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.
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