Optical absorption beyond the Yablonovitch limit with light funnel arrays

Abstract

Light trapping in the spectral range of visible to the near infrared is important for a plethora of energy-related photonic devices. The study numerically examines light trapping in arrays of subwavelength silicon light funnels (LF arrays) realized on silicon-on-insulator (SOI) wafers. We demonstrate the possibility of light trapping beyond the Yablonovitch limit and ~5% enhancement beyond the limit is shown. The SOI wafers are used for two reasons: firstly, SOI wafers introduce two bottom interfaces which allow efficient optical coupling between the LF arrays and the underlying substrates and, secondly, the potential for the realization of energy harvesting photonic devices realized on SOI wafers. Strong light trapping and high absorption in LF array-substrate complexes is shown for relatively short LF arrays. The strong absorption peaks correspond to a high optical intensity in the arrays which is 3–4 orders of magnitude higher than in ambient. Subsequently, these absorption peaks conclude strong excitations in the substrates. We show that the overall transmission is low on account of the two bottom interfaces of the SOI geometry. Specifically, the strong absorption peaks are due to low transmission coupled with low reflectivity which suggests forward scattering by the arrays into the substrates. Next, light trapping in LF array-substrate complexes is examined for higher LF arrays, and the dependency of the LF bottom diameter on the overall absorption of the complex is studied. We show that for small bottom diameter the excitation of the substrate is poor possibly due to lack of photonic states at the LF bottom. The excitation of the substrates increases as the LF bottom diameter is increased, however dramatic decrease in absorption is recorded for a bottom diameter that reflects the geometry of a nanopillar array. We show that the substrate excitation by a LF array is more efficient than the excitation by a nanopillar array as LFs provide a homogenous power spread in the substrate, whereas the substrate excitation by nanopillars is governed by an anisotropic power spread.