Design and fabrication of Si subwavelength structures for broadband antireflection in mid-infrared ranges

Broadband antireflection in infrared ranges is essential for various applications such as photovoltaics, light-emitting diodes, and optical lenses for thermal imaging. We performed numerical simulations to find the optimal design of Si subwavelength structures for broadband antireflection in the mid-infrared wavelength ranges from 3 μm to 20 μm. By using the simulation results as a design guide, we fabricated Si subwavelength structures in the form of ellipsoids and paraboloids with self-assembled silica bead monolayers as dry etch masks. The silica bead monolayers on the Si wafers in large area were prepared by colloidal lithography based on spin coating of silica beads dispersed in organic binary solvents. A two-step dry etch process with combination of isotropic and anisotropic etching enables fabrication of the Si subwavelength structures of a high aspect ratio, and we demonstrated broadband antireflection in the mid-infrared wavelength ranges.