1.3-μm CW lasing characteristics of self-assembled InGaAs-GaAsquantum dots

This paper presents the lasing properties and their temperature dependence for 1.3-μm semiconductor lasers involving self-assembled InGaAs-GaAs quantum dots as the active region. High-density 1.3-μm emission dots were successfully grown by the combination of low-rate growth and InGaAs-layer overgrowth using molecular beam epitaxy. 1.3-μm ground-level CW lasing occurring at a low threshold current of 5.4 mA at 25°C with a realistic cavity length of 300 μm and high-reflectivity coatings on both facets. The internal loss of the lasers was evaluated to be about 1.2 cm^-1 from the inclination of the plots between the external quantum efficiency and the cavity length. The ground-level modal gain per dot layer was evaluated to be 1.0 cm^-1, which closely agreed with the calculation taking into account the dot density, inhomogeneous broadening, and homogeneous broadening. The characteristic temperature of threshold currents T₀ was found to depend on cavity length and the number of dot layers in the active region of the lasers. A T₀ of 82 K was obtained near room temperature, and spontaneous emission intensity as a function of injection current indicated that the nonradiative channel degraded the temperature characteristics. A low-temperature study suggested that an infinite T₀ with a low threshold current (~1 mA) is available if the nonradiative recombination process is eliminated. The investigation in this paper asserted that the improvement in surface density and radiative efficiency of quantum dots is a key to the evolution of 1.3-μm quantum-dot lasers     

Low-threshold current density 1.3-μm InAs quantum-dot laserswith the dots-in-a-well (DWELL) structure

The wavelength of InAs quantum dots in an In_0.15Ga_0.85As quantum-well (DWELL) lasers grown on a GaAs substrate has been extended to 1.3-μm. The quantum dot lasing wavelength is sensitive to growth conditions and sample thermal history resulting in blue shifts as much as 73 nm. The room temperature threshold current density is 42.6 A cm^-2 for 7.8-mm cavity length cleaved facet lasers under pulsed operation     

高性能InAs/GaAs量子点外腔激光器

为了获得高性能的量子点外腔激光器(ECL),利用InAs/GaAs量子点Fabry-Perot(FP)腔激光器研制了光栅外腔可调谐ECL。对InAs/GaAs量子点ECL进行了一系列的性能测试,主要包括单模稳定性测试、单模调谐范围测试、阈值电流密度测试、无跳模连续调谐测试和输出功率测试。在室温条件下获得了24.6nm的连续调谐范围,覆盖波长从999.2nm到1 023.8nm,并且实现了波长无跳模连续调谐。在调谐范围内最低阈值电流密度为1 525A/cm2,而且在中心波长处获得的单模输出功率为15mW,单模边模抑制比(SMSR)高达35dB。研究结果表明,通过构建光栅外腔可以实现高性能的InAs/GaAs量子点ECL。     

Longitudinal spatial hole burning in a quantum-dot laser

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     

Low threshold current operation of self-assembled InAs/GaAs quantum dot lasers by metal organic chemical vapour deposition

Low threshold current operation of self-assembled InAs/GaAs quantum dot lasers grown by metal organic chemical vapour deposition is reported. Continuous-wave lasing at room temperature with low threshold current (6.7 mA) was achieved at the wavelength of 1.18 /spl mu/m. The threshold current of 6.7 mA is the lowest value so far achieved in quantum dot lasers grown by metal organic chemical vapour deposition. Comparison with photoluminescence spectra indicates that the observed lasing originates from the ground state of InAs quantum dots.     

Effect of carrier dynamics and temperature on two-state lasing in semiconductor quantum dot lasers

It is analytically shown that the both the charge carrier dynamics in quantum dots and their capture into the quantum dots from the matrix material have a significant effect on two-state lasing phenomenon in quantum dot lasers. In particular, the consideration of desynchronization in electron and hole capture into quantum dots allows one to describe the quenching of ground-state lasing observed at high injection currents both qualitatevely and quantitatively. At the same time, an analysis of the charge carrier dynamics in a single quantum dot allowed us to describe the temperature dependences of the emission power via the ground- and excited-state optical transitions of quantum dots.     

Theoretical calculation of lasing spectra of quantum-dot lasers:effect of homogeneous broadening of optical gain

A theoretical calculation is presented for the effect of homogeneous broadening of optical gain on lasing spectra of quantum-dot lasers. Based on a coupled set of rate equations considering both the size distribution of quantum dots and a series of longitudinal cavity modes, we show that dots with different energies start lasing independently due to their spatial localization when the gain spectrum is a delta-like function, and that the dot ensemble contributes to a narrow-line lasing collectively under large homogeneous broadening. The result explains quite excellently the experimental lasing spectra found in self-assembled InGaAs-GaAs quantum-dot lasers     

Analysis of vertical-cavity surface-emitting laser multimodebehavior

This paper investigates problems associated with multimode oscillation in vertical-cavity surface-emitting lasers (VCSELs). The multimode rate equations for transverse mode were formulated. These equations take into account carrier diffusion and gain nonuniformity in the lateral direction. It was shown that multimode transverse mode excitation is due to carrier spatial hole burning, but many factors affect the number of lasing modes. The role of gain nonuniformity distribution, carrier diffusion, and modal loss compared with mirror loss in a cavity were demonstrated by numerical solution of the multimode rate equations     

Influence of composition and anneal conditions on the optical properties of (In, Ga)As quantum dots in an (Al, Ga)As matrix

The optical properties of structures containing InGaAs quantum dots in GaAs and AlGaAs matrices grown by molecular-beam epitaxy are investigated. It is shown that increasing the In content in the quantum dots has the effect of raising the energy of carrier localization and increasing the energy distance between the ground state and the excited states of carriers in the quantum dots. An investigation of the influence of postgrowth annealing on the optical properties of the structures shows that the formation of vertically coupled quantum dots and the use of a wide-gap AlGaAs matrix enhances the thermal stability of the structures. Moreover, high-temperature (830 °C) thermal annealing can improve the quality of the AlGaAs layers in structures with vertically coupled InGaAs quantum dots in an AlGaAs matrix. The results demonstrate the feasibility of using postgrowth annealing to improve the characteristics of quantum dot lasers. Fiz. Tekh. Poluprovodn. 33, 91–96 (January 1999)     

Gain characteristics of quantum-dot injection lasers

The current dependence of the optical gain in lasers based on self-organized InGaAs quantum dots in a AlGaAs/GaAs matrix is investigated experimentally. A transition from lasing via the ground state of quantum dots to lasing via an excited state is observed. The saturated gain in the latter case is approximately four times greater than for the ground state. This result is attributable to the fourfold degeneracy of the excited level of quantum dots. The effect of the density of the quantum-dot array on the threshold characteristics is investigated. A lower-density array of dots is characterized by a lower threshold current density in the low-loss regime, because the transmission current is lower, while dense quantum-dot arrays characterized by a high saturated gain are preferable at high threshold gains. Fiz. Tekh. Poluprovodn. 33, 1111–1114 (September 1999)     

1.3-μm CW lasing of InGaAs-GaAs quantum dots at room temperaturewith a threshold current of 8 mA

We demonstrate the first 1.3-μm continuous-wave (CW) lasing at room temperature of self-assembled InGaAs-GaAs quantum dots. High-density 1.3-μm emission dots were successfully formed by the combination of low-rate growth and InGaAs-layer overgrowth methods of molecular beam epitaxy. The 1.3-μm ground-level CW lasing occurred at up to 40°C, and the threshold current of 8 mA at 25°C is less than one thirtieth of values ever reported for 1.3-μm dot pulse lasers. The achievement represents a milestone for creating quantum-dot lasers applicable to fiber-optic communication system     

Gain in injection lasers based on self-organized quantum dots

The analytical form of the dependence of the gain on pump current density for lasers with an active region based on self-organized quantum dots is derived in a simple theoretical model. The proposed model is shown to faithfully describe experimental data obtained for laser diodes based on InGaAs quantum dots in an AlGaAs/GaAs matrix, as well as InAs quantum dots in an InGaAs/InP matrix. The previously observed gain saturation and switching of the lasing from the ground state to an excited state of the quantum dots are studied. The influence of the density of quantum-dot arrays on the threshold characteristics of lasers based on them is examined on the basis of this model. Fiz. Tekh. Poluprovodn. 33, 215–223 (February 1999)     

Theory of threshold characteristics of semiconductor quantum dot lasers

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.     

Effect of waveguiding properties on the axial mode competition in stripe-geometry semiconductor lasers

Based on the multimode rate equations taking account of transverse mode and carrier density distributions, the axial mode behavior in semiconductor lasers is investigated. The axial mode stability is significantly affected by these distributions. Above threshold, because of hole burning in the carrier density distribution, the gain at any wavelength except the lasing wavelength does not maintain the value at threshold. If the nonlasing mode wavelength is located in the gain decreasing spectral region, its light output decreases with the increase of current. These characteristics are observed in the experiment for a CSP laser. For axial mode stabilization, a large built-in refractive index step is effective so long as single transverse mode operation is maintained. If the injection current region is defined separately from the guiding region, the wider width of the injection current region leads to a more stabilized axial mode.     

Effects of carrier heating on laser dynamics-a Monte Carlo study

The static and dynamic properties of semiconductor quantum-well (QW) lasers have traditionally been analyzed by using rate equations that couple cold carriers to photons in the lasing cavity. This assumption of cold carriers, however, has often been disputed because it does not account for heating due to carrier relaxation, hot phonon effects, and spectral hole burning. All these processes affect laser performance significantly by modifying the gain because gain depends on carrier temperature as well as spectral broadening. In this paper, we study the carrier dynamics of QW lasers using a Monte Carlo method and conclude that hot carrier effects in semiconductor lasers are important and need to be considered for the analysis and design of semiconductor lasers     

Low-threshold continuous-wave two-stack quantum-dot laser withreduced temperature sensitivity

Data are presented on continuous wave operation of two-stack quantum dot lasers designed with reduced temperature sensitivity in their threshold. The InAs quantum dots are designed to have a wide energy spacing (~102 meV) between the ground and first excited radiative transitions. Selectively oxidized stripe lasers have continuous wave threshold currents as low as 1.2 mA for 2 μm wide stripes and minimum threshold current densities of 26 A/cm² for 13-μ-m wide stripes. Broad area lasers have continuous wave threshold current densities as low as 40 A/cm², even for p-up mounting. Ground state lasing is obtained up to the highest temperature measured of 326 K     

Rate equation model for nonequilibrium operating conditions in aself-organized quantum-dot laser

A nonequilibrium rate equation model is presented and analyzed for the self-organized quantum dot (QD) laser. The model assumes the QD zero dimensional levels are coupled to a thermal electron distribution in the wetting layer through reservoir rate equations. By including the energy dependence of the wetting layer reservoir versus temperature, the model accounts for the spectral narrowing of the gain with increasing temperature, the negative temperature coefficient of the lasing threshold, and a reduction of the spectral hole burning with increasing temperature, all found experimentally in QD lasers     

Self-assembled quantum dots

The greatest success in semiconductor lasers has been brought by the ability to artificially structure new materials on an atomic scale by using advanced crystal growth methods such as MBE and MOVPE. Laser performance successes gained using quantum wells in optoelectronic devices can be extended by adopting quantum wire and quantum dot structures. There have been several reports of successful lasing action in semiconductor dot structures within the past year. This article reviews the recent progress in the development of quantum dot lasers.