161 Copyright © 2017, IGI Global. Copying or distributing in print or electronic forms without written permission of IGI Global is prohibited. Chapter 6 DOI: 10.4018/978-1-5225-1671-2.ch006 ABSTRACT The solar energy is able to supply humanity energy for almost another 1 billion years. Optical nano- antennas (ONAs) are an attractive technology for high efciency, and low-cost solar cells. These devices can be classifed to semiconductor nano-wires and metallic nano-antenna. Extensive studies have been carried out on ONAs to investigate their ability to harvest solar energy. Inspired by these studies, the scope of the chapter is to highlight the latest designs of the two main types of ONAs. The metallic nano- antennas are discussed based on the following points: plasmon, modeling, and performance of antenna designs using diferent confgurations and materials. Moreover, the semiconductor nano-wires are studied thoroughly in terms of photonic crystals, antenna design with diferent patterns, nano-wire forms and materials. Also, the applications of ONAs and their fabrication aspects such as diode challenges are presented in detail. Finally, three novel designs of ONAs are presented and numerically simulated to maximize the harvesting efciency. INTRODUCTION The search for alternative energy sources in the 21 st century is a growing academic and industrial pursuit. Rising costs of carbon-based fuels coupled with increased emissions has placed a greater demand on the clean energy sector. Recently, renewable energy gives strong contributions to power generation without increasing environmental pollutions. A promising energy source is photovoltaics (PVs), which has tra- ditionally been made from high purity, expensive crystalline silicon (c-Si). In addition, the PV solar cell devices are designed to absorb solar energy in the visible region (400 nm-700 nm) which constitutes 46% Optical Nano-Antennas for Energy Harvesting Salah Obayya Zewail City of Science and Technology, Egypt Nihal Fayez Fahmy Areed Zewail City of Science and Technology, Egypt & Mansoura University, Egypt Mohamed Farhat O. Hameed Zewail City of Science and Technology, Egypt & Mansoura University, Egypt Mohamed Hussein Abdelrazik Zewail City of Science and Technology, Egypt & Ain Shams University, Egypt