  Flatband photonic lattices, i.e. arrays of waveguides or resonators
displaying a flat Bloch band, offer new routes for light trapping and
distortion-free imaging. Here it is shown that flatland lattices can show
stable and cooperative laser emission when optical gain is supplied to the
system, despite the large degree of degeneracy of flatland supermodes. By
considering a quasi one-dimensional rhombic lattice of coupled semiconductor
microrings, selective pumping of the outer sublattices can induce cooperative
lasing in a supermode of the flat band
