We have fabricated nanometer-scale gold dipole antennas designed to be resonant at optical frequencies. On resonance, strong field enhancement in the antenna feed gap leads to white-light supercontinuum generation. The antenna length at resonance is considerably shorter than one-half the wavelength of the incident light. This is in contradiction to classical antenna theory but in qualitative accordance with computer simulations that take into account the finite metallic conductivity at optical frequencies. Because optical antennas link propagating radiation and confined/enhanced optical fields, they should find applications in optical characterization, manipulation of nanostructures, and optical information processing.

Subwave length-resolution optical image recording is demonstrated by moving an extremely narrow aperture along a test object equipped with fine-line structures. Details of 25-nm size can be recognized using 488-nm radiation. The result indicates a resolving power of at least λ/20 which is to be compared with the values of λ/2.3 obtainable in conventional optical microscopy.

The use ofregistered names, trademarks, etc. in this publication does not imply, even in the ahsence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. VI Preface -photonie device applications, spatial hole-burning in lasers, distributed feedback, beam deflection, amplification, and modulation.A sizeable number of researchers now work in the field of dynamic gratings, and we would like to thank them for permission to reproduce their results. We tried to list as many original papers as possible, but the selection of the material is, of course, influenced by the personal view of the authors. We apologize if we did not consider all subjects which might be important to our colleagues.We finally want to express our sincere thanks to Prof. H. Gerritsen, Brown University, USA, Dr. K. Jarasivnas, Physical Faculty of Vilnivs, and Dr . S.Odulov, Ukrainian Academy of Science, USSR, for very helpful comments on an initial version of the manuscript. Thanks are also due to the IBM publication staff, Rüschlikon , to Mrs. C. Thiel for secretarial support, and to Mrs. A. Rapp ofSpringer-Verlag. Prof. T. Tamir, the series editor, and Dr . H. Lotsch of Springer-Verlag carefully revised the text and contributed new ideas.

The interaction of a single quantum dot with a bowtie antenna is demonstrated for visible light. The antenna is generated at the apex of a Si3N4 atomic force microscopy tip by focused ion beam milling. When scanned over the quantum dot, its photoluminescence is enhanced while its excited-state lifetime is decreased. Our observations demonstrate that the relaxation channels of a single quantum emitter can be controlled by coupling to an efficiently radiating metallic nanoantenna.

In this review we describe fundamentals of scanning near-field optical microscopy with aperture probes. After the discussion of instrumentation and probe fabrication, aspects of light propagation in metal-coated, tapered optical fibers are considered. This includes transmission properties and field distributions in the vicinity of subwavelength apertures. Furthermore, the near-field optical image formation mechanism is analyzed with special emphasis on potential sources of artifacts. To underline the prospects of the technique, selected applications including amplitude and phase contrast imaging, fluorescence imaging, and Raman spectroscopy, as well as near-field optical desorption, are presented. These examples demonstrate that scanning near-field optical microscopy is no longer an exotic method but has matured into a valuable tool.

The optical probe of a scanning near-field optical microscope is shown to act as a point source of surface plasmon (SP) polaritons on gold and silver films. Plasmon excitation manifests itself by emission of light in the direction of the SP resonance angle, originating from an area with the shape of a dipole radiation pattern whose extension is given by the SP decay length. Interaction with selected, individual surface inhomogeneities gives rise to characteristic modifications of the emitted radiation, which provide detailed information about SP scattering, reflection, and interference phenomena. [S0031-9007(96)01079-4]

Near-field optical (NFO) microscopes with an auxiliary gap width regulation (shear force, tunneling) may produce images that represent the path of the probe rather than optical properties of the sample. Experimental and theoretical evidence leads us to the conclusion that many NFO results reported in the past might have been affected or even dominated by the resulting artifact. The specifications derived from such results for the different types of NFO microscopes used therefore warrant reexamination. We show that the resolving power of aperture NFO microscopes, 30–50 nm, is of genuine NFO origin but can be heavily obscured by the artifact.

Near-field optical-scanning (NFOS) microscopy or ‘‘optical stethoscopy’’ provides images with resolution in the 20-nm range, i.e., a very small fraction of an optical wavelength. Scan images of metal films with fine structures presented in this paper convincingly demonstrate this resolution capability. Design of an NFOS microscope with tunnel distance regulation, its theoretical background, application potential, and limitations are discussed.