Controlled propagation of soliton bullets in an engineered strain field

Predicting and controlling the propagation of nonlinear responses in materials is critical to a range of fields, from the manipulation of single electrons in quantum optics to the understanding of crack propagation and failure of quasi-brittle materials. Solitons, which are highly localized strain patterns that propagate and persist due to nonlinear feedback mechanisms, can be produced in liquid crystal (LC) films under high-frequency AC electric fields. In previous work using uniformly oriented films of LC, soliton bullets propagated in one preset direction perpendicular to the far-field orientation of the LC director. Here, we show that confinement of the LC between asymmetric surfaces and the introduction of strain can provide a versatile mechanism to modulate the propagation direction of solitons. Specifically, we find that soliton bullets propagate along two oblique axes, where the angle can be dynamically modulated with the electric field frequency. The origins of this behavior are understood through theory and simulations, where the forces driving soliton motion are analyzed. Importantly, asymmetric flexoelectric torques lead to frequency-dependent oblique trajectories in the presence of hybrid LC anchoring, with numerical simulations predicting asynchronous out-of-plane fluctuations that are verified in experiments. Overall, our results highlight the interplay between the nonlinear action of external fields and the far-field strain on soliton propagation. They also show that confinement can be used to control the direction of propagation of nonlinear signals and demonstrate how LCs can be used as model systems to test and predict the effects of nonlinear excitations in new material designs.