Patent ID: 7821691

Claim:
A Zero-order diffractive filter comprising: a first layer comprising a periodic diffractive microstructure having a period that is between 100 nm and 3000 nm; and at least one second layer, wherein said at least one second layer has an effective index of refraction n eff of less than 1.4; wherein said first layer has a refractive index that is higher than the refractive index of said at least one second layer by at least 0.2; wherein at least one of said at least one second layer is a porous layer comprising an agglomeration of inorganic nanoparticles of 0.2 g/m 2 to 40 g/m 2 forming air-filled interstitial spaces between neighboring inorganic nanoparticles; and wherein said interstitial spaces are nanopores of said porous layer, said nanopores having a pore volume of at least 20% of the entire volume of said porous layer.