Photonics is a technology that has rapidly gained recognition as one of the most important areas of modern development. Worldwide, this enterprise generates over 500 billion dollars of annual trade, which since 2015 has been accounting for employment figures of more than two million individuals. In funding circles, photonics is now widely accepted as a "key enabling domain," in which entirely new scientific and technological spheres of application are keenly anticipated. Indeed, it is often remarked that the present century will become - if it is not already - one in which light and photonics will supplant the previous role of electricity and electronics in modern society. It is entirely fitting that this should be so: the term "photonics" was originally introduced to the technical community by Eugen Sänger, with just such a vision in mind. Acceptance of the term was accelerated when, in 1982, the trade publication previously entitled Optical Spectra changed its name to Photonics Spectra; its consolidation in academic circles came with the 1991 publication of Saleh and Teich’s masterwork.

Today, photonics is a field that has opened up a host of new avenues for research and applications, to the extent that in a typical year there are numerous scientific and technical conferences taking place around the globe, including major conventions such as Photonics West. While scientific advances race on in fields including quantum optics, cavity photonics, nonlinear optics, plasmonics, and metamaterials, other areas of application are emerging in solid-state lighting and displays, optical interconnect technology, and electronic chips, alongside new platforms for quantum computing, bio-imaging, solar energy capture, and many more.

True to form, just as electronics has in the past led to microelectronics, so photonics now leads to microphotonics and nanophotonics. Topics such as these, once considered a fringe of the subject, have already become the core of modern photonics. This is not simply the result of a drive towards increasing miniaturization, pursued for its own ends; it also reflects the distinctive mechanisms that become operative at sub-micron scales. Almost all of the recent innovations involving photonics would have been impossible without the disruptive breakthroughs at micron and sub-micron scales. As miniature lasers and microfabrication methods have continually evolved, parallel growth in the optical fiber industry has helped spur the continued push towards the long-sought goal of total integration in optical devices.

So, with a broad spectrum of activity and progress typifying the whole sphere of photonics, one might ask where the nature of the photon itself fits. We know that light is made of photons. How important is it that light is not simply represented as a continuous sinusoidal waveform, as seen in most optics textbooks? Even some research areas headlined as "photonics" can generate accounts that pay scant heed to the quantum nature of light - it is not hard to find work, even in this subject, in which the word "photon" fails to appear. Nonetheless, some areas, such as the burgeoning field of quantum optics and informatics, hinge upon the discrete nature of photons: indeed, the sciences of color, spectroscopy, and photochemistry, and much of the technology of optical materials, depend on this simple premise. With an increasing number of phenomena that absolutely require full use of the photon concept, it is appropriate to look into the full meaning and significance of the photon.

The content of this book is now outlined. In Chapter 1, we begin with an overview of the history that eventually led to a photon-based understanding of optical phenomena, paving the way for the later development of the fully-fledged quantum theory of light. Chapter 2 then presents a broad perspective on the properties of the photon, and the associated characteristics of photon propagation both in free space and within dispersive and complex media. This is where links to modern photonics technology first begin to feature. Chapter 3 draws out some of the most striking quantum aspects of photon science, introducing a simple operator formalism that enables links to be shown with current developments in quantum communication, computation, and informatics.

Chapter 4 introduces and develops the modern field theory that underpins our whole understanding of light, relevant across the full range of optical and electrodynamic applications. Revisiting some of the simple concepts such as energy, linear momentum, and angular momentum in full quantum operator form provides a basis to understand the principles of not only conventional light, but also structured light such as optical vortices. The formal field theory also furnishes the feature known as vacuum fluctuations, whose nature and significance are far more pervasive than is commonly recognized, and whose effects have to be built into the design of nanoscale electromechanical systems.

Photonic aspects of light-matter interaction, encompassing both active and passive optical systems, are then addressed in Chapter 5. Here, it is not only the more distinctively photon-based features that prove to require the application of a quantum field treatment; this is because semi-classical descriptions fail to correctly describe even simple processes such as photoemission. The route to develop theory for arbitrary kinds of interaction is clearly spelled out. Coverage extends from linear to nonlinear optics, where links with specifically quantum features once again emerge. This chapter also addresses near-field interactions that arise in the vicinity of a material source and introduces specifically quantum-based opto-mechanical effects, such as those used in cold atom physics (again linking with the technology of platforms for quantum computation). We conclude with a brief summary and outlook.

Throughout the chapters that follow, we have been at pains to develop and deploy a representation of theory that is internally consistent across very different fields of optics - whose basis is to be found at the heart of an even more diverse range of modern technologies. We have aimed to deliver, in a slender volume, a treatment that is fresh, concise, expedient, and reasonably complete, showcasing a huge interconnectivity with a firm focus on the key concepts, principles, and mechanisms. One of the greatest challenges has been to keep to an accessible level of theory that is also sufficient for readers to follow the latest lines of research.

We have imposed on ourselves a strict limitation, resisting numerous inclinations to go further into individual topics. In the effort to present a very concise introduction to a subject as large as this, the selection of citations to primary sources is an invidious task, and we crave forgiveness from countless scientists whose pioneering studies are not explicitly cited. Undoubtedly, many readers will want to learn more about specific topics, and to this end we have included more than 250 references, supplemented by suggestions for further reading. While the responsibility for any errors or omissions is our own, we are truly grateful to colleagues and friends who have made time to read the draft and provide feedback. In particular, we thank Kayn Forbes, Jack Ford, Jesper Gluckstad, Roger Grinter, and Garth Jones.