The cosmic microwave background (CMB) radiation observation is the primary tool for determining the global properties, content, and history of the universe. One efficient way to increase the detection sensitivity of future CMB measurement instruments is to use refractive optics due to the improved detection efficiency and the reduced space, weight, and cost. At long wavelengths from far infrared to millimeter, silicon is a superior material for manufacturing refractive optics because of its excellent transmission and low dispersion. However, due to its very high refractive index, silicon optics suffers from a 30% reflection loss per silicon-air interface. We will develop a new broadband antireflection coating technology inspired by the periodic nanostructures found on the cornea of a moth's eye. The resulting coatings can significantly reduce optical reflection from silicon refractive optics over a wide range of long wavelengths and incident angles.