Optical Nano-Antennas for Energy Harvesting

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 traditionally 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% of the solar spectrum (Malinovska, 2010). Therefore, the search for cheaper, high-performance materials for solar cells based applications is mandatory.

One of the alternative approaches to PV technology is the optical antennas that can harvest more solar energy by extending the absorbed range to the infrared (IR) region (Malinovska, 2010). The optical antenna can be fabricated by using metallic nano-particles or semiconductor nano-wires. Therefore, the optical antenna can be classified to metallic nano-antenna (rectenna) and semiconductor nano-antenna. The use of rectenna for energy harvesting depends on the fact that when an electromagnetic wave is incident on a nano-antenna, a time varying current will be induced on the antenna surface, and hence a voltage will be generated at the feeding point of the antenna. The generated wave will oscillate at the frequency of the incident wave. Consequently, in order to obtain Direct Current (DC) power, a suitable rectifier should be embedded at the feed point of antenna. These types of energy harvesting systems are called “rectennas”; which basically consist of antennas connected to a rectifier that converts the received signal to DC power and produces electricity (Malinovska 2010; Berland et al. 2001). An infrared rectenna structures based on metal–insulator–metal (MIM) diodes between dipoles (Berland et al., 2003) has been designed for operation at the operating wavelength λ=10 μm. However the efficiency of this system was less than 1% (Berland et al., 2003). A spiral nano-antenna for solar energy harvesting has been designed and fabricated at mid-infrared region by Kotter et al. (2010). In addition, Midrio et al. (2011) have designed and analyzed mono pole antenna based on nickel for the reception of thermal radiation. Vandenbosch et al. (2012) have introduced upper bounds for the solar energy harvesting efficiency of metallic nano-antennas. In this regard the silver exhibited the highest efficiencies, both in free space and on top of glass (SiO₂) substrate, with radiation efficiencies near or slightly above 90% (Vandenbosch et al., 2012). Although, the metallic nano-antenna offers outstanding harvesting efficiency over a wide frequency range, no suitable rectifier diodes are described in the literature to meet the requirements for solar energy harvesting.

Semiconductor nano-antennas have recently gained tremendous interest due to their light absorption (Hong et al., 2013). Light absorption in standing semiconductor nanowires is a complex phenomenon, with strong dependence on nano-wire dimensions, lattice arrangement and absorption coefficient of the raw materials (Hong et al., 2013). Ordered semiconductor nano-antenna structure can be fabricated through a few unattractive methods, such as lithography and polystyrene ball assembly, due to their high cost and complexity (Hong et al., 2013).