Role of Surface Mode on Light Out-Coupling Characteristics of waveguide in Three Dimensional Photonic Crystals

The radiation pattern and out-coupling efficiency of light from the end surface of interlayer (oblique) waveguides in stacked-stripe 3-D photonic crystals are numerically investigated. It is shown that the coupling to the surface mode of the 3-D photonic crystal decreases the out-coupling efficiency to free-space modes down to 60%, whereas the reflection at the surface is negligibly small. The efficiency can be improved to 90% by introducing a 2-D grating-like structure to create a surface-mode gap, where surface modes are prohibited.