Controlled Lasing in Gallium Nitride Nanowires
There is considerable interest in ultra-small coherent light sources. A strong candidate is a semiconductor-nanowire laser, where a single, monolithic nanowire functions simultaneously as an optical microcavity and active medium, leading to an extremely compact and robust laser. Recent advances in nanowire synthesis have enabled realization of optically pumped nanowire lasers in different material systems, including III-V, III-nitride, and II-VI semiconductors. However, due to the limited lasing control techniques, most of the nanowire lasers operate in naturally-occurring multi-mode and randomly polarized states. Lasing control in nanowire lasers is strongly desired for many practical applications. For instance, specifically polarized lasing is desired for atom trapping and biological detection, and single-mode lasing is crucial for applications needing high beam quality and spectral purity such as nanolithography and on-chip communications. Motivated by these practical requirements, this dissertation concentrates on the study of fundamental lasing characteristics and their control in gallium nitride (GaN) nanowire lasers. GaN nanowire lasers typically operate in a combined multi-longitudinal and multi-transverse mode state. Two schemes are introduced here for controlling the optical mode and achieving single-mode operation of the nanowire lasers. The first method involves placing two nanowires side-by-side in contact to form a coupled cavity. The coupled cavity can generate a Vernier effect, which is able to suppress both multi-longitudinal and multi-transverse mode operation, giving rise to the single-mode lasing in these nanowire lasers. For the second method, single-mode lasing is achieved by placing individual GaN nanowires onto gold substrates. The nanowire-gold contact generates a mode-dependent loss, which can strongly attenuate high-order guiding modes and ensure single-mode operation. Additionally, polarization properties of the gallium nitride nanowire lasers are studied experimentally by direct analysis of light emission from the nanowire end-facets. Linearly and elliptically polarized emissions are both obtained from a single nanowire at different pump strength, and a clear switching of the polarization states is observed with the change of optical excitation. This polarization change is attributed to a switching of transverse modes due to their difference in cavity losses. Finally, lasing polarization control is allowed by the coupling of the GaN nanowire lasers to an underlying gold substrate. The gold substrate breaks the symmetry of the nanowire geometry and generates an inherent polarization-sensitive loss. These effects allow us to demonstrate linearly polarized emission of GaN nanowire lasers, with a large extinction ratio and a fixed polarization orientation parallel to the substrate surface.
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