Fabrication of two- and three-dimensional photonic crystals of titania with submicrometer resolution by deep x-ray lithography

Two-dimensional photonic crystals of titanium dioxide are predicted to have many advantages over semiconductor photonic crystals, e.g., silicon and GaAs: in particular, low optical loss in the near infrared region used for optical communication, low thermal expansion, and a refractive index which is close to that of optical fibers. However, it is difficult to create micronanostructures in titanium dioxide, since semiconductor microfabrication techniques cannot be applied to titanium dioxide. As the first step, we calculated the photonic band gap of titanium dioxide rod slab on SiO₂. Band gap percent against thickness of the rod slab was also examined. Finally, we confirmed the most suitable structure for two-dimensional (2D) photonic crystals. A deep x-ray lithography technique was employed to create a very deep and precise template. Liquid-phase deposition was then used to faithfully deposit a tightly packed layer of titanium oxide onto the template. Finally, the template was selectively removed to obtain a photonic nanostructure. We also calculated the photonic band gap for the 3D structure of TiO₂. A template for the most appropriate structure was fabricated using the method proposed by Yablonovitch. By employing the same method, we successfully obtained the 3D structure of TiO₂. The refractive index of the obtained TiO₂ followed by heating at 700 °C was determined as being 2.5, which is close to that of the anatase phase.