Experimental observation of modulational instability in vertical-cavity surface emitting laser arrays

We report the experimental observation of transverse optical patterns in optically injected phase-coupled VCSEL arrays. The devices consist of a single VCSEL with a metallic top contact defining a 8x8 pixels array. The different semiconductor/air - metal/air index step determines the periodic variation of the cavity transmission, that enables the selection of a single laser supermode above threshold. The patterns were observed in the near field at driving currents below the laser threshold, while injecting a nearly resonant optical field. At large frequency detuning hexagonal patterns were observed, whereas rolls appeared closer to the cavity resonance. The experimental observations are explained according to a well established model (Spinelli, Tissoni, Brambilla, Prati, Lugiato, Phys.Rev.A 58, (1998) 2542) accounting for diffraction and nonlinear effects in optically injected VCSELs, and modified to allow for the spatially modulated cavity transmission.