Researchers who engineered the first optical rectenna in 2015 haven’t stopped there. They recently increased light-to-electric current conversion efficiency by two orders of magnitude, allowing the rectenna to generate millivolts of power from blue light input. This can be used to run low-power sensing equipment.
The device is able to accomplish the conversion to electric and capture thermal energy by coupling electromagnetic fields at optical frequencies to the antenna. Previous methods, which utilized the photoelectric effect as a solar panel, did not allow for the thermal energy capture.
Optical rectenna converting blue light to electricity. (Image Courtesy of Christopher Moore at Georgia Institute of Technology.)
The antenna itself functions as a half-wave rectifier. It is composed of multiwall carbon nanotubes that have a diode switch at one end. Initially, the switch is open as the light’s electromagnetic field generates oscillations of electrons in the antenna. When this alternating electron flow reaches a peak at the open end of the antenna, the diode switch closes and traps the electrons, then opens again so it can catch the next oscillation. The fluctuations in the electromagnetic field are thus transformed into a usable alternating current.
The first optical rectenna was inefficient and impractical due to the use of calcium metal at the junction between the antenna and diode. Because calcium deteriorates in air, the rectenna could only be created and operated in a vacuum. The latest update uses aluminum at the junction and a bilayer of alumina and hafnium dioxide to coat the carbon nanotubes themselves, allowing the rectenna to function in air for about a year and enhancing efficiency by increasing electron flow.
Baratunde Cola, right, believes that the design improvements his research team made to the optical rectenna can lead to applications in spaceflight. (Image courtesy of Christopher Moore at Georgia Institute of Technology.)
Baratunde Cola, associate professor at the Georgia Institute of Technology, believes the optical rectenna his research team developed will be used in two rapidly growing fields: the Internet of Things (IoT)and space travel. High-performance computing generates vast amounts of waste heat that the rectennas could partially recapture as usable energy. When it comes to space travel, the small electrical currents generated are perfect for powering the ion thrusters in spacecraft. With further improvements in efficiency, the optical rectenna could even be the technology that gets humanity to the surface of Mars.