Materials with on-demand refractive indices in the terahertz range

We demonstrate the possibility to create materials with chosen refractive indices and a strong birefringence in the terahertz range by etching of patterns with appropriate filling factors in a dielectric substrate. We show that by using deep inductive plasma etching of silicon wafers, it is possible to achieve a birefringence as high as 1.2 in an $80\mu \mathrm{m}$ thick layer. The resulting stacks were used as building blocks for a photonic crystal displaying sharp defect mode peaks in transmittance.