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In a laser with asymmetric barrier layers (ABLs) two thin barrier layers adjacent to the active region on both sides are intended to prevent bipolar population of the waveguide layers, hence, to suppress parasitic recombination in them. A theoretical model of a laser with ABLs, based on rate equations which acknowledge undesirable carrier leakage inevitable in lasers of this type implemented in practice, is proposed. Solutions to equations are obtained for the steady-state case. By the example of an InGaAs/GaAs quantum-well laser (lasing wavelength λ = 980 nm), the effect of leakages through ABLs on the device characteristics is studied. The parasitic-flux suppression ratios C of ABLs which are required to prevent the adverse effect of waveguide recombination are estimated. In the case at hand, the effect of ABLs becomes appreciable at suppression ratios of C ≥ 102. To suppress 90% of the parasitic current, C should be 2.3 × 104. The effect of ABLs on useful carrier fluxes arriving at the active region is also studied.

A theory of static (threshold and power) characteristics of novel diode lasers – quantum dot (QD) lasers with asymmetric barrier layers (ABLs) – is developed. The barrier layers are asymmetric in that they have considerably different heights for the carriers of opposite signs. The ABL located on the electron- (hole-) injecting side of the structure provides a low barrier (ideally no barrier) for electrons (holes) [so that it does not prevent electrons (holes) from easily approaching the active region] and a high barrier for holes (electrons) [so that holes (electrons) injected from the opposite side of the structure do not overcome it]. The use of ABLs should thus ideally prevent the simultaneous presence of electrons and holes (and hence parasitic electron – hole recombination) outside the QDs. It is shown in this work that in such a case of total suppression of parasitic recombination, the QD lasers with ABLs offer close-to-ideal performance: the threshold current density is below 10 A cm−2 at any temperature, the absolute value of the characteristic temperature is above 1000 K (which manifests a virtually temperature-independent operation), the internal differential quantum efficiency is practically unity, and the light – current characteristic is linear at any pump current.

The effect of out-tunneling leakage of carriers from quantum dots (QDs) on modulation bandwidth of semiconductor double tunneling-injection (DTI) QD lasers is studied. In this type of laser, as an alternative to conventional pumping, electrons and holes are injected into QDs by tunneling from two separate quantum wells (QWs) located on the opposite sides of the QD layer and separated from the latter by thin barrier layers. Fast tunneling-injection into QDs should be maintained to ensure the high modulation bandwidth. It may also seem that out-tunneling leakage from QDs (tunneling of electrons to the hole-injecting QW and holes to the electron-injecting QW), which is an undesirable process, should be kept as slow as possible (or even totally suppressed) to maximize the modulation bandwidth. However, it is shown in this paper that, due to zero-dimensional nature of QDs, the modulation bandwidth is practically unaffected by out-tunneling leakage from them—it depends only slightly on the out-tunneling time. Hence, concerning out-tunneling leakage, DTI QD lasers are robust—provided that tunneling-injection is fast, no significant effort should be necessary to suppress out-tunneling leakage in practical devices.

A search for materials suitable for implementation of 1.55 µm Al-free diode lasers based on InP with asymmetric barrier (AB) layers is conducted. It is shown that a very high (over 106) suppression ratio of the parasitic electron flux can be achieved using common III–V alloys for the ABs. Hence placing such ABs in the immediate vicinity of the active region should completely suppress the parasitic recombination in the waveguide. Several optimal AB designs are proposed that are based on one of the following alloys: Al-free GaInPSb, ternary AlInAs, or quaternary AlGaInAs with a low Al-content. As an important and beneficial byproduct of utilization of such ABs, an improvement of majority carrier capture into the active region occurs.

A self-consistent model for calculating the threshold and high-power characteristics of semiconductor quantum well lasers with asymmetric barrier layers is developed. The model, which is based on a system of rate equations, uses the universal condition of global charge neutrality in the laser structure. The electron and hole concentrations in the waveguide region and in the quantum well (QW) and the concentration of photons of stimulated emission are calculated. The local neutrality in the QW is shown to be strongly violated, especially at high injection currents. The violation of neutrality in a QW makes the electron and hole concentrations there dependent on the injection current under lasing conditions: in the structures under consideration, the electron concentration in the QW decreases while the hole concentration increases with increasing injection current. In the case of the ideal functioning of asymmetric barrier layers, when electron–hole recombination in the waveguide region is completely suppressed, the violation of neutrality in the QW has almost no effect on the dependence of the output optical power on the injection current: the quantum efficiency is close to unity and the light–current characteristic is linear. Nevertheless, the violation of neutrality in the QW causes weakening of the temperature dependence of the threshold current and, hence, an increase in the characteristic temperature T0 of the laser.

Direct modulation bandwidth and optimum dc current maximizing it are discussed for double tunneling-injection quantum dot (QD) lasers and QD lasers with asymmetric barrier layers and compared to those for conventional QD lasers.

The small-signal dynamic response of quantum dot (QD) lasers with asymmetric barrier layers (ABLs) is studied. The modulation bandwidth of this novel type of laser is shown to have a maximum as a function of the dc component of the pump current and parameters of the structure. This provides a clear path for optimizing the laser structure to enhance its bandwidth. The effect of non-instantaneous capture of electrons and holes into QDs on the modulation bandwidth is studied. The maximum bandwidth and the optimum dc injection current at which it is attained are calculated as functions of the capture cross-section. The optimum dc injection current is shown to be considerably lower in ABL QD lasers as compared to conventional QD lasers, i.e., the maximum bandwidth is easier to attain in ABL QD lasers.

Due to internal optical absorption loss, which varies with carrier density in the optical confinement region, the operating characteristics can be two valued in semiconductor quantum well lasers. Particularly, there can be two lasing thresholds and two branches in the light–current characteristic. While the internal differential quantum efficiency for the first (conventional) branch of the light–current characteristic is less than 1 and decreases with increasing pump current, that for the second (unconventional) branch is greater than 1 and nonmonotonous.

The light-current characteristic (LCC) of semiconductor quantum well lasers is theoretically studied. It is discussed here that, due to internal optical absorption loss, which depends on the electron and hole densities in the optical confinement layer, (i) roll-over of the LCC occurs with increasing injection current, and, (ii) depending on the parameters of laser structures, the LCC can have two branches, i.e. the optical emission at two different output powers will be possible within a certain range of injection currents.

The operating characteristics of semiconductor quantum-well lasers, calculated with consideration for an increase in the internal optical loss in the waveguide region with increasing pump current, are presented. The condition of global electroneutrality in the structure is used. This condition consists in that the total charge of electrons in the 2D active region (quantum well) and bulk waveguide region (optical confinement layer) is equal to the total charge of holes in these two regions. Good agreement between the calculated and experimentally determined light–current characteristics is obtained.

The spatial distribution of the intensity of the emission caused by recombination appearing at a high injection level (up to 30 kA/cm2) in the waveguide layer of a GaAs/AlGaAs laser structure with GaInP and AlGaInAs asymmetric barrier layers is studied by means of near-field scanning optical microscopy. It is found that the waveguide luminescence in such a laser, which is on the whole less intense as compared to that observed in a similar laser without asymmetric barriers, is non-uniformly distributed in the waveguide, so that the distribution maximum is shifted closer to the p-type cladding layer. This can be attributed to the ability of the GaInP barrier adjoining the quantum well on the side of the n-type cladding layer to suppress the hole transport.

The power characteristics of quantum-well lasers with asymmetric barrier layers, which represent a novel type of injection laser, are calculated on the basis of an extended model taking into account asymmetry in the filling of electron and hole states. The electron–hole asymmetry is shown to have no significant effect on the characteristics of these lasers. Even in the presence of intermediate layers (located between the quantum well and each of the two asymmetric barrier layers), where parasitic electron–hole recombination does occur, the internal differential quantum efficiency of such a laser exhibits only a weak dependence on the pump current and remains close to unity; therefore, the light–current characteristic remains linear up to high pumping levels.

Threshold characteristics of a semiconductor quantum-well (QW) laser are calculated using the global electroneutrality condition, which includes charge carriers both in the active and waveguide regions and thus presents an equality of the total charge of electrons to the total charge of holes in these two regions. It is shown that at the lasing threshold, the densities of electrons in the QW and the waveguide region are not equal to the densities of holes in these regions, i.e., the local electroneutrality condition is violated in each of the regions. Depending on the velocities of the carrier capture from the waveguide region into the QW, the electron density can be either higher or lower than the hole density (both in the QW and in the waveguide region). The charge of the carriers of each sign in the waveguide region is shown to be greater than that in the QW.

The operating characteristics of a semiconductor quantum-well laser calculated using three models are compared. These models are (i) a model not taking into account differences between the electron and hole parameters and using the electron parameters for both types of charge carriers; (ii) a model, which does not take into account differences between the electron and hole parameters and uses the hole parameters for both types of charge carriers; and (iii) a model taking into account the asymmetry between the electron and hole parameters. It is shown that, at the same velocity of electron and hole capture into an unoccupied quantum well, the laser characteristics, obtained using the three models, differ considerably. These differences are due to a difference between the filling of the electron and hole subbands in a quantum well. The electron subband is more occupied than the hole subband. As a result, at the same velocities of electron and hole capture into an empty quantum well, the effective electron-capture velocity is lower than the effective hole-capture velocity. Specifically, it is shown that for the laser structure studied the hole-capture velocity of 5 × 105 cm/s into an empty quantum well and the corresponding electron-capture velocity of 3 × 106 cm/s into an empty quantum well describe the rapid capture of these carriers, at which the light–current characteristic of the laser remains virtually linear up to high pump-current densities. However, an electron-capture velocity of 5 × 105 cm/s and a corresponding hole-capture velocity of 8.4 × 104 cm/s describe the slow capture of these carriers, causing significant sublinearity in the light–current characteristic.

Threshold and power characteristics of quantum well lasers are theoretically studied in the presence of internal optical absorption loss. Due to variation of the internal loss coefficient with electron and densities in the optical confinement layer of the laser, the light-current characteristic may have two branches, each with its own threshold. The branches merge together at the maximum operating current.

We study the possibility of realization of the asymmetric barrier layers (ABL) concept in an 808-nm Al-free GaInAsP/InGaP/GaAs semiconductor laser. Two ABLs on both sides of the active region are aimed to suppress the parasitic recombination in the optical confinement layers. It is shown that such ABL-laser can be made fully Al-free having high suppression ratios for parasitic charge carrier flows (60 and 207 times for electrons and holes, respectively, as compared to a conventional SCH heterostructure).

The feasibility of implementation of asymmetric barriers (ABs) made of common materials for completely aluminum-free diode lasers is studied. The ABs adjoining a low-dimensional active region on both sides aim to prevent bipolar population in the waveguide layers and thus to suppress parasitic recombination therein, which in turn would enhance the efficiency and temperature-stability of the device. Our search algorithm for appropriate AB materials relies on the minimization of undesired carrier flow (electrons or holes passing through the active region toward the p- or n-type doped cladding layer, respectively), while maintaining the useful flows of hole and electron injection into the active region. Using an example of an 808-nm GaInAsP laser, it is shown that the n- and p-side ABs can be made, for instance, of GaInPSb and GaInP, respectively. In such a laser, the parasitic recombination flux can be suppressed by a factor of 60 for electrons and 200 for holes. It is found that the contribution of the indirect valleys to the electron flow through the p-side AB can be significant and even decisive in some cases. The contribution of light holes to the transmission through the ABs can also be considerable. The optimal thicknesses of the AB layers are determined and the chemical composition tolerances are estimated for a given flux suppression ratio. Published by AIP Publishing.

Band offsets at the heterointerface are calculated for various combinations of InAlGaAs/AlGaAs heteropairs that can be synthesized on GaAs substrates in the layer-by-layer pseudomorphic growth mode. Patterns which make it possible to obtain an asymmetric barrier layer providing the almost obstruction-free transport of holes and the highest possible barrier height for electrons are found. The optimal compositions of both compounds (In0.232Al0.594Ga0.174As/Al0.355Ga0.645As) at which the flux of electrons across the barrier is at a minimum are determined with consideration for the critical thickness of the indium-containing quaternary solid solution.

The modulation response in double tunneling-injection quantum dot lasers is analyzed. The highest possible modulation bandwidth and the effect of noninstantaneous pumping of quantum dots on the bandwidth are discussed.

To overcome the limitations placed on the operating characteristics of diode lasers by recombination outside their active region, two novel designs were proposed for them: one using double tunneling-injection (injection of both electrons and holes) into the active region, and the other using two asymmetric barrier layers (ABLs) flanking the active region. The barrier layers are asymmetric in that they have considerably different heights for the carriers of opposite signs. The ABL located on the electron- (hole-) injecting side of the structure provides a low barrier (ideally no barrier) for electrons (holes) [so that it does not prevent electrons (holes) from easily approaching the active region] and a high barrier for holes (electrons) [so that holes (electrons) injected from the opposite side of the structure do not overcome it]. The use of ABLs should thus ideally prevent the simultaneous existence of electrons and holes (and hence parasitic electron-hole recombination) outside the active region. In this work, we calculate the threshold and power characteristics of quantum dot lasers with ABLs. We show that quantum dot lasers with ABLs offer close-to-ideal performance: low threshold current density, very high characteristic temperature (virtually temperature-independent operation), close-to-unity internal differential quantum efficiency, and linear light-current characteristic.

Using an extended theoretical model, which includes the rate equations for both electrons and holes, we have studied the output characteristics of semiconductor quantum-well lasers. We have found non-trivial dependences of electron and hole concentrations in the waveguide region of the laser on the capture velocities of both types of carriers from the waveguide region into the quantum well. We have obtained the dependences of the internal differential quantum efficiency and optical output power of the laser on the capture velocities of electrons and holes. An increase in the capture velocities has been shown to result in suppression of parasitic recombination in the waveguide region and therefore in a substantial increase in the quantum efficiency and output power.

An AlGaAs/GaAs quantum well (QW) laser is fabricated with GaInP and AlGaInAs asymmetric barrier layers (ABLs) and its light–current characteristic (LCC) is compared with that of a reference conventional QW laser without ABLs. It was found that the use of the ABLs suppresses the sublinearity of the LCC at high current densities. As a result, the maximum lasing power of 9.2 W, being limited by catastrophic optical mirror damage, is achieved at a considerably lower operating current in the laser with ABLs as compared to the reference laser (12.5 against 20.2 A). The ABL effect is associated with the suppression of the parasitic recombination in the optical confinement layer, as confirmed by a decrease of the intensity of the spontaneous emission from the layer.

A simple method for the determination of the capture velocity of charge carriers from a three-dimensional (3D) region (waveguide region) into a 2D region (quantum well) is proposed. The method is based on measurement of the threshold current density and internal differential quantum efficiency in a semiconductor laser structure. The method also allows determining the 2D carrier density in a quantum well, which is otherwise not easy to measure in a multilayer laser structure.

The upper limit for the modulation bandwidth in a double tunneling-injection (DTI) quantum dot (QD) laser is discussed. While the maximum bandwidth is the same in DTI and conventional QD lasers, the optimum dc current, at which it is obtained, is lower in a DTI laser. Different factors limiting the modulation bandwidth in a DTI QD laser are also discussed.

The light-current characteristic of a semiconductor laser with multiple quantum wells (QWs) is calculated, with the delayed capture of charge carriers from the waveguide region into the wells taken into account. It is shown that increasing the number of QWs is a more effective way to improve the power characteristics of a laser, compared with an increase in the velocity of carrier capture into each of the wells. For example, using two QWs as the active region leads to a substantial increase in the internal quantum efficiency of stimulated emission and to a significantly better linearity of the light-current characteristic of the laser, compared with a single-well structure. At the same time, using three or more QWs only slightly improves the power characteristics of the laser, compared with the double-well structure. Thus, a double-well structure is the optimal as regards high output power and simplicity of growth.

A method for enhancing the temperature stability of injection lasers that is based on introducing asymmetric barrier layers on each side of the quantum-confined active region is suggested. The asymmetric barrier layers prevent electrons from escaping from the active region into the part of the waveguide region where holes are injected and prevent holes from escaping into the part of the waveguide region where electrons are injected. Parameters of the layers that allow implementation of the asymmetric-barrier design using pseudomorphic structures grown on GaAs substrates are determined. The calculation of the threshold characteristics of these laser structures demonstrates that suppression of electron-hole recombination outside the active region attained due to the use of asymmetric barrier layers leads to a significant decrease in the threshold current and an increase in the characteristic temperature of this type of lasers.

To suppress bipolar population and hence electron–hole recombination outside quantum dots (QDs), tunneling-injection of electrons and holes into QDs from two separate quantum wells was proposed earlier. Close-to-ideal operating characteristics were predicted for such a double tunneling-injection (DTI) laser. In the Stranski–Krastanow growth mode, a two-dimensional wetting layer (WL) is initially grown followed by the formation of QDs. Due to thermal escape of carriers from QDs, there will be bipolar population and hence electron–hole recombination in the WL, even in a DTI structure. In this work, the light–current characteristic (LCC) of a DTI QD laser is studied in the presence of the WL. Since the opposite sides of a DTI structure are only connected by the current paths through QDs and the WL is located in the n-side of the structure, the only source of holes for the WL is provided by QDs. It is shown that, due to the zero-dimensional nature of QDs, the rate of the hole supply to the WL remains limited with increasing injection current. For this reason, as in the other parts of the structure outside QDs (quantum wells and optical confinement layer), the parasitic electron–hole recombination remains restricted in the WL. As a result, even in the presence of the WL, the LCC of a DTI QD laser becomes increasingly linear at high injection currents, which is a further demonstration of the potential of such a laser for high-power operation.

Operating characteristics of quantum dot (QD) lasers are discussed. Carrier-density-dependent internal loss sets an upper limit for operating temperatures and considerably reduces the characteristic temperature. Such a loss also constrains the shallowest potential well depth and the smallest tolerable size of a QD at which the lasing can be attained. In a laser with a single layer of QDs, this loss can strongly limit the output power and cause a rollover of the light-current curve (LCC). Excited-state-mediated capture of carriers from the waveguide into the QD ground-state places a fundamental limitation on ground-state lasing - the output power saturates at high injection currents. The saturation power is controlled by the transition time between the excited- and ground-state in a QD; the longest, cut-off time exists, beyond which no ground-state lasing is possible. Spatial hole burning causes the multimode generation, reduces the total optical power and contributes to nonlinearity of the LCC.

Power and spectral characteristics of lasers with asymmetric barrier layers (ABLs) and a wide waveguide are studied. The use of ABLs reduces the saturation of light-current characteristic, associated with the parasitic recombination in the waveguide.

Dynamic characteristics of double tunneling-injection (DTI) quantum dot (QD) lasers are studied. To reveal the potential of such lasers for high-speed direct modulation of their optical output by pump current, fast carrier injection into QDs and no carrier leakage from QDs are assumed. The small-signal analysis of rate equations is applied. The modulation bandwidth is calculated as a function of the dc component of the injection current density and parameters of the laser structure.

We develop a general approach to including the internal optical loss in the description of semiconductor lasers with a quantum-confined active region. We assume that the internal absorption loss coefficient is linear in the free-carrier density in the optical confinement layer and is characterized by two parameters, the constant component and the net cross section for all absorption loss processes. We show that, in any structure where the free-carrier density does not pin in the presence of light generation, the free-carrier-density dependence of internal loss gives rise to the existence of a second lasing threshold above the conventional threshold. Above the second threshold, the light-current characteristic is two-valued up to a maximum current at which the lasing is quenched. We show that the presence of internal loss narrows considerably the region of tolerable structure parameters in which the lasing is attainable; for example, the minimum cavity length is significantly increased. Our approach is quite general but the numerical examples presented are specific for quantum dot (QD) lasers. Our calculations suggest that the internal loss is likely to be a major limiting factor to lasing in short-cavity QD structures.

A comprehensive theory of threshold characteristics of quantum dot (QD) lasers, which provides a basis for optimization of their design, is reviewed. The dependences of the gain, transparency current, threshold current, characteristic temperature, and multimode generation threshold on the parameters of the QD ensemble (surface density and size dispersion of QDs), cavity (stripe length and thickness of the waveguide region), heterocontacts (band offsets), and temperature are considered in detail. The limiting characteristics of the laser (optimum structure parameters, minimum threshold current density, and characteristic temperature of the optimized structure) are discussed at length. The results of the analysis may serve as direct recommendations for the development of QD lasers that significantly outperform the semiconductor lasers currently in use.

We discuss in detail a new mechanism of nonlinearity of the light-current characteristic (LCC) in heterostructure lasers with reduced-dimensionality active regions, such as quantum wells (QWs), quantum wires (QWRs), and quantum dots (QDs). It arises from: 1) noninstantaneous carrier capture into the quantum-confined active region and 2) nonlinear (in the carrier density) recombination rate outside the active region. Because of 1), the carrier density outside the active region rises with injection current, even above threshold, and because of 2), the useful fraction of current (that ends up as output light) decreases. We derive a universal closed-form expression for the internal differential quantum efficiency /spl eta//sub int/ that holds true for QD, QWR, and QW lasers. This expression directly relates the power and threshold characteristics. The key parameter, controlling /spl eta//sub int/ and limiting both the output power and the LCC linearity, is the ratio of the threshold values of the recombination current outside the active region to the carrier capture current into the active region. Analysis of the LCC shape is shown to provide a method for revealing the dominant recombination channel outside the active region. A critical dependence of the power characteristics on the laser structure parameters is revealed. While the new mechanism and our formal expressions describing it are universal, we illustrate it by detailed exemplary calculations specific to QD lasers. These calculations suggest a clear path for improvement of their power characteristics. In properly optimized QD lasers, the LCC is linear and the internal quantum efficiency is close to unity up to very high injection-current densities (15 kA/cm/sup 2/). Output powers in excess of 10 W at /spl eta//sub int/ higher than 95% are shown to be attainable in broad-area devices. Our results indicate that QD lasers may possess an advantage for high-power applications.

Different approaches to the design of a genuinely temperature-insensitive quantum dot (QD) laser are proposed. Suppression of the parasitic recombination outside the QDs, which is the dominant source of the temperature dependence of the threshold current in the conventional design of a QD laser, is accomplished either by tunneling injection of carriers into the QDs or by band-gap engineering. Elimination of this recombination channel alone enhances the characteristic temperatures T0 above 1000 K. Remaining sources of temperature dependence (recombination from higher QD levels, inhomogeneous line broadening, and violation of charge neutrality in QDs) are studied. Tunnelinginjection structures are shown to offer an additional advantage of suppressed effects of inhomogeneous broadening and neutrality violation. 2002 Elsevier Science Ltd. All rights reserved.

We propose a genuinely temperature-insensitive quantum dot (QD) laser. Our approach is based on direct injection of carriers into the QDs, resulting in a strong depletion of minority carriers in the regions outside the QDs. Recombination in these regions, which is the dominant source of the temperature dependence, is thereby suppressed, raising the characteristic temperature T/sub 0/ above 1500 K. Still further enhancement of T/sub 0/ results from the resonant nature of tunneling injection, which reduces the inhomogeneous line broadening by selectively cutting off the nonlasing QDs.

Experimental and theoretical study was made of injection lasers based on InAs/GaAs quantum dots (QDs) formed by the activated alloy phase separation and emitting at about 1.3 /spl mu/m. Electroluminescence and gain spectra were investigated. The maximum modal gain is measured experimentally using two different techniques. Threshold current densities as low as 22 A cm/sup -2/ per QD sheet were achieved. A step-like switch from ground- to excited-state transition lasing was observed with an increasing cavity loss. The characteristic temperatures for a sample with four cleaved sides and a 2-mm long stripe device at 300 K were 140 and 83 K, respectively. Single lateral-mode continuous-wave (CW) operation with the maximum output power of 210 mW was realized. Threshold characteristics of a laser were simulated taking into account radiative recombination in QDs, the wetting layer, and the optical confinement layer. The dependence of the threshold current density on the cavity length was shown to be extremely sensitive to the QD-array parameters determining the maximum gain for ground- and excited-state transitions and to the waveguide design. Our analysis reveals that nonradiative recombination channels may play an important role in the laser operation.

The modulation bandwidth of double tunneling-injection (DTI) quantum dot (QD) lasers is studied, taking into account noninstantaneous pumping of QDs. In this advanced type of semiconductor lasers, carriers are first captured from the bulk waveguide region into two-dimensional regions (quantum wells [QWs]); then they tunnel from the QWs into zero-dimensional regions (QDs). The two processes are noninstantaneous and, thus, could delay the delivery of the carriers to the QDs. Here, the modulation bandwidth of DTI QD lasers is calculated as a function of two characteristic times (the capture time from the waveguide region into the QW and the tunneling time from the QW into the QD ensemble) and is shown to increase as either of these times is reduced. The capture and tunneling times of 1 and 0.1 ps, respectively, are shown to characterize fast capture and tunneling processes; as the capture and tunneling times are brought below 1 and 0.1 ps, the bandwidth remains almost unchanged and close to its upper limit.

Detailed theoretical analysis of longitudinal spatial hole burning in quantum-dot (QD) lasers is given. Unlike conventional semiconductor lasers, escape of thermally excited carriers from QDs, rather than diffusion, is shown to control the smoothing-out of the spatially nonuniform population inversion and multimode generation in QD lasers. The multimode generation threshold is calculated as a function of structure parameters (surface density of QDs, QD size dispersion, and cavity length) and temperature. A decrease in the QD size dispersion is shown to increase considerably the relative multimode generation threshold. The maximum tolerable QD size dispersion and the minimum tolerable cavity length, at which lasing is possible to attain, are shown to exist. Concurrent with the increase of threshold current, an increase of the multimode generation threshold is shown to occur with a rise in temperature. Ways to optimize the QD laser, aimed at maximizing the multimode generation threshold, are outlined.

An analytical model of the effect of heating on the high-power and high-temperature operation of semiconductor multiple quantum well lasers (MQWLs) is developed. Both the lattice heating and the carrier heating in the active region are shown to play an important role. The lattice heating predominates at high injection currents, while the carrier heating prevails at low currents. The maximum output power and the corresponding injection current are shown to be decreasing functions of temperature. The ways to increase the maximum output power of MQWLs are discussed. The effect of the series resistance on the maximum output power is investigated. Optimization of MQWLs with respect to the QW number and the cavity length is carried out. The results are illustrated by the example of a ridge MQW structure lasing at 1.3 µm. The theoretical and experimental dependences are compared.

The role of thermal carrier ejection from quantum dots and free carrier diffusion in the burning of spatial holes in semiconductor quantum dot lasers is analyzed. The balance of the spatially inhomogeneous population inversion in the longitudinal direction of the cavity is shown to be controlled by thermal ejection from quantum dots. Because of this circumstance, hole burning in quantum dot lasers can show up more strongly and the threshold for multimode lasing can be lower than in semiconductor lasers with three-dimensional active regions or quantum-well lasers. The threshold for multimode lasing is determined as a function of the dispersion in the quantum dot size, cavity length, and temperature for structures that have been optimized to minimize the threshold current density of the fundamental mode.

Excited-state-mediated capture of carriers from the waveguide into the lasing ground-state in quantum dots (QDs) is studied. Such a two-step capture places a fundamental limitation on ground-state lasing-the output power saturates at high injection currents. The saturation power is controlled by the transition time between the excited- and ground-state in a QD. The longest, cut-off transition time exists, beyond which no ground-state lasing is possible

Light-current characteristic (LCC) of a novel type of quantum well (QW) lasers-QW lasers with asymmetric barrier layers (ABLs)-is studied. The ABLs (one on each side of the QW) prevent electrons from entering the hole-injecting side of the structure and holes from entering the electron-injecting side. The use of ABLs thus suppresses the parasitic electron-hole recombination outside the QW and eliminates the mechanism of sublinearity of the LCC in conventional lasers associated with this recombination and with the carrier capture delay into the QW. As a result, no matter how slow is the carrier capture into the QW, the LCC of an ABL QW laser is virtually linear. In an ABL laser containing indent layers between the QW and each of the ABLs (parasitic recombination still occurs in these thin layers), even in the case of slow capture of carriers into the QW, the LCC is also considerably more linear than in a reference conventional QW laser. (C) 2013 AIP Publishing LLC. Russia (Federation). Ministry of Education and Science - Agreement No. 8518

We consider direct and indirect (excited-state-mediated) capture of carriers from the waveguide region into the lasing ground state in quantum dots (QDs) and calculate the modulation response of a QD laser. We show that, when only indirect capture is involved, the excited-to-ground-state relaxation delay strongly limits the ground-state modulation bandwidth of the laser-at the longest tolerable relaxation time, the bandwidth becomes zero. When direct capture is also involved, the effect of excited-to-ground-state relaxation is less significant and the modulation bandwidth is considerably higher. (C) 2013 AIP Publishing LLC. U.S. Army Research Office (Grant No. W911-NF-08-1-0462) China Scholarship Council Russian Foundation for Basic Research (Grant No. 11-02-00573) Program "Fundamental Research in Nanotechnology and Nanomaterials" of the Presidium of the Russian Academy of Sciences

We develop a comprehensive rate equations model for semiconductor quantum dot solar cells (QDSCs). The model is based on the continuity equations with a proper account for quantum dots (QDs). A general analytical expression for the total current density is obtained, and the current-voltage characteristic is studied for several specific situations. The degradation in the open circuit voltage of the QDSC is shown to be due to strong spontaneous radiative recombination in QDs. Due to small absorption coefficient of the QD ensemble, the improvement in the short circuit current density is negligible if only one QD layer is used. If spontaneous radiative recombination would be suppressed in QDs, a QDSC with multiple QD layers would have significantly higher short circuit current density and power conversion efficiency than its conventional counterpart. The effects of photoexcitation of carriers from discrete-energy states in QDs to continuum-energy states are discussed. An extended model, which includes excited states in QDs, is also introduced.

We present an overview of our theoretical and experimental work on a novel type of semiconductor lasers - quantum well (QW) lasers with asymmetric barrier layers (ABLs). Our experimental work supports our theoretical derivations — ABL QW lasers demonstrate superior operating characteristics as compared to conventional QW lasers. In particular, the threshold current is lower and more temperature-stable, the light-current characteristic is more linear, and the wall-plug efficiency is higher in ABL lasers.

We show that the free-carrier-density dependence of internal optical loss gives rise, in general, to the existence of a second lasing threshold above the conventional threshold. Above the second threshold, the light-current characteristic is two-valued up to a maximum current at which the lasing is quenched.

We fabricated and tested a quantum well laser with asymmetric barrier layers. Such a laser has been proposed earlier to suppress bipolar carrier population in the optical confinement layer and thus to improve temperature-stability of the threshold current. As compared to the conventional reference laser structure, our laser with asymmetric barrier layers demonstrates reduced internal optical loss, lower threshold current density at elevated temperatures, and higher characteristic temperature (143 vs. 99K at 20 degrees C). (C) 2012 American Institute of Physics. [doi: 10.1063/1.3676085] Federal Target Program "Scientific and Scientific-Pedagogical Personnel of Innovative Russia" (arrangement no. 1.5 "Team research led by invited scientists," Grant No. 02.740.11.5161) (FLASH) from the Danish Council for Independent Research Grant No. 274-09-0247 European Commission for funding through the Marie Curie Incoming International Fellowship (Project No. 252890)

It is demonstrated that the use of asymmetric barrier layers in a waveguide of a diode laser suppress non-linearity of light-current characteristic and thus improve its power characteristics under high current injection. The results are presented for 850-nm AlGaAs/GaAs broad-area lasers with GaInP and AlInGaAs asymmetric barriers. Published version