Spontaneous nucleation of localized peaks in a multistable nonlinear system

In a nonlinear optical experiment we report a unique class of localized structures, which appears as localized peaks of a pattern nucleating over another pattern. We show that this occurs when the system is driven through three pattern branches of solutions, accompanied by the appearance of localized peaks with two different amplitudes. Spontaneous creation and motion of localized peaks are triggered by amplitude and phase fluctuations of the underlying pattern. The scenario is universal and applies whenever a subcritical bifurcation exists between two different pattern solutions.