  We develop an efficient method to derive a class of surface states in
photonic superlattices. In a kind of infinite bichromatic superlattices
satisfying some specific conditions, we obtain a finite portion of their in-gap
states, which are superpositions of finite numbers of their unstable Bloch
waves. By using these unstable in-gap states, we construct exactly several
stable surface states near various interfaces in photonic superlattices. We
analytically explore the parametric dependence of these exact surface states.
Our analysis provides an exact demonstration for the existence of surface
states and would be also helpful to understand surface states in other lattice
systems.
