Patent ID: 7697395

Claim:
An optical device, comprising: a sub-wavelength grating formed in at least one of a plurality of regions on a substrate with a refractive index of n 1 , wherein when an incident light beam with a predetermined polarization direction is used, the sub-wavelength grating has a refractive index indicated by the formula np = ⁢ t ⁡ ( n ⁢ ⁢ 1 ) 2 + ( 1 - t ) ⁢ ( n ⁢ ⁢ 2 ) 2 ⁢ ⁢ or ⁢ ⁢ ns = ⁢ 1 t ( n ⁢ ⁢ 1 ) 2 + 1 - t ( n ⁢ ⁢ 2 ) 2 where n 1 , n 2 represent refractive indexes of materials at tops and bottoms in the grating periodic structure, and the refractive index n 1 , a grating height, or a duty ratio t of said sub-wavelength grating are selected such that a light path of an incident first light beam passing through the sub-wavelength grating is the same as a light path of the first light beam passing through regions neighboring the at-least-one region, wherein the sub-wavelength grating is not formed in the neighboring regions.