Simulation and characterization of thermal and optical effects in surface-emitting lasers

Doktorsavhandling, 2025

III-nitride surface-emitting lasers are highly attractive as compact and efficient light sources across the visible and ultraviolet (UV) spectrum, with applications in high-brightness displays, photolithography, UV curing, and disinfection. Despite these prospects, their development has been slowed by challenges related to current injection, optical confinement, cavity length control, material defect density, and thermal management. 

Blue electrically injected vertical-cavity surface-emitting lasers (VCSELs) have come the farthest, with state-of-the-art devices now achieving wall-plug efficiencies of 27.6% and output powers in the tens of milliwatts, placing them on the cusp of commercialization. One remaining hurdle is in demonstrating a robust way of realizing single mode operation and polarization control. In this thesis, we develop fabrication-friendly design concepts using localized shallow grating structures as viable strategies to achieve both. 

In the ultraviolet, only optically pumped VCSELs have been realized so far, representing an important first step toward electrically injected devices. The poor thermal conductivity of AlGaN, however, means that resistive Joule heating will cause severe internal heating under electrical injection. Our simulations show that directly transferring GaN-based VCSEL designs to the UVC regime results in prohibitively high thermal resistance, but that introducing intracavity heat spreaders can lower device temperatures to lasing-viable levels. We also develop design concepts for built-in wavelength stabilization, culminating in the experimental demonstration of optically pumped UVB VCSELs with the most inherently temperature-stable emission wavelength reported to date for any VCSEL no matter material system.

The thesis also addresses UV photonic-crystal surface-emitting lasers (PCSELs). A three-dimensional coupled-wave theory framework was implemented
to analyze finite-size loss mechanisms, and was complemented by the development of a new k-space weighted loss estimation method, which enables rapid analytical evaluation of vertical and lateral losses from infinite-structure band diagrams. These approaches provide powerful tools for understanding loss mechanisms in finite UV PCSELs, where a trade-off between low losses and efficient current spreading must be carefully managed. Together, these contributions advance the understanding of thermal and optical effects in III-nitride surface emitting lasers and provide strategies and modeling frameworks to support the development of high-performance blue VCSELs, and electrically injected UV VCSELs and PCSELs.