Principles and properties of phononic crystal waveguides

Strongly confined waveguiding is one of the main applications of phononic crystals that can be achieved at any frequency and scale. Phononic crystal waveguides replace the cladding of classical homogeneous waveguides by a crystal possessing a complete phononic bandgap. We review the different material systems used to implement phononic crystal waveguides and how waveguiding is obtained by confining waves in a core or by coupling defects along a given direction. Finally, we introduce topological principles to design defect-less waveguides by exploiting the symmetry of crystals.