Enhanced Security Platform via Active Multi-band Metasurface with Tunable Heliconical Liquid Crystal Modulators

Abstract

Metasurfaces that are composed of nanostructures with subwavelength-scale (meta-atoms) have the ability to precisely modulate phase and amplitude of incident electromagnetic waves based on the design of meta-atoms (e.g, organization, shape, size, materials). The encoded information on metasurfaces can be decrypted and be shown as optical signals only upon specific keys such as polarization, amplitude, wavelength, and incidnet angle of light [1]. Due to infinite degrees of freedoms of light, therfore, the metasurfaces have been proposed as a promising high security platform beyond previous security systems with a limited number of encryption. To enhance optical security, previous works demonstrated bifunctional metasurfaces that were fully decrypted by combination of multiple keys (e.g. polarization and wavelength) [2, 3]. Moreover, we reported active metasurface incorporated with liquid crystals (LCs) that can modulate optical keys [3, 4]. Based on this, we propose herein the enhanced security platform via active multi-band metasurface integrated with tunable heliconical LC modulators that can control reflection wavelength (Fig. 1a). The heliconical LCs can reflect specific wavelength via Bragg reflection due to pitch between internal quasi-layers [5]. The wavelegnth can be tuned from ultraviolet to infrared range by electric field (Fig. 1b) that changes the cone angle of LC molecules associated with the pitch. Therefore, metasurfaces with heliconical LC modulators are doubly encrypted by incident and reflection wavelength, which can be only decrypted when the both wavelength match. This method provides high security platform with active LC modulators. The heliconical LCs can be designed with various stimuli such as temperature and chiral dopants, thus we envisage that the proposed simple method can broaden security and anticounterfeiting platform.