Nano‐photonic devices are playing increasingly important roles in optical communication and optical sensor systems. By engineering the nano‐photonic structures, for example, by fine tuning the photonic band diagram of photonic crystal waveguides, one can slow down the group velocity of the photons by two orders of magnitude, which can significantly increase the light‐matter interaction. In this presentation, I will discuss the design and fabrication of an innovative photonic crystal slot waveguide on silicon‐on‐insulator (SOI) wafers, with special emphasis on coupling light from conventional optical fibers into slow light enhanced nanophotonic waveguide. Based on this ultra‐efficient platform, we have developed highly compact (300μm) and ultrasensitive on‐chip optical sensors for water quality monitoring (50ppb xylene in water) and green‐house gas detection (100ppm methane in nitrogen). When the slow light enhanced nanophotonic waveguide is combined with other innovative materials, we can create various photonic devices with enhanced functionalities for a broad spectrum of applications in board level optical interconnect, radio frequency (RF) photonic communication, electromagnetic wave detection, and bio‐molecule sensing. I will show the state‐of‐the‐art design of a nano‐photonic modulator using E‐O polymer infiltrated silicon photonic crystal slot waveguide with unprecedented efficiency, and experimental demonstration of 735pm/V in‐device E‐O coefficient and 0.44V‐mm of VπL, which is ten times better than the best results of our competitors. The application of photonic bandgap nano‐devices for surface enhanced Raman scattering is also demonstrated to achieve 10 times additional enhancement factors. In summary, photonic bandgap nanodevices have demonstrated extremely high potential in many communication and sensing areas, and will continue to broaden its application in many emerging fields through interdisciplinary research.