Perturbative approach to diatomic lattices
  By using a small rotation approach, we show that it is possible to obtain well behaved perturbed solutions for the amplitude of the electromagnetic field propagating in a photonic waveguide array. This array mimics the propagation of a quasiparticle among the sites of an infinite one-dimensional chain. 
Francisco Soto-Eguibar and H\'ector Manuel Moya-Cessa
quant-ph