Broadband solar absorption with silicon metamaterials driven by strong proximity effects

Absorption of the solar radiation over a wide spectral range is of utmost importance to applications related to the harvesting of solar energy. We numerically demonstrate broadband solar absorption enhancement employing a metamaterial in the form of arrays composed of subwavelength silicon truncated inverted cones, henceforth referred to as light funnel (LF) arrays. We show that the broadband absorption efficiency of an unoptimized LF array is 36% greater compared with an optically-maximized NP array. We show that photon trapping in LF arrays is motivated by proximity effects related to the optical overlap between LFs. We make the distinction between two types of optical overlap: weak overlap in which the coupling between the sparse array modes and the impinging illumination increases with array densification, and strong overlap where the array densification introduces new highly absorbing modes. We show that in nanopillar (NP) arrays the optical intensity inside the NPs decreases upon densification and the overall increase in absorptivity is due to increase in filling ratio (as was also shown by others), while the densification of LF arrays increases the optical intensity inside the individual LF and with the concurrent increase in filling ratio concludes light trapping much superior to that of NP arrays. Light trapping governed by strong proximity effects was not reported to date, and we believe it is an important paradigm for miniaturized lab-on-chip technologies.