# Nanophotonic Hierarchical Holograms: Demonstration of Hierarchical Applications Based on Nanophotonics

#### Abstract

Recently, Italy’s National Committee for Cultural Heritage found some microscopic codes in Mona Lisa’s pupils by using a magnifying glass (Lorenzi, 2010). Experts have pointed out that the codes may represent several messages, including the initials of Leonardo Da Vinci, "LV". On the other hand, how and why such microscopic messages were embedded in her pupils has not been revealed yet. The most interesting part of this other Da Vinci code is that, although techniques for microscopic fabrication and retrieval had not been generally established in the early 16th century, the concept of embedding secret messages in a macro-scale view already existed—or as we now say, “The best place to hide a leaf is in a forest”. Present-day techniques for realizing this concept involve the ideas of covertness and overtness. The former means not showing something openly, and the latter means the opposite. As Da Vinci showed 500 years ago in Mona Lisa’s pupils, microscopic optical techniques are suitable for embedding secret messages in a macro-scale optical observation, because the hierarchical structure inherent between different levels of the optical scale can be implemented simply, and the levels are functionally independent of each other. For instance, confidential information can be hidden in any of the physical attributes of light, such as phase, wavelength, spatial frequency, or polarization, so that one kind of anti-counterfeiting is represented (Javidi et al., 1994; Refregier et al., 1995; Rakuljic et al., 1992). Holography, which generates natural three-dimensional images consisting of a number of diffracted light beams, is one of the most common anti-counterfeiting optical techniques (Renesse et al., 1998). In the case of a volume hologram, the surface of the hologram is ingeniously designed into a complicated structure that diffracts incident light in specific directions. A number of diffracted light beams can form an arbitrary three-dimensional image. Because these structures are generally recognized as being difficult to duplicate,

### Cite this paper

@inproceedings{Tate2017NanophotonicHH, title={Nanophotonic Hierarchical Holograms: Demonstration of Hierarchical Applications Based on Nanophotonics}, author={Naoya Tate and Makoto Naruse and Takashi Yatsui and Tadashi Kawazoe and Morihisa Hoga and Yasuyuki Ohyagi and Motoichi Ohtsu}, year={2017} }