Excitability and localized structures are universal phenomena, observed in various systems. Both possess interesting properties for potential applications, especially in optics. Excitability is the intrinsic property of the neuron defining its response to an external stimulus: for a sub-threshold stimulus the neuron stays quiescent; for a super-threshold stimulus, it emits a well-calibrated pulse independent on the strength of the stimulus. Localized structures in optics are bright intensity peaks coexisting with a homogeneous low intensity background. They appear in the transverse section of spatially-extended laser resonators. We study the occurrence of these nonlinear phenomena in semiconductor microcavities since the applications in all-optical processing of information are promising. Moreover we investigate the possibility of a novel kind of localized structure which stands at the intersection of these two phenomena: the excitable localized structures. On the one hand we study the properties of localized structures arising from a cavity soliton laser composed of two mutually coupled broad-area VCSELs in a LSA (Laser with Saturable Absorber) configuration . We report on the observation of a random and localized emission of pulses which we claim to be the first experimental evidence of noise-triggered excitable localized structures whose excitability is induced by inhomogeneities and drift. On the other hand we demonstrate the excitability in an optically injected laser by showing the control of excitable pulses by means of an external perturbation. We also perform numerical simulations which reveal the existence of excitable localized structures in a model of broad area VCSEL with saturable absorber.
