The interaction of light with matter is a central topic in both fundamental science and applied technology. At the heart of this interaction lies the absorption or emission of a photon by an electronic transition in for example an atom, molecule or semiconductor. Because such quantum emitters are generally much smaller than the wavelength of light, they interact slowly and omnidirectionally with light, limiting their absorption and emission. At radio frequencies similar issues were encountered and addressed long ago. Electrical circuits radiate little because they are much smaller than the corresponding wavelength. To enable wireless communication, they are connected to antennas that have dimensions in the order of the wavelength. These antennas are designed to effectively convert electrical signals into radiation and vice versa. The same concept can be applied in optics. The central idea of this thesis is that the interaction of a quantum emitter with light can be improved by near-field coupling it to the resonant plasmon modes of a metal nano-particle, which then acts as an optical antenna. In this way, excitation and emission rates can be enhanced, and the angular, polarization, and spectral dependence controlled. Chapter 1 of this thesis outlines these concepts and introduces optical antennas for single emitters. The experimental demonstration of optical antennas requires the near-field coupling of a single emitter to a resonant optical antenna. We fabricated optical monopole antennas on scanning probes, so that they can be precisely positioned near single fluorescent molecules. In this way we directly mapped the changes in the excitation and emission of a single quantum emitter as it is scanned near the antenna. Chapter 2 presents the results for the excitation part of the interaction. The enhanced excitation field at the antenna is highly confined (within 25 nm); the emitter only interacts with the antenna mode over this short distance. The antenna resonances were probed directly in the near-field and show that the antenna is indeed an optical analog of a monopole antenna. The experiments in Chapter 3 demonstrate how the antenna controls the emission. If the emitter is placed at the right position and if the antenna is tuned to resonance, the angular emission of the coupled system is determined by the antenna mode, regardless of the orientation of the emitter. In Chapter 4, we exploit that fact. We demonstrate, theoretically and experimentally, that the radiation from a single emitter coupled to a multi-element optical Yagi-Uda antenna is highly directed. We show that by reciprocity such a high directivity both enhances the excitation field and the collection efficiency. An intuitive way to understand optical antennas is as cavities for surface plasmon polaritons. In chapter 5, I present an extended description of the interaction of dipolar emitters with radiation through nano-rod antenna modes, by treating the antenna as a cavity. The results demonstrate how the properties of the antenna modes evolve from macroscopic perfectly conducting antennas to nanoscale plasmonic antennas, and highlight the similarities and differences between optical and conventional antennas. The results presented in this thesis show that optical antennas provide a new way to link single emitters to light. By designing the antenna the absorption and emission properties of the emitter can be tailored. More generally, optical antennas enhance and control light-matter interaction on the nano-scale, making them promising tools for applications in topics as diverse as high resolution near-field scanning optical microscopy, non-linear optics and spectroscopy, and photovoltaic devices.

This consistent and systematic review of recent advances in optical antenna theory and practice brings together leading experts in the fields of electrical engineering, nano-optics and nano-photonics, physical chemistry and nanofabrication. Fundamental concepts and functionalities relevant to optical antennas are explained, together with key principles for optical antenna modelling, design and characterisation. Recognising the tremendous potential of this technology, practical applications are also outlined. Presenting a clear translation of the concepts of radio antenna design, near-field optics and field-enhanced spectroscopy into optical antennas, this interdisciplinary book is an indispensable resource for researchers and graduate students in engineering, optics and photonics, physics and chemistry.

"Optical antennas are analogs of their radiowave and microwave counterparts, and can be defined as devices that serve to efficiently convert free-propagating optical radiation to localized energy, and vice-versa. Colloidal metal nanoparticles with their strong plasmonic optical response offer a convenient realization of optical antennas. Such nanoparticle antennas serve to spatially enhance and localize fields, and modify the excitation rate and the radiative decay rate when placed close to single emitters (molecules, quantum dots, etc.). In addition, they can also cause undesirable losses, leading to an increase in the non-radiative decay rates of these emitters. This interplay of rates can lead to a strong modification of the emission characteristics over the intrinsic behavior. We study photoemission from single emitters coupled to antennas of different geometries made from colloidal metal nanoparticles. We demonstrate enhancements of fluorescence from single quantum emitters by a factor 10 to 100, with the highest enhancements resulting for molecules with very low intrinsic quantum yields. Such enhancements afford an improvement in resolution for fluorescence imaging down to [lambda]/40. We also investigate changes to fluorescence blinking of a colloidal quantum dots (QD) coupled to an antenna, as a function of antenna-QD distance. We find that power-law blinking is preserved unaltered even as the antenna drastically modifies the excitonic decay rate in the QD, and reduces the blinking probability. This resilience of the power-law to change provides evidence that blinking statistics are not swayed by environment-induced variations in kinetics, and offers clues towards identifying the as-yet unknown mechanism behind universal fluorescence intermittency. Finally, in analogy with traditional electromagnetic antennas, we excite prototypical optical antennas using electrons (current) instead of photons (fields). We excite localized plasmons using low energy tunneling electrons, which are then converted to propagating far-field photons. We demonstrate electrically excited photon emission from a smooth gold film, an extended gold nanowire and from isolated gold nanoparticles. We thus show that electron tunneling provides a non-optical, voltage-controlled and low-energy pathway for photoemission on the nanoscale"--Page vi-vii.

The emission of light is at the heart of both fundamental science and technological applications. At its origin lie electronic transitions in nanoscale materials such as molecules, atoms and semiconductors. The interaction of light with such single quantum emitters is inefficient because of their point-like character. Efficient interfaces between light and nanoscale matter are therefore necessary. Inspired by the effective communication between small electronic circuits enabled by radio-frequency antenna technology, an emitter can be addressed efficiently with a nanoantenna, an optical element that converts localized energy into propagating radiation. The control of light emission with such optical antennas is the topic of this Thesis. By coupling an emitter to a metal antenna, the emission properties are determined by the antenna mode in direction, transition rates, polarization, and spectrum. In Chapter 1, we set out the basic concepts of optical antenna theory. To couple an emitter to an antenna, it must be within its near field. In Chapter 2, we introduce a nanofabrication method to place quantum dots on metal nanostructures with high spatial accuracy. The resulting emitter-antenna systems are imaged by confocal microscopy and their angular radiation patterns directly recorded. This combination of experimental methods allows us to study any optical antenna. A metal wire is the canonical antenna design and the basis to understand and construct other optical antennas. Through selective coupling of a quantum dot to the resonant modes of a nanowire, we demonstrate in Chapter 3 that the emission of a dipolar source can be converted controllably into higher multipolar radiation. We describe the antenna as a standing-wave resonator for plasmons and reproduce its emission with a multipolar expansion. An aperture in a metal film can be regarded as the complementary structure of a wire. In Chapter 4, we address the emission of light through a rectangular nanoaperture as an antenna problem. We demonstrate, explicitly, that resonant nanoslot antennas display a magnetic dipole response. Such antennas offer an efficient interface between emitters and surface plasmons. The excitation or detection of a dipolar emitter from the far field involves large solid angles. To address quantum emitters efficiently, a low divergence of their radiation patterns is needed. To this end, in Chapter 5 we develop and realize unidirectional optical antennas. We show how the emission of a quantum emitter is directed by multi-element Yagi-Uda and log-periodic optical antennas and demonstrate directional operation of a single-element design based on a splitring resonator. Light emission usually occurs through electric dipole transitions because multipolar emission rates are orders of magnitude slower. In some materials, however, multipolar optical transitions do occur. In Chapter 6, we assess through simulations the feasibility of enhancing magnetic dipole and electric quadrupole transitions with several realistic nanoantenna designs. The results in this Thesis demonstrate the potential of optical antennas as elements to control light on the nanoscale, based on radio and microwave antenna engineering. Within this powerful paradigm, the interaction of light with nanoscale matter can be tailored with complete flexibility. Such a degree of control over light emission and absorption may have a practical impact in spectroscopy, sensing, display technologies, lighting, photovoltaics, and general optical and optoelectronic devices.

The linear and nonlinear resonance behaviour of optical antennas (metallic nanostructures showing resonance behaviour at optical frequencies) made of gold and aluminum using electron-beam lithography is investigated. Specifically, it is of interest how the emission behaviour is changed by the coupling of two antenna arms via a small gap. Experimental techniques applied include dark-field spectroscopy and two-photon luminescence.

This book explains how to design, analyse and test cylindrical antenna arrays from a practical engineering standpoint. Written by three of the leading engineers in the field, this book is destined to become established as the basic reference in the field for many years to come.