Optical Guiding and Feedback in Gallium Nitride Based Vertical Cavity Surface Emitting Lasers

Licentiatavhandling, 2015

The gallium-nitride (GaN) semiconductor material has been the core of the revolutionary breakthroughs during the last two decades in the lighting industry, by enabling manufacturing of efficient blue light emitting diodes (LEDs), for which the 2014 Nobel prize in physics was awarded. The GaN technology has further led to violet edge-emitting lasers (EELs), enabling the Blu-ray disk technology, and also to the commercialization of directly green emitting EELs. A natural next step is the realization of GaN-based vertical-cavity surface-emitting lasers (VCSELs), which has proved to be a challenging task. The first electrically injected GaN-based VCSEL was announced in 2008, more than a decade after the first reports on its EEL counterpart. Still today only four groups in the world have demonstrated lasing under continuous-wave operation in a blue VCSEL. Some of the major challenges to realize GaN-based VCSELs are the lack of two lattice matched materials for forming high reflectivity distributed Bragg reflectors (DBRs), the poor current spreading capabilities in p-doped GaN, and the difficulty to achieve current and optical confinement. In this work we have addressed those issues. We have developed the novel concept of TiO2/air high contrast gratings (HCGs) to achieve high reflectivity over a broad wavelength range. The HCGs show a high reflectivity (>95%) over a 25 nm wavelength span, and a very good agreement between simulated and measured reflectivity spectra has been achieved. By using our in-house developed VCSEL simulation tools we have studied existing GaN-based VCSEL designs and shown that the approach taken by most groups to confine the current to the center of the device (transverse direction), yields an optical resonator that is weakly antiguiding with very high optical losses and thereby high threshold currents. These anti-guided devices have total losses that are typically 100-200% higher than in our newly proposed structures, in which current confinement and optically guiding can be achieved simultaneously. These structures have already been implemented by two of the world's leading groups in the area.