Near-Field Optical Excitations in Silicon Subwavelength Light Funnel Arrays for Broadband Absorption of the Solar Radiation

Broadband absorption is pivotal for the realization of green energy based on solar energy. Decoration of photovoltaic cells with arrays of subwavelength formations provides an efficient means for broadband absorption in thin films. Surface arrays of silicon light funnels (LFs) have been suggested as a promising platform to produce broadband absorption that is considerably superior to other subwavelength arrays such as the well-known nanopillar (NP) arrays. 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 using a near-field scanning optical microscopy. It is shown 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. Also, the experimental near-field of the arrays follows the numerically calculated absorption cross section of the array-nested NPs/LFs. Therefore, 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 absorption cross-sectional enhancement coupled with a higher filling ratio in compact arrays produces broadband absorption that is significantly greater than that of NP arrays.