Optically selective photomask based on field-induced periodic defect arrays in ferroelectric nematics

Controllable periodic topological defects in liquid crystals (LCs) are valuable for applications in self-assembly and optical devices. In this work, we demonstrate the application of field-induced defect arrays created in the ferroelectric nematic (NF) LC phase as an optically selective photomask. These uniform stable 2D defect patterns form under out-of-plane electric fields in LC cells with bare electrodes, simplifying the fabrication process. The studied LCs require a low threshold voltage (∼2.5 V at 0.5 kHz) to induce pattern formation, which is controlled by voltage rather than field. The periodicity of defect patterns is 100 to 400 times larger than the cell gap and varies with both field parameters and cell gap. Notably, applied frequency significantly influences the periodicity; increasing the frequency from 0.5 to 30 kHz decreases the periodicity from 1.64 to 0.45 mm. Furthermore, we use these periodic domains as a photomask to generate continuous photoalignment patterns in a photosensitive layer, which subsequently serve as a template to form periodic phase arrays from reactive mesogen. Our experimental findings show that spatial characteristics of the resulting phase arrays can be precisely tuned by controlling the domain size of the photomask and the polarization of incident light.