is the efficiency with which power received by the diode is rectified.
To be able to receive a signal, the antenna has to be of the same length scale of the wavelength of the incoming wave. At IR frequency, wavelengths are of micrometer, which indicate the necessity of building the antenna, and rectifier in this length scale range. Shockley and Queisser in 1961 put an upper boundary on the efficiency at around 30% for single-junction solar cells and 55% for multi-junction solar cells. Energy conversion efficiency limit for a rectenna with an ideal rectifying element is only limited by the Landsberg limit of 93,3%, which is a strictly thermodynamic radiation conversion limit. Table 7 shows the comparison between Solar cells and Plasmonics.
Solar Cell: | Rectenna: |
Narrow band Can be improved by multi junction cells. | Capture the EM waves in broadband antennas: • Operates all day and night. |
Operate in daylight only | The technology far less expensive than photovoltaics |
Expensive | Very high efficiencies with full wave rectification (> 80%). |
Low efficiency: • Typical efficiency of commercial arrays: 15-20%. • Maximum efficiency in research labs: 50% | Absorption at all frequencies. |
Low absorption at low frequencies. | Omnidirectional. |
Sensitive to direction of incident light. | |
Table 7: Comparison between Solar Cells and Rectenna
4.3 Metals behavior at infrared frequencies and Drude Model
The metal type plays an important role in determining the efficiency and losses in the nanoantenna. At infrared and optical frequencies, the permittivity becomes complex and frequency dependent. In this section the relative permittivity response of some metals such as Gold (Au), Silver (Ag), Copper (Cu), Aluminum (Al) and Nickel (Ni) are investigated at MID-IR region of electromagnetic. At high wave frequencies, the behavior of metal is between perfect metal and perfect dielectric. The metal contains electromagnetic field like the dielectric, and losses like the conductor. Fields and currents can exist inside the metal volume in the infrared frequencies. Consequently, the selection of the metal type used for realizing nantennas is much more critical than the case of microwave antennas. We use to describe the electrical conduction properties of materials (particularly metals) with the Drude Model. In the Drude model, metals are characterized by a cloud of free electrons that are not bound to a particular atomic nucleus but are free to move about within the metal lattice. The model also includes frictional damping that describes the resistance to movement felt by the electrons. The frequency dependent complex permittivity and the electrical conductivity of metal in the terahertz frequency range is described using the Drude model given by [8]