Light-induced isotropic pen for generation of topological solitons and hopfion–toron transition in frustrated chiral nematic films

Chiral nematic liquid crystals are capable of hosting a variety of non-trivial orientational configurations, such as skyrmions and hopfions; however, their on-demand generation still remains a challenging task. In this study, we investigate the generation and subsequent relaxation of predefined topological structures in frustrated chiral nematic films using a low-power light beam. The light beam, absorbed in the bulk of a dye-doped liquid crystal, induces the formation of an isotropic region. The motion of this isotropic region, followed by the isotropic-to-nematic phase transition, results in the excitation of orientational structures. In particular, we demonstrate the creation and characterization of topological solitons, open-ended cholesteric finger fragments, and cholesteric finger loops. The specific type of cholesteric finger formed is determined by the speed and trajectory of the light beam, as well as the presence of colloidal inclusions, which act as nucleation sites for particular orientational configurations. Following numerical simulations of the director field, we identify one of the finger loops as a hopfion that gradually relaxes into a toron. The presented approach offers an efficient way for the spatially controlled generation of predetermined extended orientational patterns and metastable topological solitons in chiral nematic films.