A rectenna is a combination of an antenna and a rectifier and converts alternating current (AC) into direct current (DC). An optical rectenna converts electromagnetic fields at optical frequencies into electrical current.

In this work, by a team of researchers led by Baratunde Cola at the Georgia Institute of Technology, the antenna is an array of vertically aligned multiwalled carbon nanotube (MWCNT) diodes with open ends.

“We first made this device in 2015 and have now improved on it,” explains Cola. “We did this by sequentially depositing two different oxides (Al2O3 and HfO2) on the aligned CNT arrays and then deposited a top metal layer film at the tips of the array. We made electrical contact with leads connected to the metal films and the substrate on which we grew the CNTs.”

Efficient broadband electromagnetic absorbers

The devices work by exciting AC waves in the CNTs with light, heat or any energy in the form of electromagnetic waves, he tells nanotechweb.org. CNTs are efficient broadband electromagnetic absorbers and the arrays are particularly good at increasing the antenna’s light absorbing efficiency.

“Since the AC waves move on the scale of femtoseconds, the diodes must open and close on this timescale too if they are to act as switches. The only diodes that can do this are tunnelling diodes that work thanks to quantum mechanical tunnelling, so these are ones we employed.”

The double insulator structure helps optimize the efficiency of this tunnelling and when the AC electron wave produced in the CNTs reaches the diode junction, it opens and electrons pass through. Many of these electrons are then trapped as the wave moves back in the opposite direction as the junction quickly closes. This produces a positive unidirectional flow of electrons that generates current and voltage in the millivolt range, which is enough to do useful work in the circuit and operate low-power devices.

Powering IoT devices

Since the principle for converting heat into electricity is the same as that for light, Cola says that the rectenna could power IoT devices. This would be especially good if it were to harvest wasted heat (from industrial pipes and smoke stacks, for example).

“With the growth of IoT, the need for power sources is huge and our technology could harvest radio-frequency energy, heat and light, making it the most versatile choice for powering a connected world,” he adds.

The team, reporting its work in Advanced Electronic Materials DOI: 10.1002/aelm.201700446, says that it is now continuing to further optimise the rectennas’ performance and efficiency.