Surfaces and interfaces of all kinds determine our primary sensory experience of the world we inhabit, and our perception of solidity is very largely based on an interpretation of surface data. In the mind of the scientist, a process of abstraction has led to the familiar modeling of surfaces as two-dimensional boundaries between different parts of otherwise three-dimensional systems. Indeed, such 'ideal' interfaces satisfactorily engage with simple macroscopic models to describe mechanical, acoustical or optical phenomena as they appear to human perception. Such connections work because, on the length scale of a meter (and a few orders of magnitude above or below), the various dynamical responses of material systems are dominated by the bulk properties of their components.
The reliability of this perspective has been significantly undermined by the advent of nanotechnology. As the reader will find in most introductory presentations, the prefix nano derives from the Greek for 'dwarf', and the new usage follows its earlier adoption in quantitative unit terminology to denote a factor 10-9 : indeed, nanotechnology deals with systems where the scale of typical lengths is from one up to some tens of nanometers (1nm = 10-9 meter). To see why this makes a difference to the relationship between surface and bulk properties, it is instructive to reflect on the deeper physical meaning of such a length scale, which corresponds to a typical length of a few atomic bonds.1 This means that in nanomaterials - materials whose dimensions are confined to the nanometer scale - atoms within the 'bulk' are never more than a few bond lengths away from an interface of some kind. Consequently it is no longer possible to consider any intrinsic property of such a material as independent from its boundary properties. Moreover, in some sense we could say that in shifting focus from the macro to the nano-world, we find materials shifting from primarily bulk-like to surface-like attributes, and it is their surface rather than bulk properties that determine system behavior.
There is a whole realm of fascinating phenomena associated with surfaces; although they afford stimulating opportunities to creative minds, their understanding it is by no means an easy endeavor: "God created all matter - but the surfaces are the work of the Devil" was Wolfgang Pauli's reported view. In this book, we touch on a number of recently developed concepts relating to the nanoscale optical and photonic properties of surfaces, without any attempt at completeness. Some of the ideas are still speculative, as in the case of Chapter 1 which addresses the interaction of light with moving interfaces. Such a concept is particularly intriguing in connection with photonic structures where light can be significantly slowed down - and where it is even possible to conceive superluminal effects. Chapter 2 steers the subject matter to topics of already proven practical application, the use of evanescent optical waves for sensing applications. Of course, the device implementation of any form of nanoscale response is ultimately subject to the development of efficient and inexpensive fabrication techniques, to achieve the necessary nanostructures. In Chapter 3, in the context of potential applications in biology, attention focuses on a case in point, porous silicon, which has captured much scientific attention over the last fifteen years and now seems well placed for significant commercial exploitation. A survey of integrated optoelectronic circuits in Chapter 4 puts us back in the context of one of the most relevant field of applications, where optics and nanotechnology are strongly pushing research and development. Metallic nanostructures are also of major interest, especially for phenomena related to surface plasmons (mobile charge oscillations at metallic-dielectric interfaces): Chapter 5 reviews the properties and applications of gold quantum dots. Finally, Chapter 6 concludes with another topic at the adventurous forefront of the subject, concerning the highly unusual optical properties of surfaces fabricated to develop a chiral topography.
We belive that the interplay of surface properties and optics will, in the years to come, more and more emerge as a broad but distinctive field of photonics, one that is rich both in its fundamental interest and its potential for creating major applications. We hope, with this book, to stimulate that interest and the creativity in our readers.
Zeno Gaburro, Trento
David L. Andrews, Norwich
|1|| The bond length scale is more appropriately located in the Angstrom range (1A = 0.1nm). |