Optical Coupling of Semiconductor Nanowires

Abstract

The continued growth of computational demand has shifted performance and energy-efficiency bottlenecks from individual processing units toward data movement within and between computing systems, where electronic interconnects are increasingly limited by energy consumption and bandwidth. Photonic integrated circuits offer a promising route to alleviate these limitations by enabling high-bandwidth, low-energy optical interconnects. As photonic systems are pushed toward higher integration densities, optical functionality must be realized within increasingly confined volumes, giving rise to strong interactions between neighboring photonic elements. Semiconductor nanowires, which combine subwavelength optical confinement with micrometer-scale propagation lengths, have emerged as versatile building blocks for nanoscale photonic systems, including active elements such as nanowire lasers. In densely integrated architectures, optical coupling between nanowires becomes unavoidable and can fundamentally modify device behavior, necessitating a detailed understanding of how such interactions influence nanoscale light sources. In this thesis, optical coupling between semiconductor nanowires is investigated experimentally and numerically, with a particular focus on the lasing regime. A bottom-up experimental approach is employed, in which deliberately designed nanowire assemblies with well-defined geometries are used to isolate coupling effects. This strategy enables systematic investigations of coupling in configurations ranging from individual nanowires to nanowire pairs and small clusters. Two complementary material platforms are studied: zinc oxide nanowires arranged laterally on a substrate and indium phosphide nanowires grown vertically with lithographically defined positioning. Optical coupling is examined through experimentally accessible observables, including emission spectra, far-field radiation patterns, and energy transfer between neighboring nanowires under optical excitation. By systematically varying nanowire geometry and local excitation conditions, this approach enables coupling effects to be identified and compared across the two complementary material platforms and nanowire configurations investigated in this thesis. Numerical simulations based on eigenmode analysis and finite-difference time-domain methods are used alongside the experiments to interpret the observed behavior and to assess the role of geometry, symmetry, and fabrication tolerances. In zinc oxide nanowire pairs, coupling leads to full and partial frequency locking, master–slave behavior, and controlled transitions between coupled and decoupled lasing states. Selective local excitation density is shown to provide an effective control parameter for selecting the collective state, and in one specific geometry enables single-mode lasing through coupling-induced mode suppression. In indium phosphide nanowire systems, deliberately designed vertical nanowire pairs support coupled supermodes with strongly modified far-field emission, resulting in pronounced directionality that is well reproduced by numerical simulations. The experimental observation of multiple supermodes, together with extended structures such as nanowire triplets and arrays, demonstrates that coupled nanowire geometries can be realized reproducibly using selective-area epitaxy within realistic fabrication tolerances. Taken together, these findings establish optical coupling as a powerful design tool for engineering collective emission and directionality in nanoscale laser systems, and identify semiconductor nanowires as versatile building blocks for active directional photonic structures and nanoscale metasurfaces.