  We report the design of silica-based 1D phononic crystals (PnCs) with
topologically distinct complete phononic bandgaps (PnBGs) and the observation
of a topologically protected state of elastic waves at their interface. By
choosing different structural parameters of unit cells, two PnCs can possess a
common PnBG with different topological nature. At the interface between the two
PnCs, a topological interface mode with a quality factor of ~5,650 is observed
in the PnBG. Spatial confinement of the interface mode is also confirmed by
using photoelastic imaging technique. Such topologically protected elastic
states are potentially applicable for constructing novel phononic devices.
