Omnidirectional reflection from nanocolumnar TiO2 films

Abstract

Anisotropic properties of columnar nanoporous thin films were utilized to design and fabricate interference mirrors with lossless omnidirectional reflection in the visible spectral range. Index graded columnar films with distributed Bragg reflector (DBR), sinusoidal, and Gaussian refractive index profiles were studied using finite-difference frequency-domain and finite-difference time-domain methods, with an emphasis on maximizing the omnidirectional reflection bandwidth. Titanium dioxide columnar films with sixteen period sinusoidal refractive index profile were fabricated using the glancing angle deposition technique and characterized by angle resolved transmittance measurements. Simulations and experimental measurements have shown the presence of the omnidirectional reflection band up to 5% wide for a film with a maximum refractive index nmax=2.3 and refractive index contrast Δn=0.8. Simulations further showed that with the optimal choice of the refractive index variation range, the omnidirectional reflection band can reach 10.5% width in TiO2 films with a sinusoidal index profile, 14.5% with a DBR index profile, and 12% with a Gaussian profile. Due to the optical anisotropy of the columnar films, the reflection bandwidth exceeded the corresponding value, observed in isotropic analogs, by a factor of three to four depending on the choice of the refractive index profile.