Abstract
Structural colour prints (SCPs) produce colour from the interaction of light with nanostructures, rather than from potentially harmful inks or dyes found in conventional prints. SCPs not only provide a sustainable option in printing, but also offer much higher pixel resolution (~ 100,000 dots per inch compared to ~ 1,000 dots per inch in conventional prints). As such, SCPs are advantageous for security applications where easy-to-verify image features and difficult-to-copy nanostructures are demanded to combat the counterfeiting of physical goods. However, most SCPs are only designed to work at a fixed angle (i.e. normal angle of incidence, sample rotation angle, and viewing angle), which limits their information capacity. Increasing this capacity will allow multiple sets of information to be encoded and individually decoded in a single SCP. To increase the information storage capacity in an SCP, previous studies have used optical polarization methods, but they have not effectively achieved multiple sets of information. This thesis hypothesizes that rotation and observation angle-dependent SCPs have much higher information capacity than their conventional counterparts. The thesis begins by examining how multiple sets of hidden colour information are encoded in an SCP by strategic design of diffraction gratings and control over their geometric parameters (height, periodicity, orientation, and angle of incidence). The thesis proceeds to examine how SCPs combined with microlens arrays (MLAs) produce an optical moiré effect that distinctly magnifies and reveals overlapping arrays of colourful micropatterns, based on the relative rotation angle between the SCP and MLA. The thesis then examines how another combination of SCP and MLA creates a “light field print” that displays different perspectives of a 3D image seen by a naked-eye observer from varying observation angles. To summarize, the rotation and observation angle-dependent SCPs reported in this thesis find their significance in: • Encoding up to 225 sets of information, instead of 1 or 2 sets of information in conventional SCPs. • Their combination with MLAs to produce special optical effects such as glasses-free 3D, not found in conventional SCPs. • Requiring only white light illumination, whereas specialized optics such as lasers and polarization filters used in previous works are not necessary. These attributes can enhance SCPs for visual authentication and security applications to protect physical goods against counterfeiting.