**4. Nanoantennas**

Mirrors and lenses are usually utilized to control light propagation. However, they are unable to concentrate the light in a tiny area (smaller than λ/2), whereas antennas can easily confine the electromagnetic wave in subwavelength (beyond the diffraction limit). The urgent need to localize the light beyond the diffraction limit has motivated the researchers and helped toward the development of nanoantennas. With the rapid growth of nanotechnology techniques, scientists are now able to fabricate nanoantennas in the order of 10 nm using E-beam lithography [14, 15]. The dimensions of nanoantenna must be in the order of the incident light wavelength to ensure efficient performance. Light/matter interaction has been exploited extensively in many applications such as photovoltaics, microscopy, and THz sensing.

The main role that nanoantenna plays in solar rectennas is to receive external fields and confine the energy at its feed gap to be rectified by a nanodiode. The technological advances in the development of a new generation of nanodiodes such as point-contact diodes have contributed significantly to the emergence of solar rectennas in its modern form [16]. **Figure 3** demonstrates numerous fabricated nanoantennas for various applications.

The performance of nanoantennas in solar rectennas is measured by their ability to efficiently concentrate the received solar energy at the feed gap of the antenna.

**183**

**Figure 6.**

*Bowtie nanoarray configuration [19].*

**Figure 5.**

*Solar Rectennas: Analysis and Design*

field at the antenna's gap [18].

*DOI: http://dx.doi.org/10.5772/intechopen.89216*

The electric field generated at the feed gap varies from one type of antenna to another depending on the characteristics of the antenna itself. Thus, the confined electric field can be enhanced by choosing the proper antenna type for this application or by gathering a number of antennas in one rectenna system forming an antenna array. A comparison between different types of nanoantennas is presented in **Figures 4** and **5**, where the figure of merit is the value of the received electric

Another way to increase the captured electric field is to arrange several antenna elements in an array form. **Figure 6** shows an eight-element bowtie nanoarray as suggested in [19], where the concentration in the feed gap is also illustrated, while **Figure 7** shows the variation of the electric field with increasing the wavelength. The nanoarray

exhibits multiple resonances with maximum capturing at longer wavelengths.

*Electric field variation versus wavelength for different nanoantennas [17].*

**Figure 4.**

*Concentration of the electric field at the feed gap of different nanoantennas [17].*

#### *Solar Rectennas: Analysis and Design DOI: http://dx.doi.org/10.5772/intechopen.89216*

*Recent Wireless Power Transfer Technologies*

nanoantennas for various applications.

diffraction limit). The urgent need to localize the light beyond the diffraction limit has motivated the researchers and helped toward the development of nanoantennas. With the rapid growth of nanotechnology techniques, scientists are now able to fabricate nanoantennas in the order of 10 nm using E-beam lithography [14, 15]. The dimensions of nanoantenna must be in the order of the incident light wavelength to ensure efficient performance. Light/matter interaction has been exploited extensively in many applications such as photovoltaics, microscopy, and THz sensing. The main role that nanoantenna plays in solar rectennas is to receive external fields and confine the energy at its feed gap to be rectified by a nanodiode. The technological advances in the development of a new generation of nanodiodes such as point-contact diodes have contributed significantly to the emergence of solar rectennas in its modern form [16]. **Figure 3** demonstrates numerous fabricated

The performance of nanoantennas in solar rectennas is measured by their ability to efficiently concentrate the received solar energy at the feed gap of the antenna.

**182**

**Figure 4.**

**Figure 3.**

*Concentration of the electric field at the feed gap of different nanoantennas [17].*

*Fabricated nanoantennas: (a) dipole, (b) bowtie, (c) log-periodic, and (d) spiral.*

The electric field generated at the feed gap varies from one type of antenna to another depending on the characteristics of the antenna itself. Thus, the confined electric field can be enhanced by choosing the proper antenna type for this application or by gathering a number of antennas in one rectenna system forming an antenna array. A comparison between different types of nanoantennas is presented in **Figures 4** and **5**, where the figure of merit is the value of the received electric field at the antenna's gap [18].

Another way to increase the captured electric field is to arrange several antenna elements in an array form. **Figure 6** shows an eight-element bowtie nanoarray as suggested in [19], where the concentration in the feed gap is also illustrated, while **Figure 7** shows the variation of the electric field with increasing the wavelength. The nanoarray exhibits multiple resonances with maximum capturing at longer wavelengths.

**Figure 5.** *Electric field variation versus wavelength for different nanoantennas [17].*

**Figure 6.** *Bowtie nanoarray configuration [19].*

**Figure 7.**

*Electric field variation with wavelength for bowtie nanoarray and single bowtie of the same footprint area [19].*
