The team are harvesting this heat by treating it as high-frequency electromagnetic waves. According to KAUST, using appropriately designed antennas, collected waves are sent to a rectifier that converts alternating signals to direct current charge for batteries or power devices.
KAUST says that putting these 'rectenna' designs into practice has proved challenging because infrared emissions have very small wavelengths and need micro- or nanoscale antennas that are not easy to fabricate or test. The infrared waves also oscillate thousands of times faster than a typical semiconductor can move electrons through its junction.
"There is no commercial diode in the world that can operate at such high frequency," explains Atif Shamim, project leader from KAUST. "That's why we turned to quantum tunneling."
Tunneling devices, such as metal-insulator-metal (MIM) diodes, rectify infrared waves into current by moving electrons through a small barrier. MIM diodes are able to handle high-frequency signals on the order of femtoseconds because the barrier is only a nanometer thin.
To generate the intense fields needed for tunnelling, the team used a 'bowtie-shaped' nano-antenna that sandwiches the thin insulator film between two slightly overlapped metallic arms.
"The most challenging part was the nanoscale overlap of the two antenna arms, which required very precise alignment," says postdoctoral researcher, Gaurav Jayaswal.
By choosing metals with different work functions, the team explains that the new MIM diode could catch the infrared waves with zero applied voltage, a passive feature that switches the device on only when needed.
Experiments with infrared exposure revealed the bowtie harvested energy solely from the radiation and not from thermal effects, as evidenced by a polarisation-dependent output voltage.
"This is just the beginning – a proof of concept," concludes Shamim. "We could have millions of such devices connected to boost overall electricity generation."
