Experimental evidence of bouncing localized structures in a nonlinear optical system is reported. Oscillations in the position of the localized states are described by a consistent amplitude equation, which we call the Lifshitz normal form equation, in analogy with phase transitions. Localized structures are shown to arise close to the Lifshtiz point, where non-variational terms drive the dynamics into complex and oscillatory behaviors. [11,12,13] and cavity solitons in lasers [14]. Localized states are patterns which extend only over a small portion of a spatially extended and homogeneous system [15]. Different mechanisms leading to stable localization have been proposed [16]. Among these, two main classes of localized structures have to be distinguished, namely those localized structures arising as solutions of a quintic Swift-Hohenberg like equation [16] and those that are stabilized by nonvariational terms in the subcritical Ginzburg-Landau equation [17]. The main difference between the two cases is that the first-type localized structures have a characteristic size which is fixed by the pinning mechanism over the underlying pattern or by spatial damped oscillations between homogenous states [16,18], whereas the second-type ones have no intrinsic spatial length, their size being selected by non-variational effects and going to infinity when dissipation goes to zero. In both cases, non-variational effects may lead to dynamical behaviors of localized structures [19]. Variational models based on a generalized Swift-Hohenberg equation have been proposed to describe the appearance of localized structures in nonlinear optics [20]. However, a generalization including non-variational terms is generically expected to apply even in optics, as happens, for instance, in semiconductor laser instabilities [21], giving rise to dynamical behaviors of localized structures, such as propagation and oscillations of their positions [22].
PacsWe report here an experimental evidence of localized structures dynamics in a Liquid-Crystal-Light-Valve (LCLV) with optical feedback. It is already known that, in the simultaneous presence of bistability and pattern forming diffractive feedback, the LCLV system shows localized structures [12,23,24,25]. Recently, rotation of localized structures along concentric rings have been reported in the case of a rotation angle introduced in the feedback loop [26]. Here, we fix a zero rotation angle and we show a new dynamical behavior, the bouncing of two adjacent localized structures, that is not related to imposed boundary conditions but is instead a direct consequence of the non-variational character of the system under study. Theoretically, we show that the LCLV system has several branches of bistability connecting an homogeneous state to a patterned one and we derive an amplitude equation accounting for the appearance of localized structures. This is a one-dimensional model, that we call the Lifshitz normal form equation [22], characterizing the dynamics of localized structures close to each po...