**2. Hierarchical nanophotonic system**

#### **2.1 Nanophotonics**

Nanophotonics is a novel technology that utilizes the optical near-field, the electromagnetic field that mediates the interactions between closely spaced nanometric matter (Ohtsu et al., 2008). As shown in Fig. 1(a), optical near-fields are the elementary surface excitations on nanometric particles, which are induced by incident propagating light. By exploiting optical near-field interactions, nanophotonics has broken through the integration density restrictions imposed on conventional optical devices by the diffraction limit of light (Fig. 1(b)). This higher integration density has enabled realization of *quantitative* innovations in photonic devices and optical fabrication technologies (Nishida et al., 2007; Ozbay et al., 2006). Moreover, *qualitative* innovations have been accomplished by utilizing novel functions and phenomena made possible by optical near-field interactions that are otherwise unachievable with conventional propagating light (Ohtsu et al., 2008).

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

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

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

Nanophotonics is a novel technology that utilizes the optical near-field, the electromagnetic field that mediates the interactions between closely spaced nanometric matter (Ohtsu et al., 2008). As shown in Fig. 1(a), optical near-fields are the elementary surface excitations on nanometric particles, which are induced by incident propagating light. By exploiting optical near-field interactions, nanophotonics has broken through the integration density restrictions imposed on conventional optical devices by the diffraction limit of light (Fig. 1(b)). This higher integration density has enabled realization of *quantitative* innovations in photonic devices and optical fabrication technologies (Nishida et al., 2007; Ozbay et al., 2006). Moreover, *qualitative* innovations have been accomplished by utilizing novel functions and phenomena made possible by optical near-field interactions that are otherwise

utilized, yet allowing novel functionalities to be added to the hologram.

unachievable with conventional propagating light (Ohtsu et al., 2008).

**2. Hierarchical nanophotonic system** 

appearance is quite difficult.

diffraction efficiency.

**2.1 Nanophotonics** 

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* innovations can be achieved.

### **2.2 Hierarchy based on nanophotonics**

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

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 is mass-producable.
