**4. Summary**

In this chapter, we described the basic concept of a nanophotonic hierarchical hologram and a nanophotonic code embedded in a hologram as an implementation of a hierarchical hologram. One of the most notable characteristics of our proposed approach is embedding a nanophotonic code within the patterns of a hologram composed of one-dimensional grid structures; it yields clear polarization dependence compared with an isolated nanophotonic code that is not embedded within a grid structure. These features were successfully demonstrated both numerically and experimentally.

In future research, the relation between the retrieved optical intensity distributions and the design of the nanometric structures may come to be understood, including their environmental conditions. Such insights should allow us to propose, for instance, an optimized strategy for implementing nanophotonic codes, or a strategy that is robust against errors that possibly occur in the fabrication and/or retrieval processes (Naruse et al., 2009).

From the point of view of practical use of these concepts in novel security devices, because embedding and retrieval of a nanophotonic code require highly advanced technical knowhow, this approach can also improve the strength of anti-counterfeiting measures. The number of layers can be increased in the near-mode observation to further extend the hierarchical function. The optical near-field interaction between multiple nanometric structures produces a characteristic spatial distribution depending on the size, alignment, etc. Therefore, various optical signal patterns can be observed depending on the size of the fiber probe, and another layer can be added in near-mode observation (Naruse et al., 2007; Naruse et al., 2005). A simpler method for achieving such a hierarchical function is required, without using optical fiber probe tips. The scale for such retrieval might be set at the submillimeter range, as represented by *optical far-field* in Fig. 3. These aspects are currently being investigated by the authors.

Finally, the concept of hierarchy based on nanophotonics may produce innovations not only in optical security but also for other optical applications, such as lenses and jewellery. Adding extra functions in this way creates value-added media with only a slight impact on the primary functions. However, a trade-off occurs between the nanometric fabrication conditions (e.g., size and pitch) and the impact on the primary functions. Adequate discussion is needed to investigate these trade-offs in each application for their practical use.
