High quality photonic crystal waveguide filters based on mode-gap effect

We demonstrate that the side-wall profiles of high-aspect-ratio two-dimensional (2D) photonic crystals (PhCs) in InP-based materials can be made vertical by reshaping through annealing. The annealing reduces depth and shape irregularities which are inherent to the etch-process. The efficacy of the reshaping is demonstrated by comparing the optical properties of PhC waveguides having as-etched and reshaped PhC-hole geometries. Spectral characteristics of ministop-bands (MSBs), due to coupling of third and fifth order modes with the fundamental mode, are used to qualify PhC fabrication. We demonstrate high optical quality filters based on the MSB effect (first and fifth order modes) and also use the spectral characteristics as a quality indicator of PhC fabrication. The MSBs transmission spectrum shows very sharp cut-offs for reshaped PhC waveguides. It is proposed that the reshaping process using annealing may also be beneficial for other PhC devices, nanostructure geometries and materials.