Standing wave-mediated molecular reorientation and spontaneous formation of tunable, concentric defect arrays in liquid crystal cells

Standing waves are involved in various fundamental, natural phenomena over a wide range of scales, that is, from electron orbitals to planetary oscillations. Interactions between standing waves and matter are of interest. Here we demonstrate spontaneous, nematodynamic standing waves in a nonlinear liquid crystal medium and their interactions with director fields. The standing waves hold unique features such as diagonal nodal lines, self-adaption to broad resonance frequencies and a step-like increase in the wavelength with the increasing frequency. Furthermore, the standing wave causes unexpected phenomena that can be used in various applications; for example, it can manipulate the molecular orientation to form tunable, periodic defect arrays with concentric director profiles, which can serve as an optical vortex array inducer or tunable micro-liquid crystal lens array. The standing wave also exerts asymmetric mechanical pressure that can relocate small particles. The results lead to new approaches in director manipulation, colloidal assembly and singular optics. Liquid-crystal cells pressured into forming standing waves show remarkable periodic patterns and particle-moving capabilities. Jang-Kun Song from Korea's Sungkyunkwan University and colleagues used repeating rectangular electric pulses to turn a randomly textured liquid crystal into an ordered two-dimensional array of spiral vortices free from the restrictions of typical resonances. Nonlinear effects helped stable periodic features emerge through a broad, adjustable range of frequencies. Close scrutiny of low-frequency patterns revealed that the standing waves maneuver the molecules of the liquid crystal into microdomains with vortex structures. By adding a chiral dopant to the liquid crystal, the team eliminated the small zones and produced a uniform defect array capable of inducing optical vortices or acting as tunable microlenses. The pressure exerted by the waves could also relocate crystal rods perpendicularly away from the field pulses. Spontaneous nematodynamic standing waves in liquid crystal medium is demonstrated. The standing wave manipulates the molecular orientation to form tunable periodic defect arrays with concentric director profiles, which can serve as optical vortex array inducer or as tunable micro-liquid crystal lens array. In addition, the standing waves hold unique features such as diagonal nodal lines, self-adaption to broad resonance frequencies, and a step-like increase of wavelength with increasing frequency. It also exerts asymmetric mechanical pressure that can relocate small particles. The results lead to new approaches in director manipulation, colloidal assembly, and singular optics.