Investigation of proximity effects in light funnel arrays using near field optical microscopy

Broadband absorption is pivotal for the realization of green energy based on solar energy. Decoration of photovoltaic cells, for example, with arrays of subwavelength formations provides an efficient means for broadband absorption in thin films. Surface arrays of silicon light funnels (LF) were suggested as a promising platform to produce broadband absorption that is considerably superior to that of the well-known nanopillar (NP) arrays, for example. The current study explores the underlying mechanism of broadband absorption in LF arrays. To this end the optical near-field of LF and NP arrays is experimentally probed. We show that in LF arrays the near-field increases as the array period decreases in contrast with NP arrays in which the near-field decreases with decreasing array period. For example, in diluted arrays the near-field intensity of the NP array is ~2.4 times higher than that of the LF array, whereas for compact arrays the near[1]field intensity of the LF array is ~2.4 times higher than that of the NP array. Also, it is shown that the arrays near-field follows the numerically calculated absorption cross[1]section of the individual NPs/LFs nested in the arrays. Therefore, it is suggested that the origin to the broadband absorption in compact LF arrays is due to field overlap of adjacent LFs which increases the absorption cross-section of the individual LFs composing the array. This enhancement in the absorption cross section and the higher filling ratio in compact arrays produce broadband absorption that is significantly higher than that of optically-optimized NP arrays.