On carrier injection and gain dynamics in quantum well lasers

A detailed carrier dynamics model for quantum well lasers is presented. The model describes the transport of carriers using full continuity equations and the gain by rate equations for each well separately, and it also takes into account electron-hole interactions which modify the energy band structure. To this end, the model includes Poisson and Schrodinger equations. The model is solved in steady state where it yields nonuniform carrier distributions along the crystal growth axis. Dynamically, the model is solved in the time domain, yielding the evolution of carriers in time and space and highlighting a new effect, photon-assisted carrier transport. The model is also solved in the small-signal regime where the phase lag in gain between wells is determined     

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)     

Absorption, carrier lifetime, and gain in InAs-GaAs quantum-dot infrared photodetectors

Quantum-dot infrared photodetectors (QDIPs) are being studied extensively for mid-wavelength and long-wavelength infrared detection because they offer normal-incidence, high-temperature, multispectral operation. Intersubband absorption, carrier lifetime, and gain are parameters that need to be better characterized, understood, and controlled in order to realize high-performance QDIPs. An eight-band k/spl middot/p model is used to calculate polarization-dependent intersubband absorption. The calculated trend in absorption has been compared with measured data. In addition, a Monte-Carlo simulation is used to calculate the effective carrier lifetime in detectors, allowing the calculation of gain in QDIPs as a function of bias. The calculated gain values can be fitted well with experimental data, revealing that the gain in these devices consists of two mechanisms: photoconductive gain and avalanche gain, where the latter is less dominant at normal operating biases.     

High-modal gain 1300-nm In(Ga)As-GaAs quantum-dot lasers

A semiconductor laser containing seven InAs-InGaAs stacked quantum-dot (QD) layers was grown by molecular beam epitaxy. Shallow mesa ridge-waveguide lasers with stripe width of 120 /spl mu/m were fabricated and tested. A high modal gain of 41 cm/sup -1/ was obtained at room temperature corresponding to a modal gain of /spl sim/6 cm/sup -1/ per QD layer, which is very promising to enable the realization of 1.3-/spl mu/m ultrashort cavity devices such as vertical-cavity surface-emitting lasers. Ground state laser action was achieved for a 360-/spl mu/m-cavity length with as-cleaved facets. The transparency current density per QD layer and internal quantum efficiency were 13 A/cm/sup 2/ and 67%, respectively.     

Tunneling injection lasers: a new class of lasers with reduced hotcarrier effects

In conventional quantum-well lasers, carriers are injected into the quantum wells with quite high energies. We have investigated quantum-well lasers in which electrons are injected into the quantum-well ground state through tunneling. The tunneling injection lasers are shown to have negligible gain compression, superior high-temperature performance, lower Auger recombination and wavelength chirp, and better modulation characteristics when compared to conventional lasers. The underlying physical principles behind the superior performance are also explored, and calculations and measurements of relaxation times in quantum wells have been made. Experimental results are presented for lasers made with a variety of material systems, InGaAs-GaAs-AlGaAs, InGaAs-GaAs-InGaAsP-InGaP, and InGaAs-InGaAsP-InP, for different applications. Both single quantum-well and multiple quantum-well tunneling injection lasers are demonstrated     

Nonlinear gain suppression in semiconductor lasers due to carrier heating

A simple model is presented for carrier heating in semiconductor lasers from which the temperature dynamics of the electron and hole distributions can be calculated. Analytical expressions for two new contributions to the nonlinear gain coefficient, in are derived, which reflect carrier heating due to stimulated emission and free carrier absorption. In typical cases, carrier heating and spectral holeburning are found to give comparable contributions to nonlinear gain suppression. The results are in good agreement with recent measurements on InGaAsP laser diodes.<>     

Frequency stabilization and narrowing of optical pulses from CW GaAs injection lasers

Self-induced intensity pulsations of continuously operating GaAs injection lasers have been frequency stabilized and narrowed by applying to the laser microwave feedback signals derived from the electrical and optical outputs of the laser itself. The width of the optical pulses has been reduced to less than 180 ps at a pulse rate whose spectral width was simultaneously reduced to less than 30 kHz. Significant differences between electrical and optical methods of feedback are demonstrated and discussed.     

The gain and carrier density in semiconductor lasers understeady-state and transient conditions

The carrier distribution functions in a semiconductor crystal in the presence of a strong optical field are obtained. These are used to derive expressions for the gain dependence on the carrier density and on the optical intensity-the gain suppression effect. A general expression for high-order nonlinear gain coefficients is obtained. This formalism is used to describe the carrier and power dynamics in semiconductor lasers above and below threshold in the static and transient regimes     

Intermodal injection locking and gain profile measurement of GaAlAs lasers

Injection locking of a GaAlAs laser by coupling light into a nonlasing Fabry-Perot mode separated by up to 22 mode spacings (60 Å) from the free-running lasing mode is demonstrated. The gain spectra for operation above threshold are determined from measurements of the wavelength dependence of the injected power required for locking. These results are compared to gain curves deduced from spontaneous emission spectra measured slightly below threshold. For the greatest wavelength separation, the gain difference was found to be about 4 cm^-1, with injected power levels of 1 percent of the total slave laser power required to obtain 75 percent of the total energy at the master laser frequency.     

Nonlinear gain effects due to carrier heating and spectralholeburning in strained-quantum-well lasers

The authors present numerical results for the nonlinear gain effects due to carrier heating and spectral holeburning in 50 Å strained In_xGa_1-xAs/Al_0.3Ga_0.7 As quantum-well lasers. Calculations are performed on the basis of a 4×4 matrix system consisting of the usual Kohn-Luttinger Hamiltonian and a strain Hamiltonian for the valence band structure. In addition, the authors perform a small-signal analysis based on four dynamic equations for the photon density, carrier density, and two supplementary equations for the electron and hole energy densities to obtain information about nonlinear gain coefficients. The results indicate that the nonlinear gain is enhanced with the strain mainly due to the rapid increase of the carrier heating effect as the carrier density at the lasing threshold decreases, and that carrier heating is about five times as important compared to spectral holeburning     

Asymmetric gain induced by longitudinal spatial carrier burning in semiconductor lasers

Nonlinear gain caused by dielectric corrugation resulting from the cavity standing wave of a lasing mode in semiconductor lasers is investigated using the perturbation approach. The results show that the nonlinear gain spectrum is asymmetric when the linewidth enhancement factor alpha not=0, and the possibility of single mode operation is greater at alpha =0.<>     

Novel design scheme for high-speed MQW lasers with enhanceddifferential gain and reduced carrier transport effect

We present a theoretical analysis exploring the optimum design of high-speed multiple-quantum-well (MQW) lasers for 1.55-μm operation. Various combinations of well and barrier materials are examined for lattice-matched, strained-layered (SL), and strain-compensated (SC) MQW lasers with InGaAsP and InGaAlAs barriers. The gain characteristics are investigated for these MQW lasers with various barrier bandgap wavelengths and are used to evaluate the modulation characteristics based on the carrier dynamics model which includes a set of Poisson, continuity, and rate equations. The importance of band engineering aimed at simultaneously reducing the carrier transport effect and enhancing the differential gain is described. It is shown that SC-MQW lasers with InGaAlAs barriers have an advantage in reducing the density of states in the valence band by reducing the overlap integral between the heavy- and light-hole wave functions, which effect has previously been discarded as a minor correction in designing conventional InGaAsP-based MQW lasers. Furthermore, the hole transport rate across the barriers can be drastically reduced in SC-MQW lasers due to the reduced effective barrier height for the holes. Based on this novel design scheme, a 3-dB bandwidth approaching 70 GHz is expected for 20-well SC-MQW lasers with InGaAlAs barriers as a result of both the large differential gain and reduced transport effect     

Temperature dependence of the gain of lasers based on quantum-dot arrays with an inhomogeneously broadened density of states

A model making it possible to describe analytically the temperature dependence of the optical gain and threshold current density in lasers based on quantum-dot arrays characterized by inhomogeneous broadening of the density of states is proposed. At high temperatures the dependence obtained is universal, i.e., it is determined exclusively by the broadening of the density of states, not by its specific form. Fiz. Tekh. Poluprovodn. 33, 1395–1400 (November 1999)     

Enhanced carrier injection efficiency from lateral current injection in multiple-quantum-well DFB lasers

A bidimensional simulation shows that the lateral carrier injection in etched active region of multiple-quantum-well DFB lasers represents a large fraction of the injected current, leading to improved carrier homogenization in the wells. This increases the average carrier density substantially and provides a much higher gain for the same injected current, depending on detailed device structure.     

Saturation behavior of the optical gain in GaAs injection lasers

A recent analysis of the saturation behavior of the optical gain in semiconductor lasers has predicted a strong spectral hole-burning effect. With an improved treatment of the intraband relaxation in the analysis the effect disappears entirely.     

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     

Carrier distribution, gain, and lasing in 1.3-/spl mu/m InAs-InGaAs quantum-dot lasers

In this paper, we present the results of a theoretical model built to describe the temperature-dependent lasing characteristics of InAs-InGaAs quantum-dot (QD) lasers operating at 1.3 /spl mu/m. From the model, we find that traditional carrier distribution theories are inadequate to describe the performance of these lasers. We therefore introduce an improved model that allows for both free carriers and excitons in the dots. The new model provides threshold current and characteristic temperature T/sub 0/ values that are in good agreement with experimental data. The results of our modeling reveal that, while it is the excitons that mainly contribute to the gain, the ratio of excitons to free carriers significantly affect the T/sub 0/ of QD lasers. Our model results also indicate that the wetting layer current plays little role in QD laser performance. In addition, the model correctly predicts other experimental observations such as; increased T/sub 0/ for increased number of QD layers and p-doped structures, and the oscillatory behavior of T/sub 0/, lending further credibility to the model.     

Optimizing carrier balance of a red quantum-dot light-emitting electrochemical cell with a carrier injection layer of cationic Ir(III) complex

Solid-state light-emitting electrochemical cells (LECs) with promising features of solution processability, low-voltage operation and compatibility with inert cathode metals have shown great potential in display and lighting applications in recent years. Among the reported emissive materials for LECs, ionic transition metal complexes (iTMCs) have relatively higher electroluminescence (EL) efficiencies due to their phosphorescent property. However, the red iTMCs generally exhibit moderate color saturation and low emission efficiency, limiting their display applications. To improve color saturation and device efficiency of red LECs, efficient quantum dots (QDs) with narrow emission bandwidth are good alternative emissive materials. In this work, efficient and saturated red QD LECs employing iTMC carrier injection layers to provide in situ electrochemical doping are demonstrated. The thicknesses of iTMC and red-QD layers are systematically adjusted to achieve the best carrier balance. In the optimized device, the iTMC carrier injection layer facilitates hole injection into the red-QD layer while electrons are injected from the cathode into the red-QD layer directly since the electron injection barrier is low. The Commission Internationale de I'Eclairage (CIE) coordinates of the EL spectra approach the red standard point of National Television System Committee (NTSC). High external quantum efficiency and current efficiency reaching 9.7% and 16.1 cd A⁻¹, respectively. These results confirm superior carrier balance in such a simple iTMC/QD bilayer device structure. Furthermore, compared with iTMC LECs, less degree of device efficiency roll-off upon increasing device current is observed in QD LECs since a shorter excited-state lifetime of fluorescent QDs reduces the probability of collision exciton quenching. Saturated and efficient red EL with mitigated efficiency roll-off from red-QD LECs employing iTMC carrier injection layers confirms that they are good candidates of saturated light sources for displays.