The authors report on the nanowires-like and nanodots-like lasing behaviors in addition to multiple-wavelength
interband transitions from InAs/InAlGaAs quantum dash (Qdash) lasers in the range of ~1550 nm. The presence of
lasing actions simultaneously from two different dash ensembles, after postgrowth intermixing for crystalline quality
improvement, indicate the absence of optical phonon emission due to the small variation in quantized interband
transition energies. This effect is reproducible and shows different lasing characteristics from its quantum dot and
quantum wire laser counterparts. Furthermore, the small energy spacing of only 25 nm (at center lasing wavelength of
~1550 nm) and the subsequent quenching of higher energy transition states at higher bias level in Qdash lasers suggest
the absence of excited-state transition in highly inhomogeneous self-assembled Qdash structures. However, the
appearance of a second lasing line in a certain range of high injection level, which is due to the presence of different
sizes of dash assembles, corresponds to the transition from smaller size of Qdash ensembles in different planar active
medium. This unique transition mechanism will affect the carrier dynamics, relaxation process in particular and further
indicates localized finite carrier lifetime in all sizes of Qdash ensembles. These phenomena will lead to important
consequences for the ground-state lasing efficiency and frequency modulation response of Qdash devices. In addition,
these imply that proper manipulation of the Qdash ensembles will potentially result in localized nanolasers from
individual ensemble and thus contributing towards enormously large envelope lasing coverage from semiconductor
devices.
The advantages and applications of chalcogenide glass (ChG) thin film photoresists for grayscale lithography are
demonstrated. It is shown that the ChG films can be used to make ultrathin (~600 nm), high-resolution grayscale
patterns, which can find their application, for example, in IR optics. Unlike polymer photoresists, the IR transparent
ChG patterns can be useful as such on the surface, or be used to transfer the etched pattern into silicon or other
substrates. Even if the ChG is used as an etch mask for the silicon substrate, its greater hardness can achieve a greater
transfer ratio than that obtained with organic photoresists. The suitability of ChG photoresists is demonstrated with
inexpensive and reliable fabrication of ultrathin Fresnel lenses that are transparent in the visible as well as in the IR
region. The optical functionality of the Fresnel lenses is confirmed. Application of silver photodissolution in grayscale
lithography for MEMS applications is also shown. The process consists of the following steps: ChG film deposition, Ag
film deposition, irradiation through a grayscale mask, removal of the excess Ag and the transfer of the pattern to Si by
dry etching. A substrate to ChG thickness etching ratio of ~ 10 is obtained for the transfer of patterns into silicon, more
than a five fold increase compared to traditional polymer photoresist.
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.
This paper reports on the measurement and analysis of the coherence function for broadband emitters such as
superluminescent diodes (SLDs) and novel broadband laser diodes (BLDs) from self-assembled InGaAs/GaAs quantumdot
(QD) and InAs/InP quantum-dash (Qdash) structures that emit at center wavelengths of 1150nm and 1650nm,
respectively. Using the fiber-based spectral interferometry system, coherence lengths in fiber of 23 μm and 48 μm have
been measured from the QD and Qdash BLDs. Larger spectral bandwidth of 137 nm and 78 nm have been measured
from the QD and Qdash SLDs that yield coherence lengths in fiber of 3 μm and 10 μm, respectively. The coherence
function of both BLDs and SLD reveals negligible secondary coherence subpeaks and sidelobes indicating the
possibility of using these broadband sources to produce low artifacts optical coherence tomography (OCT) images.
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