Propagation Loss of Line-Defect Photonic Crystal Slab Waveguides

Photonic crystal slab waveguides are created by inserting a linear defect in two-dimensional (2-D) periodic dielectric structures of finite height. Photonic crystals provide 2-D in-plane bandgaps through which light cannot propagate, however, the fact that the waveguide modes must be index-confined in the vertical direction implies that the propagation loss is strongly dependent on the out-of-plane radiation loss. We present a fully three-dimensional finite-difference time-domain numerical model for calculating the out-of-plane radiation loss in photonic crystal slab waveguides. The propagation loss of the single-line defect waveguide in 2-D triangular lattice photonic crystals is calculated for suspended membranes, oxidized lower claddings, and deeply etched structures. The results show that low-loss waveguides are achievable for sufficiently suspended membranes and oxidized lower cladding structures. The roles of the photonic crystal in out-of-plane loss of the waveguide modes are further analyzed. It is predicted that the out-of-plane radiation loss can be reduced by shifting one side of the photonic crystal cladding by one-half period with respect to the other sides along the propagation direction.

I. Introduction

Two-dimensional (2-D) photonic crystal defect waveguides have been the subject of active research [1]–[3] due to their potential to be a basic building block for densely integrated optics. They are most simply formed by inserting a linear defect in a 2-D triangular lattice of air holes in a dielectric slab. Photonic crystals provide a 2-D in-plane frequency bandgap for the guided modes of the slab. In the vertical direction, light is localized to the slab by means of total internal reflection. Although the in-plane photonic bandgap prohibits light to propagate anywhere but along the linear defect, a much weaker index confinement in the vertical direction raises the concern for the out-of-plane radiation loss.