PhD Oral Exam - Gaowen Chen, Electrical and Computer Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

Quantum-dot (QD) and quantum-dash (Qdash) semiconductor lasers have attracted significant attention for optical communication and interconnect systems due to their discrete density of states, reduced linewidth enhancement factor, and improved temperature stability compared with conventional quantum-well devices. These advantages make them promising candidates for emerging applications such as wavelength-division multiplexing, optical frequency comb generation, and high-speed optical interconnects in co-packaged optics architectures. However, the performance of QD/Qdash lasers is strongly influenced by complex interactions between nanostructure morphology, strain distribution, carrier transport, and optical cavity dynamics. A comprehensive framework that links nanostructure-level physics to device-level performance remains an important research challenge.

This dissertation investigates the physics and device applications of QD and Qdash gain media for Fabry–Pérot lasers and vertical-cavity surface-emitting lasers (VCSELs). An integrated multi-physics simulation framework is developed to bridge electronic-structure modeling and dynamic laser behavior. The framework combines optical cavity eigenmode analysis, eight-band k·p electronic structure calculations, and travelling-wave time-domain (TDTW) carrier–photon dynamics, enabling self-consistent modeling of optical confinement, transition energies, dipole matrix elements, and carrier transport processes. Additional coupling mechanisms, including vertical stacking interactions and lateral carrier tunneling, are incorporated into the carrier dynamics to capture ensemble effects in densely packed nanostructure systems.

Using this framework, the dissertation first investigates vertical coupling effects in chirped stacked QD and Qdash active regions. It is shown that strain redistribution and electronic coupling between stacked layers strongly influence the distribution of optical transition energies. Depending on the stacking sequence, vertical coupling can either compress or expand the ensemble gain spectrum. Device-level simulations demonstrate that descending stacked structures enhance optical bandwidth and enable stable broadband mode-locking in Fabry–Pérot lasers without increasing the number of active layers.

The second part of the study focuses on lateral coupling effects in dense Qdash ensembles. By constructing a parallel Qdash pair model, the role of interdash tunneling in carrier redistribution is quantified. The results show that electron tunneling from higher-energy to lower-energy nanostructures can concentrate carriers into dominant gain states, leading to reduced threshold current in long-cavity Fabry–Pérot lasers. The impact of lateral coupling is found to depend strongly on device geometry and carrier density, highlighting the importance of morphology-aware device design.

Finally, coordinated bandgap and strain engineering strategies are investigated for high-speed C-band Qdash VCSELs. By introducing composition-engineered Qdashes and optimized confinement layers, emission wavelength control is decoupled from nanostructure height, enabling improved carrier confinement and enhanced differential gain. Time-domain simulations predict significant improvements in modulation bandwidth and large-signal eye-diagram performance compared with conventional Qdash VCSEL structures.

Overall, this dissertation establishes a physics-driven design framework linking nanostructure engineering, carrier dynamics, and optical cavity design in QD/Qdash semiconductor lasers. The results provide insights into coupling mechanisms in nanostructured gain media and demonstrate how these effects can be exploited to realize broadband, low-threshold, and high-speed laser devices for next-generation optical communication systems.