14 [30] S. Mossmann, A. Schulze, D. Witthaut and H.J. Korsch, J. Phys. A: Math. Gen. 38 3381 (2005). [31] Analytical results can be also provided for a more general two-dimensional lattice model, in which nearest diagonal coupling terms are accounted for in the Hamiltonian (38) (see Ref.[30]). However, the resulting equations are very cumbersome, and this case will not be considered here. [32] T. Pertsch, U. Peschel, F. Lederer, J. Burghoff, M. Will, S. Nolte, and A. T¨ unnermann, Opt. Lett. 29 , 468 (2004). [33] D.H. Dunlap, V.M. Kenkre, Phys. Rev. B 34 (1986) 3625. [29] H.J. Korsch and S. Mossmann, Phys. Lett. A 317 , 54 (2003). [27] A.E. Siegman, Lasers (University Science, Mill Valley, California, 1986). [28] H.L. Haroutyunyan and G. Nienhuis, Phys. Rev. A 64 , 033424 (2001). [26] A. Szameit, F. Dreisow, M. Heinrich, T. Pertsch, S. Nolte, A. T¨ unnermann, E. Suran, F. Louradour, A. Barth´ el´ emy, and S. Longhi, Appl. Phys. Lett. 93 , 181109 (2008). sider beams with a bounded variance, for which the cor- responding spectrum F ( k ) is not singular.