Introduction

Light passing in a small aperture has been the subject of intense scientific interest since the very first introduction of the concept of diffraction by Grimaldi in 1665. This interest is directly sustained by two facts: an aperture in an opaque screen is probably the simplest optical element, and its interaction with electromagnetic radiation leads to a wide range of physical phenomena. As the fundamental comprehension of electromagnetism as well as fabrication techniques evolved during the twentieth century, the interest turned towards apertures of subwavelength dimensions. Bethe gave the first theory of diffraction by an idealized subwavelength aperture in a thin perfect metal layer [17], predicting extremely small transmitted powers as the aperture diameter decreased far below the radiation wavelength. These predictions were refuted by the observation of the so-called extraordinary optical transmission phenomenon by Ebbesen and co-workers in 1998 [23], which in turn stimulated much fundamental research and technology development around subwavelength apertures and nano-optics over the last decade [65]. It is not the aim of this chapter to review the transmission of light through subwavelength apertures. Comprehensive reviews can be found in Refs. [880, 881]. Instead, this chapter will focus on subwavelength apertures to reversibly convert freely propagating optical radiation into localized energy, and tailor light–matter interaction at the nanoscale. This goes within the rapidly growing field of optical antennas [36, 202], which forms the core of this book.