**1. Introduction**

340 Holograms – Recording Materials and Applications

Yang H-G.; & Kim E-S. (1996). Practical Image Encryption Sscheme by Real-Valued Data,"

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,

<sup>\*</sup> Makoto Naruse1,2, Takashi Yatsui1, Tadashi Kawazoe1, Morihisa Hoga3, Yasuyuki Ohyagi3,

Yoko Sekine3, Tokuhiro Fukuyama3, Mitsuru Kitamura3 and Motoichi Ohtsu1

*<sup>1</sup>The University of Tokyo,Japan* 

*<sup>2</sup>National Institute of Information and Communications Technology, Japan* 

*<sup>3</sup>Dai Nippon Printing Co. Ltd., Japan* 

Nanophotonic Hierarchical Holograms:

innovations can be achieved.

is mass-producable.

**3.1 Concept** 

**2.2 Hierarchy based on nanophotonics** 

device in the form of a *nanophotonic hierarchical system* (Fig. 2).

**3. Nanophotonic hierarchical hologram** 

Demonstration of Hierarchical Applications Based on Nanophotonics 343

Fig. 1. (a) Generation of optical near-fields, and (b) development of nanophotonics. Because optical near-fields do not involve the optical diffraction limit and they exhibit characteristic features that depend on direct interaction with materials, both *quantitative* and *qualitative*

Hierarchy in optical near-fields is one of the most appealing attributes for making innovative devices and systems based on nanophotonics. Naruse et al. investigated the hierarchy within the scale of optical near-fields, whose distribution is represented by a Yukawa function (Ohtsu et al., 2008), by investigating the size of materials and their associated optical near-fields (Naruse et al., 2005). The optical near-field response at a given scale is the result of interactions between the retrieval probe and the nanometric materials, and it is correlated with the materials involved at that scale. This feature has been exploited in various applications, for example, hierarchical optical memories where shape-engineered nanostructures provide twolayer responses in optical near-fields (Naruse et al., 2008). Moreover, besides the sizes of the materials, the shape, alignment, and composition are also important physical properties for engineering hierarchical systems. By suitable arrangement of such properties, several characteristic distributions of optical near-fields can be revealed (Naruse et al., 2008; Tate et al., APB2009; Tate et al., OptExp2009). These characteristics exert a large influence on the retrieval, and it means that various retrieval layers can be independently implemented in the same

From the point of view of the optical security, each layer is defined as an independent information layer. This enables a security layer structure in which nano-scale layers implement covertness and macro-scale layers implement overtness. The former is technically difficult to access and is non-duplicatable, whereas the latter is easy to access and

We can see the hierarchy of optical near-fields and far-fields because optical near-field interactions are distinguishable with propagating light. This characteristic feature has led to hierarchical optical system designs, such as *nanophotonic hierarchical holograms* (Tate et al., 2008), where independent functions are associated with both optical near- and far-fields in

the same device. Figure 3 shows the basic concept of the hierarchical hologram.

holograms have been widely used in the anti-counterfeiting of bills, credit cards, etc. However, conventional anti-counterfeiting methods based on the physical appearance of holograms are less than 100% secure (McGrew et al., 1990). Although they provide ease of authentication, adding another security feature without causing any deterioration in the appearance is quite difficult.

Many existing optical devices and systems, not just holography, operate based on the phenomena of *propagating* light. Therefore, their performance is generally limited by the diffraction of light (Zhdanov et al., 1998). The critical difficulty in improving the function of conventional holograms is that they are also bounded by the diffraction limit. However, with recent advances in nanophotonics, especially in systems utilizing optical near-field interactions, several optical devices and systems can be designed at densities beyond those conventionally constrained by the diffraction limit (Ohtsu et al., 2008). Because several physical parameters of propagating light are not affected by nanometric structures, the conventional optical responses in the optical far-field are not affected by these structures either. Essentially, this means that another functional layer in the optical near-field regime can be added to conventional optical devices and systems without any effect on their primary quality, such as reflectance, absorptance, refractive index, or diffraction efficiency.

Here, we propose a *nanophotonic hierarchical hologram* as a typical demonstration of this concept. The nanophotonic hierarchical hologram is a functionally improved version of a conventional hologram that works in both the optical far- and near-fields (Tate et al., 2008). Moreover, a *nanophotonic code*, which is physically a subwavelength-scale shape-engineered metal nanostructure, is embedded in the hierarchical hologram to implement a near-mode function (Tate et al., 2010). In this chapter, the basic concept of the nanophotonic hierarchical hologram with embedded nanophotonic codes and the fabrication of a sample device are described. In particular, since the proposed approach involves embedding a nanophotonic code *within* the patterns of the hologram, which is basically composed of one-dimensional grating structures, clear polarization dependence is found compared with the case where it is not embedded within a hologram or an arrayed structure. There are also other benefits with the proposed approach: a major benefit is that the existing industrial facilities and fabrication technologies that have been developed for conventional holograms can be fully utilized, yet allowing novel functionalities to be added to the hologram.
