Annotation of the results obtained in 2016
In field of diode lasers with narrow angular beam divergence based on a concept of coupled optical cavities, we performed simulations and revealed conditions, which have to be satisfied in order to ensure high stability of beam divergence and other beam parameters of such lasers with respect to variation of external factors (such as ambient temperature and injection current) as well as unintentional modification of layered structure (thickness and composition of epitaxial layers). It was shown that the first condition is met if the thickness of the active waveguide is sufficiently broad, so that the effective refractive index of the fundamental mode is close to the refractive index of the waveguide material. The second condition can be satisfied in properly adopted parameters of the laser design (thickness of the passive waveguide, thickness and height of the optical barrier). In the optimized structure the necessary accuracy of the structure parameters can be easily achieved by means of epitaxial growth methods. A design of a diode laser with coupled optical cavities was developed to comprise two passive waveguides. This gives an opportunity to suppress two parasitic optical modes (instead of one in previously developed design) and, as a result, to enhance further the thickness of the active waveguide, thereby providing further narrowing of the beam divergence. In experimentally realized diode lasers of this kind with the active waveguide as broad as ~5 micrometers, lasing was achieved on the fundamental transverse mode. This leads to realization of the vertical beam divergence as narrow as 13 degrees. High stability of beam divergence with respect to variation of ambient temperature and injection current was experimentally demonstrated for diode lasers with coupled optical cavities up to at least 90oC and 8 thresholds, respectively. In field of diode lasers with asymmetric barrier layers (ABL), we fulfilled investigation of diode lasers operating in regime of spontaneous emission at high injection currents (above 20 kA/cm2) by means of scanning near-field optical microscopy (SNOM). These studies revealed that the near-field luminescence associated with carrier recombination in the waveguide of the ABL-laser is asymmetrically distributed across the waveguide. The luminescence maximum is shifted toward the p-type doped cladding layer. At the same time, the waveguide luminescence in the test structure is symmetrically distributed with respect to the waveguide center. The results achieved can be explained taking into consideration that spatial distribution of an optical mode, which wavelength is correspond to the waveguide recombination, is affected by spatial profile of the imaginary part of the refractive index caused by intraband absorption. Higher concentration of electrons and hole in the waveguide leads to weaker absorption. Asymmetric profile of SNOM intensity in the ABL-laser structure reflects the fact that InGaP barrier located near the n-type doped cladding layer is capable of blocking the hole transport. At the same time, AlGaInAs barrier located near the p-type doped cladding layer can bock the electron transport in less extent. As a result, bipolar population is suppressed in the left-side hand of the waveguide, whereas the right-hand side of the waveguide is populated by both electrons and holes. We developed a design of an ABL-laser suitable for operation in the 808-nm spectral range. The structure is based on results of previously fulfilled modeling of the electron transport through the asymmetric potential barrier formed by AlGaInAs/AlGaAs heterojunctions. The modeling revealed that interaction of two factors, both affected by indium composition in the barrier layer, (i.e. bandedge discontinuity at the heterojunction and critical thickness of dislocation-free growth), jointly set the lowest transparency for layers with indium mole fraction of about 22-23%. In contrast to previously studied structures, this ABL lasers were synthesized by molecular beam epitaxy. We investigated diode lasers with the stripe width of 50 micrometers and various cavity length and found that the lasing wavelength is practically unaffected by the presence of asymmetric barrier layers being in the required spectral interval. Simultaneously, ABL-diode lasers of all studied lengths demonstrated a significant reduction (by 1.5-2 times) of the threshold current density as compared to the test lasers without the ABLs. In field of diode lasers with quantum dot active region synthesized on InP substrates, we proposed and realized a novel type of quantum dots, namely InAs quantum dots capped with a thin layer of low-temperature InP, which are capable of achieving room temperature emission in the 1.5-micrometer spectral range. Correlation of structural and optical parameter of such quantum dots with regimes of their growth was studied. It was found that the optimal temperature of QD deposition of 485oC corresponds to the brightest photoluminescence with peak wavelength of 1.53 micrometer. Using InAs quantum dots capped with a thin GaAs capping layer, we realized ridge-waveguide diode lasers operating in the spectral range around 1.5 micrometer in the 20-60оС temperature interval. A record-high (for InP-based quantum dot lasers of this spectral range) temperature stability of the threshold current was achieved in those lasers (characteristic temperature as high as 205 K was obtained in the 20-50oC temperature region). It was revealed that the characteristic temperature in quantum dot lasers of this kind increases with increasing the bandgap of the waveguide material. We developed a design of an InP-based diode laser operating in the 1.5-micrometer spectra interval, comprising an asymmetric barrier layers in the waveguide (InAlAs at the p-type clad side and InP at the n-type clad side) in combination with an asymmetric waveguide (we used InGaAsP and InAlGaAs quaternary alloys, respectively). In these lasers, the room temperature threshold current density was reduced by several times (down to 400-1200 A/cm2, depending on the cavity length); the internal optical loss was decreased by 1.5 times (down to 2.6 cm-1) as well. In field of tilted-wave lasers, we demonstrated that linear behavior of the light-current curve of such lasers is preserved in pulse operation regime up to 20 А, whereas the maximal output power can reach 40 W. The far-field beam pattern remains unchanged with increasing the injection current (approximately to 15 A). Further increase of the injection current leads to step-like change of the angular position of the main beam lobes (from 24.8 to 23.5 degrees) caused by lasing mode hopping. It was shown that temperature variation of the far-field pattern is also characterized by a step-like behavior. A possibility to reduce the total vertical beam divergence (i.e., an angle between two main lobes of the output beam pattern) down to 20 degrees by means of increase of the active waveguide thickness was theoretically proven and experimentally demonstrated. It was shown that the concept of a tilted-wave laser can be supplemented with asymmetric barrier layers leading to an improvement of laser temperature stability. In broad stripe diode lasers of this kind grown by MOCVD, continuous-wave operation regime up to 120oC was achieved, whereas the characteristic temperature estimated in the 20-70oC temperature interval was improved up to 150 K. All the other peculiarities of tiled-wave lasers were kept undisturbed. In field of quantum-dot based lasers, including quantum dot lasers with modulation p-type doping, we proposed an analytic approach for modeling of quantum dot laser performance. The model predicted that the charge neutrality is broken in quantum dots, including modulation QD strictures with modulation doping of the active region. Use of modulation doping of quantum dots with acceptor impurities gives an opportunity to suppress a transition to the excited-state lasing owing to an impact of impurities on rate of charge carrier capture into quantum dots. The results of modeling are in a good agreement with the experimental data. It was experimentally shown that, in quantum dot lasers with moderate cavity length (0.6 mm), an increase of level of modulation p-type doping results in increase (from 0.85 to 2.21 W) of the maximal output power, which can be achieved on the ground-state optical transition, owing to suppression of lasing emission on the first excited-state optical transition. In shorter cavity lengths, similar behavior was observed for the 1st and the 2nd excited-state optical transitions. At the same time, use of the modulation p-type doping makes it possible to realize ground-state lasing even in those cases, when in undoped quantum dot lasers lasing starts on the excited-state transition. Effect of modulation p-type doping on internal optical loss was estimated to be 0.06 cm-1 multiplied by surface density of acceptors normalized to the surface density of quantum dots. It was shown that, at high levels of modulation p-type doping (5x10^17 cm-3) and/or in diode lasers with long cavity lengths (1 mm), growth of internal optical loss dominates and lead to certain reduction of the output power. In direction of development of international scientific cooperation, we organized an international scientific team comprising 9 foreign and 9 Russian researchers. Scientific results achieved in 2016 were published in 8 articles in Web of Science journals (including 6 in English language journals and 2 in Russian language journal having an English version). The results achieved in 2016 were also presented at 1 Russian and 5 international scientific conferences in form of 13 conference presentations (including one invited talk). Within the project scope, we organized two international events which can be considered as a scientific school for young researchers. These are the 3rd international school-and-conference entitled “Saint-Petersburg OPEN 2016” (on Optoelectronics, Photonics, and Nanostructures) and a seminar with international participation and tutorial elements for young researchers entitled “Relevant issues of physics and technology of semiconductor nanomaterials”. As compared to 2015, number of participants of “Saint-Petersburg OPEN” approximately doubled and reached about 300 persons from 13 countries. All the project tasks planned for this stage were completed, all expected results were achieved and the stage goals were reached.

 

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Annotation of the results obtained in 2014
In the current year of the project implementation the tasks can be divided into three major groups: - development of emitting near 1.55 um InP-based lasers with quantum dots and asymmetric barrier layers; - development of high-power high-brightness lasers with asymmetric barrier layers operating in the 0.9-1 um spectral range; - development of international scientific collaboration. Within the framework of the first workpackage we fulfilled band structure modeling for various combinations of AlGaInAsP semiconductor materials which can be epitaxially synthesized on an InP substrate in peseudomorphic or lattice-matched growth regimes. Conduction-band and valence-band offsets at the heterointerface of the barrier layer and the waveguide layer were calculated taking into account the effect of chemical composition, bandgap bowing and strain-induced band edge shifts. It was found that there exist specific compositions of quaternary AlInGaAs alloys lattice matched to InP that are capable of realizing (in combination with InP layers) an asymmetric potential barrier suitable for usage as n-side ABL in an 1.55-um diode laser grown on an InP substrate. It was demonstrated that Al0.272In0.529Ga0.199As layer makes the conduction band barrier lower than kT, whereas the potential barrier for holes exceeds 200 meV. It was also shown that there exist such quaternary GaInPAs alloys lattice matched to an InP substrate that are capable of realizing (in combination with In0.52Al0.48As layers) an asymmetric barrier layer suitable for p-side ABL. It was shown that Ga0.277In0.723P0.505As0.495 layer makes the barrier for holes lower than kT, whereas the electron barrier is as high as 290 meV. Thus, material compositions were found that can be used for synthesis of barrier layers possessing a significant degree of asymmetry of the barrier heights. It should be emphasized that these ABLs are lattice-matched to an InP substrate. On the basis of band structure parameters achieved , a layered structure was developed for a diode laser on an InP substrate, with quantum dot active region emitting around 1.55um comprising two asymmetric barrier layers on each side of the active region – one on p-side of the active region for blocking electron escape and the other on n-side for blocking holes.Using metal-organic vapor-phase epitaxy two separate confinement laser heterostructures were grown. A standard SCH structure of rectangular waveguide design (without ABL) was used in one laser structure, whereas another laser structure contains AlInGaP/InP and GaInPAs/InAlAs asymmetric barrier layers. The active region represents an array of InAs quantum dots emitting around 1.55 um. Laser diodes with four-cleaved facets and with edge-emitting stripe geometry were fabricated; their characteristics were preliminarily evaluated. It was demonstrated that the asymmetric barriers do not create obstacles for injection of charge carriers. This leads to similar values of turn-on voltages and specific series resistance in the standard laser structure and the ABL structure. Room temperature lasing via ground state optical transition of quantum dots was achieved in asymmetric barrier lasers. The lowest threshold current density was measured to be 2 kA/cm2. The maximal optical gain of quantum dots was estimated to be lower than 20 cm-1. This indicates that a larger number of quantum dot layers should be successively deposited and/or optimization of the waveguide structure should be undertaken in future for increasing the optical confinement factor. With the purpose of development of high-power high-brightness lasers with asymmetric barrier layers operating in the 0.9-1 um spectral range we conducted bandedge discontinuity modeling for heterojunctions made of various AlInGaAsP semiconductor materials that can be epitaxially deposited on a GaAs substrate in pseudomorphic or lattice-matched growth regime. Calculation was done for the barrier layer – waveguiding layer heterointerface taking into account effect of chemical composition, bandgap bowing and strain-induced modification of the band structure. It was found that there are certain quaternary AlInGaAs alloys (with AlAs mole fraction around 40-45% and InAs mole fraction around 28-32%) that can be used (in combination with an AlGaAs waveguide layer) for realization of asymmetric barrier layers suitable for p-side ABL of a diode laser, i.e. making sufficiently high barrier for electrons and negligible barrier for holes. The effect of chemical composition on the barrier height asymmetry and the critical thickness of the pseudomorphic growth mode was analyzed. It was shown that quaternary Al0.44In0.3Ga0.26As alloys forms a barrier for electrons of about 83 meV; at the same time, the barrier for holes is only 25 meV. The critical layer thickness is 8.2 nm. It was found that ternary In0.5Ga0.5P alloy can be used as an n-side asymmetric barrier layer. This semiconductor material is characterized by a very low degree of lattice mismatch (about 0.1%) with respect to the GaAs substrate, which makes it suitable for growth of thick high quality epitaxial layers. The estimated conduction band offset at the heterointerface is only 8.7 meV, whereas the valence band offset at the ABL-waveguide interface is as high as 260 meV. Thus, specific material compositions were found that can be used in GaAs-based 0.9-1 um diode laser as barrier layers characterized by high degree of height asymmetry. Two laser structures where synthesized based on the tilted wave laser design with an epitaxial passive waveguide layer. The structures have a passive GaAs waveguide thickness of either 10.8 or 26 um. Stripe laser diodes were fabricated and their characteristics were evaluated. It was demonstrated that far the field pattern is of high brightness with two lobes, each having an angular width as small as 2 degrees. More than 80% of the output power is concentrated in these narrow beams. The peak power conversion efficiency of these diode lasers is close to 50%. The linear regime of light-current curve persists to the power level exceeding 4 W. Within the framework of efficient international scientific cooperation development, an international scientific team was established comprising 9 foreign researchers (3 team members from USA, 2 from Germany, 1 from Denmark, 3 from Finland) and 9 Russian researchers, of which 6 are under the age of 39 (4 young Candidates of Sciences and 2 PhD students). All international researchers spent at least one month in Russia personally involved in the research. An All-Russian symposium with foreign participation Semiconductor Lasers: Physics and Technology was organized (http://www.ioffe.ru/lasers14_/ http://www.spbau.ru/main/events/1209). 107 Russian participants (of which 64 participants are young Russian researchers, PhD students and students) and 10 international participants took part in the symposium. Members of the project international scientific team delivered 6 presentations at the symposium. 8 more seminars were organized within the regular symposium program.

 

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Annotation of the results obtained in 2015
The international scientific team in 2015 comprises 9 foreign researchers and 9 Russian researchers (of which 6 researchers are not older than 39). All international researchers spent at least one month in Russia personally involved in the research. In 2015 the project results were presented in 10 scientific publications appeared in Web of Science journals (of which 7 and 3 articles were published in English language and Russian language journals, respectively) as well as in 9 conference papers presented at 4 international and 3 national scientific events. The research activity can be divided into several main directions as follows: - Diode lasers with asymmetric barrier layers on GaAs substrates; - Laser structures with broadened waveguide and reduced beam divergence; - Tilted-cavity lasers; - Laser structures with quantum dots on InP substrates; - Optimization of the asymmetric barrier layers; - Lasers grown on GaAs substrates with dense quantum dot array - Development of international scientific collaboration AlGaAs/GaAs diode lasers with asymmetric barrier layers have been studied under high current excitation (10 thresholds) at room and elevated temperatures. The results achieved demonstrate that the asymmetric barriers do not deteriorate current-voltage characteristics of the lasers. In the 20-75oC temperature interval the asymmetric barrier lasers, as compared to the reference laser structure, have demonstrated an increase of the output power (up to 29% at 10 thresholds) as well as increase of the power conversion efficiency by more than 8% owing to the nonlinear behavior suppression in light-current characteristics. These improvements become even more evident in lasers capable of operating at higher optical levels as were achieved in asymmetric barrier lasers with 0.8-um-thick optical waveguide. For example, at 8W the required operation current decreases by 44% as compared to the reference. The maximal output power in excess of 9W, being limited by catastrophic optical mirror damage, was reported. It was shown that the mirror damage threshold does not degrade in laser structures with the asymmetric barriers. Spontaneous emission spectra of the waveguide luminescence at 20-70oC were studied to demonstrate that the asymmetric barriers does suppress the parasitic carrier recombination in the laser waveguide layer. A novel laser structure with improved vertical beam divergence based on optically coupled active and passive waveguide has been proposed and approved. A laser with 2.5-um-thick waveguide has shown stable single-mode operation under pulse excitation up to 20 thresholds with beam divergence less than 21.6o and under continuous-wave operation up to 8 thresholds with beam divergence in the 20.9-24.4o interval. Other device parameters (such as lasing threshold and slope efficiency) were not deteriorated. It has been confirmed for the first time that an angular distribution of light intensity along the vertical axis of a tilted-cavity laser remains double-lobed and, besides, each lobe is characterized by very narrow beam divergence (full width at half maximum is about 2o for 26-um-thick passive waveguide) in a wide range of operation parameters: current changes beyond 25 thrtesholds and temperature is varied within 20-60oC interval. Moreover, it was found that the temperature stability of threshold characteristics does not degrade in lasers based on thick epitaxially grown passive waveguide (10.8-26um). We found that double-lobed far field pattern of a tilted-cavity laser can be focused into a single-lobed pattern by using a cylindrical lens. It was shown that the lateral distance between neighboring tilted-cavity lasers, at which emission of the laser array remains coherent, is increased as the waveguide becomes thicker (the critical lateral period increases from about 100 to 400 um as the thickness changes from 10 to 26 um). We developed a novel method of quantum dot laser structure formation on InP substrate to cover the 1.5-um spectral window. The method is based on usage of thin (<2 ML) GaAs capping layer to cover InAs quantum islands. Such quantum dots are responsible for significant increase of the maximal optical gain (up to 34 cm-1) and decrease of the threshold current density (down to 1.86 ka/cm2) as was observed in broad-area diode lasers operating at room temperature. We also fabricated ridge-waveguide lasers with 2-um-wide aperture; at room temperature an output power in linear regime of light-current dependence exceeds 10 mW whereas the characteristic temperature is 170K. The latter value is among the highest one ever reported for quantum dot based lasers synthesized on InP substrates. It was shown that the difference of electron and hole parameters (most of all, slower capture of holes, as compared to that of electrons, to the quantum-sized active region) results in formation of sub-regions with an uncompensated charge. In its turn, this phenomenon affects various laser characteristics, for example, optical gain achievable on the ground- and excited-state optical transitions. Controlling the charge in the central layers by using modulation delta-doping technique, it is possible to vary relative intensities of the ground-state and excited-state spectral components in lasing spectrum. In particular, by introducing a plane of acceptor dopants leads to an enhancement of the ground-state power, while donor dopants have the opposite effect. Spatial hole burning, that takes place along the laser resonator axis under the condition of slow in-plane diffusion of charge carriers, is responsible for formation of broad flat-topped lasing spectra, which furthermore are characterized by low relative intensity noise of longitudinal modes. Heterojunction band edge offsets have been calculated for InAlGaAs layers of various chemical composition, which can be used as asymmetric barrier layers in AlGaAs laser waveguide. epitaxially grown on a GaAs substrate under pseudomorphic growth regime. It was found that the highest energy barrier in conduction band (whereas the hole energy barrier is kept equal to the thermal energy) is achieved for such composition, at which the band structure of the quaternary alloy changes from direct to indirect that. Taking into account a critical layer thickness limitation for indium content in the quaternary alloy, optimal chemical compositions of the barrier and waveguide layers have been found (In0.232Al0.594Ga0.174As / Al0.355Ga0.645As) that results in the minimal electron flux through the barrier. The electron energy barrier reaches 117 meV. We have studied device performance of a diode laser based on a single layer of In0.4Ga0.6As/GaAs quantum dots, which can be readily incorporated into an asymmetric barrier layer laser structure. It was shown that the surface density of such quantum dots is approximately one order of magnitude larger as compared to a typical density of InAs or InAs/InGaAs quantum dots. Record-high values (among single-layer quantum dot lasers) of the optical gain (60 cm-1) and the maximal lasing temperature (163oC) have been achieved. Two international events with the scope of the project, which can be characterized as scientific schools for young scientists, were organized in 2015: School and Conference with international participation "Saint-Petersburg OPEN 2015" (Optoelectronics, Photonics, Engineering and Nanostructures) and Seminar with international participation “Relevant Issues in Physics and Technology of Semiconductor Nanostructures and Lasers”. 165 Russian participants (of which 158 participants are Russian young researchers, PhD students and students) and 13 foreign participants took part at the School and Conference. The seminar held 62 Russian participants (of which 55 participants were Russian young researchers, PhD students and students) and four foreign participants.

 

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