Kaleidoscopic stress-colored birefringent metamaterial for digital multiplexed encryption

Optical metamaterials that can interact with light in a sophisticated way are in increasing demand for multiplexed optical encryption. Current designs rely on creating delicate micro-structures of specific materials, which require complex device fabrication. Here, we devise a mechanism to fabricate birefringent metamaterials with their optical functions arising from spatially programmable stresses in common photosensitive resins. Typically, volume shrinkage occurs during the photocuring, which leads to random, undesirable, yet inevitable internal stress. In contrast, our approach digitally manipulates the stress magnitude/direction via sequential patterned exposure, converting pixelated shrinkage stress into engineered optical anisotropy. Consequently, multiple distinct birefringent colored patterns can be created on a single material. These color patterns and the associated information are invisible under regular light but can be independently decoded under a rotating polarization field, similar to a kaleidoscope. Our birefringent metamaterials are simple and fast to produce yet exhibit unusual versatility due to the digital light fabrication.