Figure 7: Rectenna equivalent circuit.
The word “rectenna” is the result of joining “rectifying circuit” and “antenna”. The rectenna is a passive element with a diode that can receive and rectify MID- IR power to DC, it can operate without any power source. The first block are the nanoantennas. When an antenna is excited into a resonance mode, it induces a cyclic plasma movement of free electrons in the metal antenna. The electrons freely flow along the antenna, generating alternating current at the same frequency as the resonance. Subsequently the AC current will be rectified to energize the load. Wideband Nanoantennas tuned in MID-IR, should be able: to concentrate the incoming radiation, a wide acceptance angle and to be able efficiently capture all polarizations of light. Several wideband antennas (spiral, log periodic, bowtie) have been investigated in [].The matching between the antenna and the rectifier is provided by low-pass filter. Its aim is suppressing possible unwanted higher harmonics rejected by the nonlinear diode. These could come back to the nanoantenna resulting in power losses. The diode is the most important device to achieve high-energy conversion efficiency. These types of rectifier should be coplanar and coupled to the nanoantenna. The requirements for an ultra-high speed diode used to obtain a DC signal are responsivity, small size, small turn on voltage and efficient performance at MID-IR frequency. Impedance matching between the rectenna system and the load is required to transfer the maximum power to load. This is possible when the load impedance is the conjugation of the rectenna impedance. Finally, the last block DC pass filter between diode and load provides a DC path to the load by separating the high frequency components from the DC signal.
4.2 Efficiency limits
The energy conversion efficiency of a rectenna structure essentially it depends on two elements, the nanoantenna and the diode which implies nanoantenna efficiency, impedance matching between nanoantenna and rectifying element and the coupling efficiency of the rectifier to the load. Therefore, the product of the efficiencies of each step determines the rectification efficiency (η) of a Rectenna as follows: