We present the development of theoretical model based on multi-population rate equation to assess the
broadband lasing emission in addition to the derivative optical gain and chirp characteristics from the supercontinuum
InGaAs/GaAs self-assembled quantum-dot (QD) interband laser. The model incorporates the peculiar characteristics
such as inhomogeneous broadening of the QD transition energies due to the size and composition fluctuation,
homogeneous broadening due to the finite carrier lifetime in each confined energy states, and the presence of continuum
states in wetting layer. We showed that the theoretical model agrees well with the experimental data of broadband QD
laser. From the model, the broadband lasing characteristics can be ascribed to the large dispersion of QD with varying
energy sub-bands and the change of de-phasing rate. These interesting characteristics can be attributed to the carrier
localization in different dots that result in a system without a global Fermi function and thus an inhomogeneously
broadened gain spectrum. Furthermore, our simulation results predict that the linewidth enhancement factor (α = 2) from
the ground state (GS) in this new class of semiconductor lasers is slightly larger but in the same order of magnitude as
the values obtained in conventional QD lasers. The calculated gain spectrum shows similar magnitude order of material
differential gain (~10-16 cm2) and material differential refractive index (~10-20 cm3) as compared to conventional QD
lasers. The comparable derivative characteristics of broadband QD laser shows its competency in providing low
frequency chirping as well as a platform for monolithic integration operation.
We demonstrate the widened broadband emission of self-assembled quantum dash laser using impurity-free vacancy
induced disordering (IFVD) technique. The 100 nm blueshifted lasers exhibit higher internal quantum efficiency and
lower threshold current densities than the as-grown devices. The laser emission from multiple groups of quantum-dash
(Qdash) families convoluted with multiple orders of subband energy levels within a single Qdash ensemble is
experimentally observed. However, the suppression of laser emission in short wavelength and the progressive redshift of
peak emission with injection current from devices with short cavity length occur. These effects have been attributed to
the nonequilibrium carrier distribution and energy exchange among different sizes of Qdash ensembles. In addition, we
perform the far-field lateral mode measurements from the fabricated as-grown Qdash laser. The analysis of mode
patterns indicate that Qdash lasers exhibit gradual broadening of beam divergence (FWHM of 3.4° to 10.8°) with
increasing injection current. However, these beam divergence angles are still narrower than the quantum well (QW) laser
(FWHM ~13°) at an injection up to 2.5 x Jth. Qdash laser exhibits an improved output beam quality, therefore reduced
filamentation, as compared to the QW laser, owing to the inherent characteristics from quantum-dot (Qdot) laser, where
injected carriers are confined by the lateral energy barriers as Qdots are disconnected laterally and are cladded by larger
bandgap materials. Our results imply a highly attractive wavelength trimming method, well suited for improved
performance, and monolithic Qdash integration of optoelectronics components.
Quantum-confined Stark effect in zero-dimensional semiconductor quantum-dot (QD) has attracted considerable interest
due to the potential applications in electro-optic modulation and quantum computing. Composition interdiffusion occurs
easily during the high temperature epitaxial growth or ex situ annealing treatment, therefore understanding the effects of
interdiffusion is essential for device design and modeling. However, relatively little attention has been devoted to a
systematic study of this effect. In this paper, the effects of isotropic interdiffusion on the optical transition energy of
self-assembled InAs/GaAs QD structure under an electric field have been investigated theoretically. Our
three-dimensional QD calculation is based on coupled QDs with different shapes arranged periodically in a tetragonal
superlattice, taking into account the finite band offset, valence-band mixing, strain, and effective mass anisotropicity.
The electron and hole Hamiltonians with the interdiffusion effect are solved in the momentum space domain. Our results
show that isotropic three-dimensional In-Ga interdiffusion will makes the Stark shift become more symmetry about F=0 in asymmetric lens-shaped and pyramidal QDs, implying the reduced build-in dipole momentum. The
interdiffusion also leads to enhanced Stark shift with more prominent effects to QDs that are under larger electric fields.
Authors report the demonstration of the emission wavelength tuning of InAs quantum-dashes within InAlGaAs
quantum-wells grown on InP substrate, that gives the initial wavelength emission at ~1.65 &mgr;m. The impurity-free
dielectric cap annealing and the nitrogen ion-implantation induced intermixing techniques have been implemented to
spatially control the group-III intermixing in the structure, which produces differential bandgap shift of 80 nm and 112
nm, respectively. Transmission electron microscopy, optical and electrical characterizations have been performed to
evaluate the quality of the intermixed QD material and bandgap tuned devices. Compared to the control (nonintermixed)
lasers, the light-current characteristics for the over 125 nm wavelength shifted QD lasers are not
significantly changed suggesting that the quality of the intermixed material is well-preserved. The intermixed lasers
exhibit the narrow linewidth as compared to the as-grown due to the improved QD homogeneity. The integrity of the QD
material is retained after intermixing suggesting the potential application for the planar integration of multiple
active/passive QD-based devices on a single InP chip.
We investigated the multiple cations intermixing in InAs/InGaAlAs quantum dot-in-well laser structure grown on InP substrate using impurity-free vacancy disordering (IFVD) technique. Selective control of the bandgap shifts has been achieved using SiO2 and SixNy annealing caps. A differential wavelength shift of 76 nm has been observed after a rapid thermal annealing step at 750 oC for 30 s. In contrast to most IFVD results in other materials, we observed a larger bandgap shift from the SixNy capped samples than from the SiO2 capped samples. Based on theoretical calculations, we attribute this to the different effective interdiffusion rates of group-III cations. The demonstrated intermixing process provides an effective method for fine tuning the bandgap of InAs QDs around 1.55 μm as an alternative to the growth manipulation, as well as for realizing photonics integrated circuits.
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