The physics of the near-field

Over the last decade, extensive exploitation of the different kinds of near-fields existing spontaneously or artificially in immediate proximity to the surface of materials has generated a considerable amount of new exciting developments. In this review the main physical properties of these peculiar fields are revisited. In a first stage, following a unified pedagogical model, we recall that the concept of near-field is not restricted to specific research areas, but actually covers numerous domains of contemporary physics (electronics, photonics, interatomic forces, phononics, . . .). To a great extent, it will be shown that it mainly concerns phenomena involving evanescent fields (electronic density surface wave, evanescent light, local electrostatic and magnetic fields, . . .) or localized interatomic or molecular interactions. In fact, the practical exploitation of these waves and local interactions was latent for a long time in physics until the beginning of the 1980s which was marked by the emergence and the success of local probe-based methods (STM, SFM, SNOM). Nowadays, various theoretical approaches and powerful numerical methods well suited to near-field physics are described in the literature. In the second part of this review, different original aspects of the near-field will be discussed with the intent of realizing control and optimization of its properties. In particular, the physics hidden inside the inverse decay length parameter (eta) associated with all near-field concepts will be analysed in detail. This analysis may serve as a general framework for the design of physical or chemical compounds (photonic and electronic) able to control this fundamental parameter. We conclude the review by reconsidering an old and fundamental problem that can be summarized by the question, ʹWhat happens in the near-field interaction zone?ʹ. Actually, this problem has been largely unaddressed in the near-field literature because what is needed in most practical situations is just the transmission coefficient of the whole device. However, when some dissipative elements interact with the near-field, this reasoning appears to be somewhat limited. In order to get more insight into this challenging question, we briefly give a stateof-the-art review of the relation between tunnelling events and energy dissipation inside the near-field.