Any device that sends out a Wi-Fi signal also emits terahertz waves — electromagnetic waves with a frequency somewhere between microwaves and infrared light. These high-frequency radiation waves, known as T-rays, are also produced by almost anything that registers a temperature including human bodies and inanimate objects. If harnessed, the concentrated power of terahertz waves could potentially serve as an alternate energy source; however, to date, terahertz waves are wasted energy, as there has been no practical way to capture and convert them into any usable form.
Physicists have created a blueprint for a device that can convert ambient terahertz waves into a direct current — a form of electricity that powers many household electronics. The design takes advantage of the quantum mechanical or atomic behavior of the carbon material graphene. By combining graphene with another material — in this case, boron nitride — the electrons in graphene should skew their motion toward a common direction. Any incoming terahertz waves should “shuttle” graphene’s electrons to flow through the material in a single direction as a direct current.
A few experimental technologies that have been able to convert terahertz waves into DC current do so only at ultracold temperatures — setups that would be difficult to implement in practical applications. Instead of turning electromagnetic waves into a DC current by applying an external electric field in a device, the researchers found that at a quantum mechanical level, a material’s own electrons could be induced to flow in one direction in order to steer incoming terahertz waves into a DC current.
Such a material would have to be very clean, or free of impurities, in order for the electrons in the material to flow through without scattering off irregularities in the material. Graphene was the ideal starting material.
To direct graphene’s electrons to flow in one direction, the material’s inherent symmetry, or what physicists call “inversion,” would need to be broken. Normally, graphene’s electrons feel an equal force between them, meaning that any incoming energy would scatter the electrons in all directions symmetrically.
Electrons are driven by incoming terahertz waves to skew in one direction and this skew motion generates a DC current if graphene were relatively pure. If too many impurities did exist in graphene, they would act as obstacles in the path of electron clouds, causing these clouds to scatter in all directions rather than moving as one.
The team also found that the stronger the incoming terahertz energy, the more of that energy a device can convert to DC current. This means that any device that converts Trays should also include a way to concentrate those waves before they enter the device. The team then drew up a blueprint for a terahertz rectifier that consists of a small square of graphene that sits atop a layer of boron nitride and is sandwiched within an antenna that would collect and concentrate ambient terahertz radiation, boosting its signal enough to convert it into a DC current.
The “high-frequency rectification” design should be able to work at room temperature, versus the ultracold temperatures required for previous terahertz rectifiers and detectors. In the near future, terahertz rectifiers may be used, for instance, to wirelessly power implants in a patient’s body without requiring surgery to change an implant’s batteries. Such devices could also convert ambient Wi-Fi signals to charge up personal electronics such as laptops and cellphones.