Smaller than Light!

Low refractive index materials

If you want to the know the basic optical properties (like reflection, refraction etc) of a material, you need to know its refractive index. The index of some materials

air = 1.003 ~ 1
water = 1.33
glass = 1.5
diamond = 2.414

You can notice that denser the material the index in general increases. The lowest index among available solids is around 1.45.

But then for certain opto-electronic applications we need an index from 1.05 to 1.45 only using solids. Can we mix two materials? How about a solid with voids? If we have voids in glass, we may visualize that it will have an index between 1 and 1.5 depending on the size of the voids.

But can light be duped to think that it is one material and not a mixture of two different materials? In reality, YES. If the sizes are smaller than light itself! Here is the challenge to make small voids of sizes much smaller than light's wavelength.