Optical performance of porous TiO2 chiral thin films

Abstract

Porous thin film structures have been fabricated by physical vapor deposition at an incident flux angle that was typically greater than 80°. This deposition technique, often called glancing angle deposition (GLAD), was used to create thin films composed of isolated helical columns. Modification of the deposition parameters was used to control the porosity, the handedness, and the pitch of the helical structure. The high porosity of the GLAD film (>50%) permits fluids, and in particular liquid crystals (LC), to be incorporated into the voids of the nanostructure. We present the results of a study assessing the effect of film material, chiral morphology, and liquid crystalline material on the optical performance of helical GLAD films. Films fabricated from TiO₂, a high refractive index material, exhibited strong optical rotation of linearly polarized light and selective reflection of circularly polarized light. By increasing the number of turns of the helix the chiral optical response was enhanced, and by tailoring the pitch of the helical columns, the wavelength-dependence of the reflection band was tuned to preferentially reflect red, green, or blue light.