Optical properties of porous helical thin films and the effects of post-deposition annealing

Abstract

Porous thin films have been fabricated by physical vapor deposition at an extremely oblique angle of incidence (85°). This deposition technique, called glancing angle deposition (GLAD), was used to create thin films composed of isolated helical columns. By investigating a variety of dielectrics, we found that helical GLAD films fabricated from titanium dioxide produce the strongest chiral optical response because of its large refractive index. Further improvements were made by using post-deposition annealing to form anatase and rutile polycrystalline phases of TiO₂. By tailoring the pitch of the helical structures, the circular Bragg reflection band was tuned to preferentially reflect red, green, and blue light. The high porosity of a GLAD film (>50%) permits liquid crystals (LC) to be incorporated into the pores of the helical nanostructure, which creates chiral alignment in otherwise non-chiral LCs. This technique improves circular Bragg reflection and can create addressable hybrid materials with potential applications to high-efficiency reflective displays.