Minimisation of out-of-plane losses in slab photonic crystals by optimising vertical heterostructure

Planar line-defect photonic crystal waveguides with a weak vertical refractive index contrast (substrate-type) suffer from large propagation losses in the order of 600-1000-dB/cm. A possibility to reduce propagation losses is to optimise the vertical layer stack for a given hole shape. This strategy is pursued to reduce the propagation losses numerically and experimentally. It is demonstrated that the loss figure can vary by a factor of about 2 for relatively small changes in the core layer thickness.