**1. Introduction**

In recent years, silicon-based integrated photonic devices have been developing rapidly. In particular, integrated optical antenna arrays have broad application prospect in many fields, such as optical communication, light detection and ranging (LiDAR), vehicle autonomous driving, security monitoring, and display advertising [1–5]. Nanoantenna is a key part of the optical antenna array for converting guided light and free space light with specific directivity. Based on the light interference principle, beam steering is realized by controlling the phase of the light radiated by each nanoantenna in the optical antenna array. In order to realize optical phase control, the concept of optical phased array (OPA) is proposed [6–8]. In the field of silicon-based photonics, OPA is a highly integrated on-chip system, which consists of light division network, phase shifters, and optical antenna array [9–11]. In optical communication, OPA is required to have high gain, narrow beam, and wide steering range. However, at present, monolithic integrated OPAs suffer from low gain, small beam steering range, and wide beam width, which are mainly due to the low radiation efficiency of the nanoantenna, the large element spacing (the spacing between adjacent antenna in the antenna array), and the limited scale of the optical antenna array [12, 13].

The most commonly used silicon-based nanoantenna in the integrated optical antenna array is dielectric grating antenna. Generally, dielectric grating antenna refers to the periodic micro/nanostructure etched on dielectric substrate. The existing dielectric grating antenna suffers from large footprint and bidirectional radiation, which result in large element spacing and waste of radiation energy of the optical nanoantenna array [14–18]. In a uniform antenna array, the element spacing larger than the operating wavelength will lead to the appearance of grating lobes in the radiation pattern of the antenna array, which will limit the steering range of the optical nanoantenna array.

In order to obtain a miniaturized optical antenna with high radiation efficiency, plasmonic nanoantenna is proposed [19–22]. Plasmonic nanoantenna is composed of metal and dielectric. When electromagnetic wave impinges on the interface of metal and dielectric, it will couple and oscillate with the surface electrons of the metal, and surface plasmon polarization (SPP) is generated. When SPP is unable to transmit along the interface and is confined, the SPP is called localized surface plasmon (LSP). LSP can confine the electromagnetic wave into a space far less than a wavelength. Based on the LSP resonance effect, electromagnetic wave will be enhanced and radiated into free space by plasmonic antenna. Taking advantage of this character, plasmonic nanoantennas can realize a tiny size [22]. However, the traditional plasmonic nanoantennas [20, 22] are fed by plasmonic waveguides, in which the impedance matching band is narrow and high loss is introduced. In addition, the traditional plasmonic nanoantenna does not radiate light along the direction perpendicular to the plane where the antenna is located, which also limits the light steering range of the optical nanoantenna array [1, 2, 20, 22].

In this chapter, we propose a plasmonic nanoantenna with sub-wavelength footprint and high gain operating at the standard optical communication wavelength of 1550 nm, i.e., 193.5 THz [23]. The proposed plasmonic nanoantenna consists of a silver block and a silicon block, and its footprint is much smaller than that of dielectric grating antenna. Unlike recent studies on the plasmonic nanoantennas, an impedance matching between the proposed plasmonic nanoantenna and a silicon waveguide is achieved in a wide band. Light is fed from the bottom of the plasmonic nanoantennas by the silicon waveguide and is radiated vertically upward without bidirectional radiation. This kind of bottom fed plasmonic nanoantenna is suitable for the expansion of the nanoantenna array. Based on the proposed plasmonic nanoantenna, two plasmonic nanoantenna arrays including 1 × 8 and 8 × 8 arrays are designed. The radiation characteristics of the plasmonic nanoantenna arrays are simulated and discussed in detail.
