We propose and demonstrate a novel visual encryption device composed of higher-order birefringent elements. When an
optical material with higher-order birefringence is placed between a pair of polarizers and illuminated by white light, it
appears only white. In contrast, when it is illuminated by monochromatic light, the transmitted intensity varies depending
cosinusoidally on the wavelength. An array of such materials can express information (e.g., letters and/or images) by
controlling the birefringence of each pixel. If birefringence phase retardation can be adjusted for a specific wavelength,
the information will be clearly displayed when it is illuminated at this wavelength. We denote this wavelength a key
wavelength. The encryption device was fabricated by controlling the amount of higher-order birefringence to achieve
high contrast only by using polarized illumination at the key wavelength. Thus, the information stored in the encryption
device can be decoded only by illuminating it at the key wavelength.
To demonstrate the validity of this encryption principle, we constructed a 3 × 3 pixel device in which commercial retarder
films were laminated. The device was illuminated by a monochromatic light. When a readout experiment was performed
using the monochromatic light at the key wavelength, the stored letter was clearly visible. On the other hand, when pixel
brightness was randomly distributed with illumination at the wavelength other than the key wavelength, the letter could
not be recognized.
Furthermore, the stored information can be easily distributed to multiple physical keys that display arbitrary images. In
this case, the birefringence phase retardation is obtained by summing the values of retardation of each pixel of the
physical keys. In the experimental device, the observed image was decoded by superimposing the two images using
different physical keys.