Near-infrared high refractive-index three-dimensional inverse woodpile photonic crystals generated by a sol-gel process

We demonstrate the formation of complete pieces of high refractive-index three-dimensional inverse woodpile photonic crystals by a simple and inexpensive sol-gel process. A mixture of the titanium dioxide precursor and the silica dioxide precursor was infiltrated multiple times into a polymeric template produced by two-photon polymerization to improve the mechanical properties of the inverse structure. After removal of the polymer template, transparent inverse photonic crystals with a refractive index of approximately 2.2 were achieved. The spectroscopic characterization reveals a stop gap in the near-infrared wavelength range, agreeing with the theoretical prediction.