Manipulation of light at the nanoscale has the promise to enable numerous technological advances in biomedical sensing, optical communications, nano-mechanics and quantum optics. As photons have vanishingly small interaction cross sections, their interactions have to be mitigated by matters (i.e. quantum emitters, molecules, electrons etc.). Waveguides and cavities are the fundamental building blocks of the optical circuits, which control or confine light to specific matters of interest. The first half of the thesis (Chapters 2 & 3) focuses on how to design various photonic nanostructures to manipulate light on nano- to micro- scale, especially to modify the light-matter interaction properties. Chapter 2 discusses how nano-slot waveguides and photonic crystal nanobeam waveguides are able to modify the emission of quantum emitters, in a different way that normal ridge waveguides are not able to. Chapter 3 focuses on a more complicated and powerful structure: the photonic crystal nanobeam cavity. The design, fabrication and characterization of the photonic crystal nanobeam cavities are described and demonstrated in detail, which lays out the foundation of the biomedical sensing applications in the second half of the thesis. The second half of the thesis (Chapters 4 & 5) focuses on the application of photonic crystal nanobeam cavities in the label-free sensing of biomedical substances. Chapter 4 demonstrates detection of solutions with different refractive index (aceton, methanol, IPA etc.), glucose concentration, single polystyrene nanoparticles and single streptavidin bio-molecules. Chapter 4 proposes a novel nonlinear optical method to further enhance the sensitivity. Chapter 4 also demonstrates high quality nanobeam cavities fabricated in polymers, that open up a new route to decrease the cost, as well as to achieve novel applications with functional polymers. The broader impact of this technology lies in its potential of commercialization of a new generation of biosensors with high sensitivity and high integration. Chapter 5 discusses progresses towards instrumentation of the nanobeam cavity sensing technology for research & development apparatus, as well as point-of-care diagnostic tools. / Engineering and Applied Sciences