An efficient and compact coupler—a device that matches a microwaveguide and a nanowaveguide—is an essential component for practical applications of nanophotonic systems. The number of coupling approaches has been rapidly increasing in the past ten years with the help of plasmonic structures and metamaterials. In this paper we overview recent as well as common solutions for nanocoupling. More specifically we consider the physical principles of operation of the devices based on a tapered waveguide section, a direct coupler, a lens, and a scatterer and support them with a number of examples. 1. Introduction Photonic components have advantages comparing to the electronic ones. Infrared and optical frequencies 1014-1015？Hz provide much broader operational bandwidth than the fastest electronic circuits. The losses in optical waveguides are smaller than in metallic wires. This is why, as D. Miller wrote, “the optical interconnects are progressively replacing wires” [1]. To achieve larger functionality on an integrated optical chip the optical components have to be miniaturized. A natural limitation, however, comes into play: the diffraction limit claims that we cannot focus light in a spot less than a half of the wavelength. The transverse size of conventional dielectric waveguides (e.g., silicon waveguides) is also limited to a half of the wavelength. Only employment of metals allows to overcome the diffraction limit and to confine a wave to a smaller area, very often at the cost of increased propagation losses. Nevertheless, the problem is not only to create efficient waveguides that provide subwavelength mode confinement, but also to make an efficient interface between free space or an optical fiber and a subwavelength nanowaveguide, that is, to focus light and launch it efficiently into the waveguide. The artistic view of the situation is depicted in Figure 1. Trying to pour water from a big bowl into a bottle with a narrow bottleneck, one would waste a lot. However, usage of a funnel simplifies the task and increases the efficiency significantly. An optical coupler plays the role of a funnel for light. Figure 1: An artistic view of the problem of coupling light from a wide microscopic fiber to a nanoscopic waveguide. Employment of a coupler, which is represented by a funnel on the figure, minimizes the losses and simplifies optical alignment. The problem of optical coupling originates from the pronounced modal mismatch between an optical fiber (a conventional single-mode telecommunication fiber has the core of 8？μm in diameter) and a nanosized waveguide,
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