High-quality photonic crystals with a nearly complete band gap obtained by direct inversion of woodpile templates with titanium dioxide

International audiencePhotonic crystal materials are based on a periodic modulation of the dielectric constant on length scales comparable to the wavelength of light. These materials can exhibit photonic band gaps; frequency regions for which the propagation of electromagnetic radiation is forbidden due to the depletion of the density of states. In order to exhibit a full band gap, 3D PCs must present a threshold refractive index contrast that depends on the crystal structure. In the case of the so-called woodpile photonic crystals this threshold is comparably low, approximately 1.9 for the direct structure. Therefore direct or inverted woodpiles made of high refractive index materials like silicon, germanium or titanium dioxide are sought after. Here we show that, by combining multiphoton lithography and atomic layer deposition, we can achieve a direct inversion of polymer templates into TiO 2 based photonic crystals. The obtained structures show remarkable optical properties in the near-infrared region with almost perfect specular reflectance, a transmission dip close to the detection limit and a Bragg length comparable to the lattice constant. Even though three-dimensional Photonic Crystals (PCs), made of high index materials such as titanium dioxide 1,2 and silicon 3 , have been fabricated for instance by successive sputtering and electron beam patterning or from opal templates 4–6 , the infiltration of a lithographically structured polymer template appears to be a particularly effective strategy for obtaining large three dimensional PCs 7,8 and for embedding functional subunits for application in optical circuits or devices 9–11. Tétreault et al. reported the first silicon replica of such a woodpile template using the silicon double inversion technique 12. A SiO 2 inverse woodpile PC was used as an intermediate structure that was subsequently infiltrated with silicon by employing high temperature chemical vapor deposi-tion (CVD). Later, the same group reported on the fabrication of silicon inverse woodpile PCs 7. In this case, a SiO 2 shell was deposited around the polymer rods of the template to preserve the log-pile structure during the silicon deposition process. By these and related approaches, silicon hollow-rod woodpile PCs 13 , waveguides 11 , hyperunifrom 14 and complete band gap PCs at telecommunication wavelengths around 1.54 μm 8 have been obtained. Another well-known high refractive index material that has been widely used in the past is titania (TiO 2 , titanium dioxide) 1,6,15. The advantages of titania, compared to silicon, are its transparency in the visible to mid-infrared region as well as the possibility to chemically wet-process. In addition, titania can be deposited at moderate temperatures around or below 100 °C. This makes titania a particularly well-suited material to infiltrate three-dimensional polymeric scaffolds directly without the need for any additional infiltration steps which tend to lead to structural deterioration. In this work we present the fabrication, structural and optical characterization of titania hollow-channel and inverse woodpile PCs. Our work reports two main achievements in the field. First, we report on the fabrication of high-index woodpile PCs in a single infiltration step process. This facilitates, as we will show, the fabricatio