Responsive spiral photonic structures for visible vapor sensing, pattern transformation and encryption

Abstract

Two photon polymerisation using direct laser writing is a burgeoning field of research, with recent focus being placed on bringing added value to microstructures, by incorporating soft, responsive polymers. Moving to the micron-scale can have a profound impact on such stimuli-responsive materials, whose speed of actuation can be increased many-fold compared to their mm-scale counterparts. Here, the fabrication of submicron 2D photonic structures, based on a vapor-responsive photoresist with a refractive index <1.55, in the visible wavelength range. The fabricated concentric spiral arrays are evaluated for their feasibility as vapor sensors by testing spectral and structural color reproducibility and reversibility under water, ethanol, isopropanol, and acetone vapors. This approach allows for the realization of predictable uniform color displays that can be modulated upon stimuli response. The transmitted colour in the dry and hydrated states can be accurately modelled. This knowledge is used to design and demonstrate structures for cloaking and image transformation. Such capability can be used for encryption and anti-counterfeiting applications.