**4. Conclusion**

In this chapter, we review the silicon-based optical nanoantennas and their applications in OPA for beam steering. In order to obtain an OPA with high gain and wide beam steering range, we propose a sub-wavelength plasmonic nanoantenna with an operating wavelength of 1550 nm. The proposed plasmonic nanoantenna consists of a silver block and a silicon block with a standard silicon waveguide for feeding light into the nanoantenna. On the basis of LSPR, the plasmonic nanoantenna radiates light vertically upward with a high gain of 8.45 dB at 1550 nm. There is a good impedance match between the plasmonic nanoantenna and the silicon waveguide in a frequency range from 176.7 to 248.5 THz. Furthermore, two nanoantenna arrays (1 × 8 and 8 × 8) with the element spacing of 0.7λ0 composed of the proposed plasmonic nanoantennas are designed, and their beam steering radiation patterns are studied in detail. The simulation results show that the 1 × 8 array can be used to realize 1-D beam steering from −44.0° to +44.0° with a gain of 14.5 dB at 1550 nm, and the 8 × 8 array can achieve a 2-D beam steering from −44.0° to +44.0° in one dimension and from −45.0° to +45.0° in the other dimension with a gain of 24.2 dB at 1550 nm.

The plasmonic nanoantenna we proposed is a good candidate for the extension of the nanoantenna array used in a large-scale OPA. Utilizing the proposed plasmonic nanoantenna, a 3-D array extend mode can be adopted to form an OPA with thousands of optical nanoantennas. We first make a 1-D OPA as a sub-layer, in which the optical power division network, phase shifters, and a 1-D plasmonic nanoantenna array are integrated in a plane. After that, such 1-D OPA layers are extended longitudinally. Therefore, a highly integrated OPA containing thousands of optical nanoantennas with sub-wavelength element spacing can be obtained theoretically to steer beam in a wide angle without grating lobes. However, the processing of the OPA with multilayer structure is limited by our micro/nanofabrication technology. We believe that with the development of micro/nanoprocessing technology, the large-scale OPA will be applied in various fields of optical communication, LiDAR, security monitoring, and display advertising, which will bring great benefits to human life.
