Based on the effective medium theory, the antireflection properties of peri
odic subwavelength surface-relief structures were analyzed and investigated
. An estimation method for effective optical index for 2-D symmetric struct
ures was given. Based on the consideration of feature size, the design conc
ept of ridges and meshes was suggested. A two-dimensional infrared antirefl
ection relief pattern was fabricated on a Si wafer, using ion beam etching
process. From experimental results, it was shown that the structures, of wh
ich the characteristics are similar to that of a single antireflection film
, obviously suppressed the reflection in infrared spectrum, and its effecti
ve refractive index is equivalent to the refractive index of the film. And
the etching depth is equivalent to the thickness of 1/4 wavelength of the f
ilm.