Optical thin-film materials are of much interest in optoelectronics as they provide the ability to guide the light generated by the optoelectronic devices. Light reflects when it strikes the interface between two materials with different refractive indices. The greater the difference between the refractive indices of two materials, the more light is reflected. Reflection-free surfaces might have a lot of uses. For example, they could make a solar cell almost perfectly absorbing to increase energy efficiency, or could make light-emitting diodes (LEDs) more transparent to deliver more light from the semiconductor to free space. We have created a new class of materials with refractive indices varying from 1.05, which is extremely close to the refractive index of air and the lowest ever reported, to the refractive index of any thin-film material by oblique-angle deposition, and have demonstrated virtually reflection-free surface on AlN by a multi-layer coating of the low-index materials. Many potential applications of this optical thin-film materials include light-absorbing layers for energy-efficient solar cells, light-transmitting layers for highly efficient LEDs, DBRs made of a single material, which are being realized in our Lab.