**3. Basics of solar rectennas**

*Recent Wireless Power Transfer Technologies*

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**Figure 1.**

performance.

*The first optical rectenna proposed by mark [8].*

that consists of nickel-nickel oxide-nickel diode is used to convert terahertz fields into electrical current. Furthermore, other research studies [13] are interested to study the impacts of geometrical parameters on the antenna

After that, there was a significant interest by researchers to study nanoantennas coupled to MIM diode for solar power-harvesting applications or THz sensing,

which cannot be covered here due to space limitations.

The structure and the operation theory of nanoantennas have been presented in this section. The same as the response of the conventional RF antenna to the electromagnetic wave, nanoantenna responds to the visible light and IR. Induced AC current, which is formed on the surface of the antenna, interacts with the incident wave and oscillates with it in the same frequency. The presence of a feeding gap in the antenna can help to collect the solar power, and then DC power is produced by rectifying the oscillated AC current with the aid of a specific diode-based rectifier.

Based on the theory of boundary conditions, the tangential electric field vanishes on the antenna surface and is equal to zero (Et = 0). This is fundamental to the traditional RF antenna, where metals are considered to have ideal electrical conductivity. In other words, Es = −Ei, where Es and Ei are scattered electric and incident electric fields, respectively.

In contrast, the operation of nanoscale antennas is based on the optical and IR regimes. In this case, metals are considered to be non-ideal conductors since they exhibit lower conductivity. Thus, the expression Et has to be taken into account. This expression can be presented by multiplying the value of surface impedance by the value of the surface current.

**Figure 2** shows the block diagram of a typical optical rectenna, in which the solar antenna receives the electromagnetic wave within a proper frequency band to deliver it to the low-pass filter (LPF) [8]. The latter, which is placed between the antenna and diode (rectifier), is used to prevent the reradiation of the higher harmonics that generated from the rectification process by the nonlinear diode. Generally, power losses result in from this reradiation.

Furthermore, the LPF matches the impedance between the antenna and the subsequent circuitry. The DC LPF smoothly delivers the rectified signal to DC and then passes it to the external load. In general, MIM diode is considered being the most common rectifier in the solar rectenna system; based on the electron tunneling process, the rectification is generally occurring through the insulator layer.

**Figure 2.** *Block diagram of optical rectenna [8].*
