Opt. Express 21, A964–A969 (2013)
Nanowire arrays have attracted attention because of their strong optical absorption — a feature that is particularly beneficial for photovoltaic applications as it allows a thinner active layer to be used, hence reducing fabrication costs. However, to date, the origin of the absorption enhancement in aperiodic arrays has not been comprehensively explained. Now, Björn Sturmberg and colleagues from Australia have numerically studied the effect of localized clustering of nanowires in such arrays. Using a combination of simulation techniques, including the finite element method and the scattering matrix method, the team found that the reduced symmetry associated with cluster formation allows external coupling of plane waves into modes that are usually dark in periodic arrays, thus significantly increasing absorption. The same conclusion was obtained with clustering of two, three, four and five nanowires, suggesting that the results are general, and are not specific to the geometry. The team pointed out that the absorption is significantly enhanced only when the gap between the nanowire surfaces is about 20% of the nanowire radius. Consequently, the arrays have to be carefully designed, rather than depending on random variation during the fabrication process.
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