Broadband absorption of the solar radiation with surface arrays of subwavelength light funnels

Surface arrays of subwavelength silicon light funnels (LFs) were recently introduced as a promising new avenue towards efficient light trapping and the broadband absorption of the solar radiation. In the current work we use finite-difference time-domain calculations to understand the nature of the optical coupling between silicon LF arrays and underlying silicon substrates. We start by examining the broadband absorption dependency of free-floating LF arrays on the LF bottom diameter, and we show that light trapping at the Yablonovitch limit can be realized by tuning the LF bottom diameter. Next we examine the optical coupling between LF arrays and underlying substrates. We show that the array-substrate complexes exhibit strong absorption peaks at the near infrared. The origin of the absorption peaks is traced to either strong optical excitation of the LF arrays (occupation of Mie modes), strong excitations of the substrates (guided modes, Fabry-Perot and mode hybridization), or excitations of both the arrays and the substrates. Finally, we demonstrate light trapping at the Yablonovitch limit with the LF array-substrate complex.