Grading absorption and enhancement in silicon nanowire arrays with thin blocks

•Absorption enhancement in silicon nanowires with thin blocks are investigated.•Grading absorption is found when the width of nanowires is small.•The enhancement mechanism caused by thin blocks is revealed.•Grading absorption and enhancement is validated to achieve by dielectric blocks.

Silicon nanowires have large aspect-ratio to keep high absorption along the incident direction, so nanowire arrays are one of potential structures for trapping light. To further enhance absorption in silicon nanowire arrays, thin blocks are arranged on the silicon substrate. It numerically demonstrates that additional blocks not only have few negative influence on absorption of nanowire arrays in the visible range, but also excite extra resonances in the near-infrared range, which make grading absorption and enhancement in the nanostructures. These excited resonances are finely examined and attributed to surface plasmon polaritons, guided modes and their coupled modes. The enhancement mechanism is further revealed by the electromagnetic fields and theoretical analysis for corresponding two-dimensional structures. The nanostructures can achieve higher absorption in silicon nanowire arrays, and grading absorption in the nanostructures will facilitate the development of other devices as novel filters, photoelectronics and so on.