  Interface states in photonic crystals usually require defects or
surface/interface decorations. We show here that one can control interface
states in 1D photonic crystals through the engineering of geometrical phase
such that interface states can be guaranteed in even or odd, or in all photonic
bandgaps. We verify experimentally the designed interface states in 1D
multilayered photonic crystals fabricated by electron beam vapor deposition. We
also obtain the geometrical phases by measuring the reflection phases at the
bandgaps of the PCs and achieve good agreement with the theory. Our approach
could provide a platform for the design of using interface states in photonic
crystals for nonlinear optic, sensing, and lasing applications
