We investigate waveguide resonators, and a coupled system based on two-dimensional silicon-on-insulator photonic crystal (PhC) slabs. First, we show a way of designing PhC waveguides and PhC resonators to overcome the radiation loss that occurs in the thickness direction in a two-dimensional system and how to couple a PhC waveguide to an optical fiber. Next, we effectively couple PhC waveguides to PhC resonators, devise a three-port-resonant-tunneling filter, and develop an ultrasmall multi-port channel-drop filter. Finally, we describe a two-beam optical switch operating with very low power that employs a PhC coupled resonator-waveguide system.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.