Omar Qasaimeh

High-speed modulation and switching characteristics of In(Ga)As-Al(Ga)As self-organized quantum-dot lasers

The dynamic characteristics, and in particular the modulation bandwidth, of high-speed semiconductor lasers are determined by intrinsic factors and extrinsic parameters. In particular, carrier transport through the heterostructure and thermalization, or quantum capture in the gain region, tend to play an important role. We have made a detailed study of carrier relaxation and quantum capture phenomena in In(Ga)As-Al(Ga)As self-organized quantum dots (QDs) and single-mode lasers incorporating such dots in the gain region through a variety of measurements. The modulation bandwidth of QD lasers is limited to 5-6 GHz at room temperature and increases to /spl sim/30 GHz only upon lowering the temperature to 100 K. This behavior is explained by considering electron-hole scattering as the dominant mechanisms of electron relaxation in QDs and the scattering rate seems to decrease with increase of temperature. The switching of the emission wavelength, from the ground state to an excited state, has been studied in coupled cavity devices. It is found that the switching speed is determined intrinsically by the relaxation time of carriers into the QD states. Fast switching from the ground to the excited state transition is observed. The electrooptic coefficients in the dots have been measured and linear coefficient /spl tau/=2.58/spl times/10/sup -11/ m/V. The characteristics of electrooptic modulators (EOMs) are also described.

Characteristics of cross-gain (XG) wavelength conversion in quantum dot semiconductor optical amplifiers

Detailed theoretical analysis of cross-gain (XG) wavelength conversion in quantum dot semiconductor optical amplifier (QD-SOA) is presented. The model takes into account the effect of the excited states, the wetting layer (WL) and the nonlinear optical gain of the dots. An analytical model of the small-signal XG conversion efficiency is also derived. The XG efficiency shows strong dependence on the escape and relaxation lifetimes between the ground and excited states. The model/analysis provides insight on the escape/relaxation lifetime of QD lasers which is very important for characterizing and understanding the performance characteristics of QD devices.

SELF-ORGANIZED IN0.4GA0.6AS QUANTUM-DOT LASERS GROWN ON SI SUBSTRATES

We report growth of self-organized In0.4Ga0.6As quantum dots on Si substrates by molecular-beam epitaxy. Low-temperature (17 K) photoluminescence spectra show that the optical properties of In0.4Ga0.6As quantum dots grown on Si are comparable to quantum dots grown on GaAs substrates. We also present preliminary characteristics of In0.4Ga0.6As quantum-dot lasers grown on Si substrates. Light versus current measurements at 80 K under pulsed bias conditions show that Ith=3.85 kA/cm2. The lasing spectral output has a peak emission wavelength of 1.013 μm and a linewidth (full width at half maximum) of ∼4 A at the threshold.

Optical gain and saturation characteristics of quantum-dot semiconductor optical amplifiers

Detailed theoretical analysis of the gain characteristics of quantum-dot semiconductor optical amplifiers (QD-SOA) is presented. An analytical expression for the optical gain is derived from the quantum dot and wetting layer rate equations. Due to the better confinement of carriers in the quantum dots, our calculation shows that large unsaturated optical gain can be obtained at low operating current. Also, we found that the output saturation intensity of QD-SOA is higher than the output saturation intensity of bulk-SOA. This fact lends itself to the design of efficient low-power SOAs.

Effect of Doping on the Optical Characteristics of Quantum-Dot Semiconductor Optical Amplifiers

The influence of p-type and n-type doping on the optical characteristics of a quantum-dot semiconductor optical amplifier (SOA) is studied using a rate equation model that takes into account the effect of the multidiscrete energy levels and the charge neutrality relation. Our calculations show that the amplifier optical gain can be greatly enhanced through p-type doping where the doping concentration should not exceed the certain level. We find that increasing the acceptor concentration increases the unsaturated optical gain but at the same time decreases the saturation density and the effective relaxation lifetime. Also our calculation reveals that the use of p-type doping will be associated with an increase in the transparency current where the increase in the transparency current depends on the incident photon energy. On the other hand, we find that it is possible to increase the saturation density and enhance the linearity of the SOA by using n-type doping.

Structural and luminescence characteristics of cycled submonolayer InAs/GaAs quantum dots with room-temperature emission at 1.3 μm

Quantum dots were grown by molecular beam epitaxy on GaAs substrates using a cycled submonolayer InAs/GaAs deposition technique. Their structural and luminescence characteristics have been compared with conventional self-organized dots. The room-temperature luminescence spectra are characterized by a ground state transition at 1.3 μm and additional transitions corresponding to excited states. Cross-sectional transmission electron microscopy indicates that no dislocations are formed if the total InAs thickness is less than 5–6 monolayers. Temperature dependence of the photoluminescence indicates that both types of quantum dots may have nonradiative defects, caused by segregation and related phenomena.

LINEAR AND QUADRATIC ELECTRO-OPTIC COEFFICIENTS OF SELF-ORGANIZED IN0.4GA0.6AS/GAAS QUANTUM DOTS

The electro-optic properties of self-organized In0.4Ga0.6As/GaAs quantum dots have been studied experimentally. Single-mode ridge waveguide structures were grown by molecular beam epitaxy with self-organized In0.4Ga0.6As/GaAs quantum dots in the guiding region. The measured linear and quadratic electro-optic coefficients are 2.58×10−11 m/V and 6.25×10−17 m2/V2, respectively, which are much higher than those obtained for bulk GaAs or quantum well structures. The measured transmission characteristics indicate that low-voltage amplitude modulators can be realized with quantum dot active regions.

Bistability and self-pulsation in quantum-dot lasers with intracavity quantum-dot saturable absorbers

Self-organized In0.4Ga0.6As/GaAs quantum-dot single-mode ridge waveguide lasers with intracavity absorber were grown by molecular beam epitaxy. Bistability in the light–current characteristics of 3 μm single-mode edge-emitting laser was obtained by controlling the intracavity absorber voltage. Self-pulsation was also observed with a center frequency of 1.6 GHz and linewidth <10 MHz.

Nonlinear optical and electro-optic properties of InAs/GaAs self-organized quantum dots

The electro-optic and nonlinear optical properties of self-organized InxGa1−xAs/GaAs quantum dots were studied experimentally. The quantum dot heterostructures were grown by molecular beam epitaxy. The measured linear and quadratic electro-optic coefficients for In0.4Ga0.6As quantum dots are 2.58×10−11 m/V and 6.25×10−17 m2/V2, respectively. For InAs quantum dots, the measured linear and quadratic electro-optic coefficients are 2.43×10−10 m/V and 3.37×10−17 m2/V2, respectively. Pump–probe differential transmission spectroscopy measurements were made on the dot samples at cryogenic temperatures with linear and circularly polarized pump and probe beams. The ground and excited state recombination times are 620 and 290 ps, respectively.

Bias-controlled wavelength switching in coupled-cavity In0.4Ga0.6As/GaAs self-organized quantum dot lasers

Efficient wavelength switching is demonstrated in an In0.4Ga0.6As/GaAs self-organized quantum dot laser with an intracavity absorber section. A wavelength shift of ∼15 nm, believed to be caused by a shift of lasing between the bound states of the quantum dot, is obtained for a bias change of 6 V.