Numerical optimization design for waveguide bends with low-loss and widebandwidth in two-dimensional photonic crystal slabs

In this paper, we demonstrate some optimization designs for waveguide bends in twodimensional photonic crystal slabs based on the effective index approximation. Four different types of waveguide bends with low-loss and wide-bandwidth are designed, which involve 60 degrees, Z-shaped, Y-shaped and 120 degrees waveguide bends in the two-dimensional triangular lattice photonic crystals. In all numerical experiments, the electromagnetic fields in the photonic crystal slab waveguides are solved by the finite element method in COMSOL software. Several strategies are applied to design slab waveguide bends with low-loss and wide-bandwidth, for example, adding some elliptical air-holes near the corner or adjusting the radii of some air-holes. For four optimized structures, the transmission efficiencies at 1550 nm are all more than 92%. As we show, this is achieved over a -0.915 dB bandwidth of more than 65 nm in all waveguide bends. The numerical results indicate that the design scheme we applied to optimize waveguide bends in photonic crystal slabs is simple and efficient, which can provide some useful guidance for further three-dimensional design.