We report on the experimental and theoretical investigation two kinds of acoustic waves in two dimensional phononic
crystal: bulk acoustic waves and surface acoustic waves. For bulk acoustic waves, the work focuses on the experimental
observation of full acoustic band gaps in a two-dimensional lattice of steel cylinders immersed in water as well as
deaf bands that cause strong attenuation in the transmission for honeycomb and triangular lattices. For surface acoustic
waves, complete acoustic band gaps found experimentally in a two-dimensional square-lattice piezoelectric phononic crystal
etched in lithium niobate will be presented. Propagation in the phononic crystal is studied by direct generation and
detection of surface waves using interdigital transducers. The complete band gap extends from 203 to 226 MHz, in good
agreement with theoretical predictions. Near the upper edge of the complete band gap, it is observed that radiation to the
bulk of the substrate dominates. This observation is explained by introducing the concept of sound line.
In this study, we design and fabricate a hollow optical waveguide with omni-directional reflectors in silicon-based materials. A groove is etched by inductive coupled plasma (ICP) with photolithographic process on (100) silicon wafer. The width of the groove is varied from 3.5 to 5.5 micrometer for different waveguide designs. The depth of the groove is 1.2 micrometers. Plasma enhanced chemical vapor deposition is used to deposit six pairs of Si/SiO<sub>2</sub>(0.111/0.258micrometers) on the samples. Finally, the top of the sample is covered by another silicon substrate on which the identical omni-directional reflector has been also deposited. By wafer bonding technology, the top omni-directional reflector can be combined with the groove to form a hollow optical waveguide. Light with the wavelength at 1.55 micrometers can be confined by the omni-directional reflectors at single mode operation. Polarization independent hollow optical waveguides can be achieved with this fabrication process.