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Optical properties and resonant cavity modes in axial InGaN/GaN nanotube microcavities

Coulon, P. -M. and Pugh, J. R. and Athanasiou, M. and Kusch, G. and Le Boulbar, E. D. and Sarua, A. and Smith, R. and Martin, R. W. and Wang, T. and Cryan, M. and Allsopp, D. W.E. and Shields, P. A. (2017) Optical properties and resonant cavity modes in axial InGaN/GaN nanotube microcavities. Optics Express, 25 (23). pp. 28246-28257. ISSN 1094-4087

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Microcavities based on group-III nitride material offer a notable platform for the investigation of light-matter interactions as well as the development of devices such as high efficiency light emitting diodes (LEDs) and low-threshold nanolasers. Disk or tube geometries in particular are attractive for low-threshold lasing applications due to their ability to support high finesse whispering gallery modes (WGMs) and small modal volumes. In this article we present the fabrication of homogenous and dense arrays of axial InGaN/GaN nanotubes via a combination of displacement Talbot lithography (DTL) for patterning and inductively coupled plasma top-down dry-etching. Optical characterization highlights the homogeneous emission from nanotube structures. Power-dependent continuous excitation reveals a non-uniform light distribution within a single nanotube, with vertical confinement between the bottom and top facets, and radial confinement within the active region. Finite-difference time-domain simulations, taking into account the particular shape of the outer diameter, indicate that the cavity mode of a single nanotube has a mixed WGM-vertical Fabry-Perot mode (FPM) nature. Additional simulations demonstrate that the improvement of the shape symmetry and dimensions primarily influence the Q-factor of the WGMs whereas the position of the active region impacts the coupling efficiency with one or a family of vertical FPMs. These results show that regular arrays of axial InGaN/GaN nanotubes can be achieved via a low-cost, fast and large-scale process based on DTL and top-down etching. These techniques open a new perspective for cost effective fabrication of nano-LED and nano-laser structures along with bio-chemical sensing applications.