We explore the use of air trenches to achieve compact high efficiency 90° waveguide bends and beamsplitters for waveguide material systems that have low refractive index and low refractive index contrast between the core and clad materials. For a single air interface, simulation results show that the optical efficiency of a waveguide bend can be increased from 78.4% to 99.2% by simply decreasing the bend angle from 90° to 60°. This can be explained by the angular spectrum of the waveguide mode optical field. For 90° bends we use a micro-genetic algorithm (μGA) with a 2-D finite difference time domain (FDTD) method to rigorously design high efficiency waveguide bends composed of multiple air trenches. Simulation results show an optical efficiency of 97.2% for an optimized bend composed of three air trenches. Similarly, a single air trench can be designed to function as a 90° beamsplitter with 98.5% total efficiency.