Various antireflective structures and methods are proposed to solve the optical loss of Si-based absorber devices. Dual-scale structures have received more concern from researchers in recent years. In this study, the finite difference time domain (FDTD) method is employed to investigate deeply the dependence of optical response on the geometric shape and size of structures. The micron cone shows lower reflectivity than other micron structures. Additionally, the lowest reflectivity region moves with the increasing height size of the cone structure. We proposed creatively a nanoripple-cone structure that maintains low reflectivity properties under varying incident angles whether in the visible region or the near-infrared region. Furthermore, the lower reflectivity is obtained with increasing micron cone and decreasing nanoripple. Finally, the dual-scale nanoripple-cone is fabricated directly and cost-effectively by a femtosecond laser instead of a two-step texture-on-texture way. The measured result shows that the high absorption above 98% extends to the mid-infrared region. This study provides directions for the fabrication of wideband Si-based absorber devices to reduce reflectivity, which exhibits a wide application potential and promotes the evolution of multi-laser processing.
Keywords: FDTD; dual-scale; femtosecond laser; nanoripple-cone; shape.