Abstract
We report on observation of laser action unique to a two-dimensional (2D) photonic lattice. When dye-solution filled in air-holes of a 2D lattice was optically-pumped, laser action without external mirrors is found to occur at a specific wavelength corresponding to a flat band-dispersion in a high-symmetry direction of the 2D lattice plane; a small group-velocity is responsible for the lasing. On further increasing the pump-fluence, another laser action is also found to occur around a peak wavelength of the spontaneous emission spectrum. This lasing is interpreted of as arising from the so-called uncoupled modes possessing a high Q-value.