InP-based photonic crystal waveguide filters

In this work ΓK oriented 2D photonic crystal (PhC) waveguides fabricated in InP-based materials are theoretically and experimentally investigated. Specifically, the paper focused on the mini-stop band (MSB) effect due to the coupling between the first and fifth order modes in PhC waveguides for filter and sensing applications. The investigated waveguides include multimode line-defect waveguides with different waveguide widths and air-fill factors, and, single-junction heretostructure waveguides. A central issue in the low-index system is the requirement of high aspect ratio etching. The quality of the etched PhC holes (shape, uniformity and depth) has a direct bearing on the optical properties of the waveguides. Present here is the state-of-the-art high aspect ratio etching of InP-PhCs highlighting the fundamental limits. A post-etch thermal process to reflow the material is investigated to re-shape the etched holes. It is shown that near cylindrical shapes can be obtained together with a dramatic improvement in the hole to hole depth uniformity and reduction in shape irregularities. These excellent geometrical characteristics of the etched PhC are shown to result in waveguides with superior performances. In particular extremely sharp MSB edges are obtained. MSB as deep as 35 dB and with sharp edges dropping in intensity over 30 dB in a ~ 4 nm wavelength change is obtained by reshaping the nano-holes. Temperature tuning experiments show clear red-shift of the MSB edges with a gradient of dλ/dT=0.1 nm/´C. The achieved transmission extinction ratio and the sharp edges together with the tune-ability make these devices attractive for sensing, tuning and modulation applications.