In response of the need for increased and faster information processing in the near future, miniaturization of optical devices has progressed  to the point that it has now almost reached the critical limit determined by the diffraction of conventional propagating light [2, 3]. Since 1990s, researchers have anticipated that optical near-field devices may be one of the first important technologies to overcome this limit; many studies have been performed in various fields such as fundamental physics in nanometric space, optical near-field microscopy and spectroscopy, optical measurement, bioimaging, nanofabrication, and nanophotonic device architecture . An optical near field is the characteristic localized electromagnetic field around a nanometric object, and its decay length, which is smaller than the wavelength of incident light, depends on the size of the object. This size dependence means that optical near fields cannot be separated from matter excitation; in nanometrics pace, the incident electromagnetic field is modified by matter excitation in an object, and the modified field also affects the object itself and another neighboring one before releasing the energy as far-field photons. This nanometric light-matter interaction must describe as a self-consistent field. The goal is to create nanometric functional devices that are free from light diffraction limits, in which such optical near fields act as information carrier and control signals. These devices are termed nanophotonic devices. The localization feature of nanophotonic devices seems to resemble electronic devices in which an electric charge always stays within the device, but in a nanophotonic device, the localized field is able to leave an object and release photons in the far field via optical near-field interaction among several nanometric objects . An important component of nanophotonic devices and nanophotonic device operations is dealing with light-matter interaction with a nanometric system, as well as dissipation of matter excitation energy toward the outer field. Since the signal is eventually detected as far-field light, nanometric light-matter interaction also needs to control the dissipation process. Hence, the inherent operation of a nanophotonic device in nanophotonics differs from conventional optical and electronic devices. The advantages of nanophotonic devices include not only miniaturization but also possibilities for novel principles of functional operations that are inherent to nanophotonics. As mentioned above, the physics of nanophotonic devices includes typical matter excited states due to optical near-field interaction, coupling between near-and far-field light, and coupling between matter excitation and phonons. Many of these characteristics have not been considered in conventional optics; the structures within nanophotonic devices may differ from those of conventional devices, since basic principles utilized differ and nanophotonic devices can accomplish functions that have not been possible to date. It is important to consider how these devices should be designed, and to learn how nanophotonic devices can coexist with other devices. In this chapter, our discussion focuses on how to use the features inherent to nanophotonics in functional device operations, what is possible, and how we can realize the possibilities. Section 1.2 explains some characteristic features of nanophotonics and provides a basic outline of nanophotonic devices.
Sangu, S., Kobayashi, K., Shojiguchi, A., Kawazoe, T., & Ohtsu, M. (2006). Theory and principles of operation of nanophotonic functional devices. In Progress in Nano-Electro-Optics V: Nanophotonic Fabrications, Devices, Systems, and Their Theoretical Bases (Vol. 117, pp. 1–62). Springer Berlin Heidelberg. https://doi.org/10.1007/978-3-540-28681-3_1