furthermore, the mechanism responsible for phase conjugation does not usually enter into the analysis. In this paper are derived the time-dependent reflectivity from a PCM created through non-degenerate four-wave mixing. The resulting laser dynamics are compared to the case of the ideal PCM, as a function of PCM mirror interaction depth, distance to the PCM, and laser current. The time-responsive PCM tends to suppress otherwise chaotic output and produces power pulses whose frequency is tunable by varying laser current or PCM reflectivity.

An asymptotic theory of Lang and Kobayashi (LK) equations describing a semiconductor laser subject to optical feedback is investigated in detail. We obtain a simple third order, nonlinear, delay-differential equation for the phase of the laser field which admits multiple branches of time-periodic intensity solutions. The theory is based on typical values of LK dimensionless parameters and assumes that the pump parameter is not too small. In this paper, we examine the validity of this assumption by considering the small pump limit. We find the same phase equation as the leading problem of our asymptotic analysis but now with a stronger damping coefficient. This phase equation fails as a correct asymptotic approximation only for very low pump, close to the lasing threshold. The approximation for this case is more complicated and reveals a stronger influence of the laser intensity.

Using a Mach-Zehnder interferometer, we have injected a perturbed Gaussian light beam into a tapered semiconductor optical amplifier. The growth rate of the amplitude of the filamentary perturbation depends on the amplifier gain, its geometry, and the fringe spacing. Numerical solutions of a relatively simple set of ordinary differential equations for the amplitude of a sinusoidal filamentary perturbation in a tapered semiconductor amplifier agree qualitatively with the results.

In this work we present a circuit model of coupled cavity semiconductor laser. The model is based on the two axial coupled semiconductor rate equations. The results obtained by this circuit model and the analysis of the rate equation in the small signal regime are discussed showing good agreement between both models.

The work presents results of the measurement of spatial and polarization far-field characteristics of the output radiation from the double-clad fiber. The beam width is found to depend on the input conditions. Polarization characteristics of the beam show existence of spatially-incoherent fields in the fiber which cause depolarization of the total output field. Theoretical interpretation of the effects is done in the framework of the modal approach. By means of numerical simulations it is shown that far-field output beam width, as well as its degree of polarization, depends on the input Gaussian beam spatial parameters due to interference between core mode and cladding modes of the fiber.

Rotation of the output speckle-pattern of multimode fiber was experimentally observed when the input radiation frequency was sweeping. Variations of the speckle-pattern were studied theoretically by means of the correlation analysis. The effect is confirmed and is shown to depend on the conditions of the guided modes excitation.

Beam anomalies in transmission of Gaussian light beams through the dielectric waveguide with incorporated periodic relief grating are investigated theoretically. The anomalies take place near the critical incidence, determined by Bragg condition, when the divergency of the incident beam is of the order or greater than the angular width of Bragg resonance. They manifest themselves as macroscopic transformation of transmitted beam shape and can be accompanied by multiple oscillations of intensity in the beam spot. The transformation of the beam shape is stipulated by interference effect due to the resonant excitation of guided modes in the waveguide by incident bounded light beam and subsequent rescattering of these modes in the direction of transmitted beam propagation.

Laser diodes and amplifiers of a nevv type based on asymmetric quantum-well heterostructures having a set of active layers different in thicknesses are considered. For such modified low-dimensional systems, in contrast to conventional laser heterostructures, the gain spectrum and a series of stimulated emission frequencies are varied through a wide range by choice of the widths and component compositions of quantum wells and barrier regions. Keywords: asymmetric quantum-well heterostructures, gain spectra, spectral broadening

We present a technique for calculating eigenenergies and eigenstates of arbitrarily shaped quantum wells in this paper. We compare the technique with some other conventional numerical method in the case of a rectangle quantum well and a step quantum well under an external electric field. The comparison shows that the technique is simple, time saving, but highly accurate. The technique is applied to the study of Stark shift, oscillator strength, and optical absorption on the conditions of variation of structure parameters and the strength of external electric field. We optimized structure for high performance asymmetric coupled quantum well optical switches.

Theory and numerical simulation of the stability and dynamic behavior of a diode laser array with nearest-neighbor coupling are discussed. The coupling coefficient is assumed to be the complex number. The linewidth enhancement factor plays the crucial role in instability development and wave field dynamics. In dependence on the values of the amplitude and phase of the coupling coefficient and of the linewidth enhancement factor the stable phase- locked operation, waves of the laser field distribution, regular oscillations, and chaotic dynamics can be observed. For different above threshold conditions as phase as amplitude chaos could be realized. The steady state and dynamic operation for a linear and ring arrays are compared. The linear stability analysis of nonlinear array modes for ring configuration is developed. Along with the wave field amplitudes the discrete Fourier spectrum was calculated. Results of simulations demonstrate that the active medium inertia plays an important role in the evolution of wave fields in the array, particularly when the characteristic variation time is on the order of the period of the relaxation oscillation for a sole laser. In the near-to-resonance conditions the increase in the intensity modulation depth took place, and the cooperative nature of oscillations became more pronounced.

We report on the application of reflectance-difference (RD) spectroscopy to the characterization of 60 degree dislocations in zincblend semiconductors. We discuss a physical model based on dislocation induced anisotropic strains which predict a RD lineshape proportional to the first energy derivative of the semiconductor reflectance spectrum. We present RD spectra for semi-insulating GaAs:Cr (100) crystals in the 1.2 - 3.5 eV energy range, which show a first derivative component in accordance to our model. From a fitting of the experimental RD spectra to the theoretical lineshape we obtain average values for the strains associated to 60 degree dislocations. We also show that for the samples reported in this paper the dislocation-induced anisotropic strain results in a normalized effective change in lattice constant in the range from 10^-5 to 10^-4.

We introduce a novel method of modeling PLZT phase modulators. Traditionally, modeling has been based upon fitting the constant quadratic electro-optic coefficient to empirical data. Our characterization has shown that the electro-optic coefficient is not a constant and that the electro-optic effect saturates at electric field strengths that exist in standard surface electrode device configurations. We have also found that the additional effects of light scattering and depolarization, which depend on the strength of the applied electric field, are significant factors for modeling device design and optimization.

This paper describes coupling and switching of optical radiation using metal-semiconductor- metal (MSM) structures, specifically in a metal-on-silicon waveguide configuration. These structures have the special advantage of being VLSI-compatible. Three different designs were successfully used to examine modulation and optical switching based upon nonlinear interactions in the silicon waveguide; a traditional Bragg reflector design and a phase-shifted structure were used to observe thermally-tunable switching of nanosecond-regime Nd:YAG pulses. Finally, a normal-incidence structure was examined which exhibited nonlinear reflectivity modulation.