Small devices such as light sensors or network components could soon be able to harvest energy from Wi-Fi and Bluetooth background signals – using a new, highly advanced component that can detect even the weakest electromagnetic waves in electricity.
Researchers have developed a highly sensitive “rectenna,” or rectifier antenna, a device that exploits the peculiarities of quantum physics to efficiently convert electromagnetic energy into direct current. The researchers used this novel approach to trapping electrons to power a commercial thermometer.
In a study published on July 24 in the journal Natural Electronics, The scientists suggested that this technology could be extended to provide Internet of Things (IoT) devices and sensors with a small portion of the excess radio frequency (RF) signals they use to communicate with each other.
Rectennas receive electromagnetic waves, such as those found in radio frequency (RF) signals such as Wi-Fi and Bluetooth, or in various wavelengths of light, and capture them as alternating current (AC) via the antenna. The device then converts this to direct current via its rectifier circuit.
It has long been known that rectennas can be used to generate small amounts of electricity; researchers have demonstrated this since the 1960s through wireless powering of model vehicles and similar experiments. In 1964, for example, the weapons manufacturer Raytheon broadcast a television program in which it demonstrated a remote-controlled helicopter with microwave propulsion.
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However, in these cases, the energy was transmitted directly to the device in the form of microwave energy. Ambient RF signals are much weaker and are not aimed directly at the devices.
In the paper, the researchers write that ambient RF signals can be well below minus 20 decibel-milliwatts, a unit of measurement used to indicate signal strength. For comparison, an average smartphone transmits signals at 27 dBm, while a microwave oven operates at 60 dBm.
To exploit the very weak ambient signals of Wi-Fi and Bluetooth networks, the researchers turned to a relatively obscure corner of quantum research.
This method, known as “spintronics,” studies the quantum spin of electrons and how it relates to magnetic fields. For their demonstration, the researchers relied on the properties of magnetic tunnel junctions (MTJs), a component consisting of a very thin layer of insulating material sandwiched between two magnetic layers. MTJs are most commonly used in hard disk drives and have been used in other types of computer memory.
RF signals can cause a displacement at MTJs, with the current of the signal affecting the spin of the electrons within the structure. This can be used to generate electricity.
The team developed a series of nanometer-sized “spin rectifiers” (SRs) made of MTJs with overall dimensions of 40 x 100 nanometers square and 80 x 200 nm2 that respond to the frequencies of common ambient electromagnetic signals such as Wi-Fi (2.4 gigahertz frequency), 4G (2.3 to 2.6 GHz) and 5G (3.5 GHz).
After demonstrating the effectiveness of their component alone, the researchers created an SR array that could power a commercially available temperature sensor with only minus 27 dBm of ambient RF.
The team hopes that this method will help reduce the carbon costs of operating wireless networks in the future by reducing battery dependence and energy consumption of sensors and other small devices.
