An improved technique for wireless power transfer technology

Going beyond the anti-laser may enable long-range wireless power transfer.


Charging a smartphone wirelessly nowadays is not a big deal. You have to put your smartphone on a charging pad. But usable long-range wireless power transfer, like from one room to another or even across the building, is still in progress.

Most of the development methods involve focusing narrow beams of energy and aiming them at their intended target. These methods have had some success but are, so far, not very efficient. And having focused electromagnetic beams flying around through the air is unsettling.

Scientists from the University of Maryland (UMD), in collaboration with a colleague at Wesleyan University in Connecticut, have developed an improved technique for wireless power transfer technology that may promise long-range power transmission without narrowly focused and directed energy beams.

Here, scientists considered the concept of anti-laser. Unlike laser, anti-laser coherently and perfectly absorbs a beam of many precisely tuned photons. It’s kind of like a laser running backward in time.

Scientists demonstrated that it is possible to design a coherent perfect absorber outside of the original time-reversed laser framework—a relaxation of some of the critical constraints in earlier work. Rather than assuming directed beams going along straight lines into an absorption target, they picked a messy calculation and not manageable to being run in reverse as expected.

UMD Professor of Physics Steven Anlage of the Quantum Materials Center (QMC) said, “We wanted to see this effect in a completely general environment where there are no constraints. We wanted a sort of random, arbitrary, complex environment, and we wanted to make perfect absorption happen under those demanding circumstances. That was the motivation for this, and we did it.”

Scientists wanted to develop a device that could receive energy from a more diffuse source, less beam, and more bath. For this, they set up their generalized anti-laser as a labyrinth of wires for electromagnetic waves to travel through. Later, they used microwaves for power transfer applications.

The labyrinth consisted of a bunch of wires and boxes connected in a purposefully disordered way. Microwaves going through this labyrinth would get so tangled up that this still wouldn’t untangle them even if it were possible to reverse time.

There was an absorber buried within the labyrinth. The team sent microwaves of different frequencies, amplitudes, and phases into the labyrinth and measured how they were transformed. Based on these measurements, they could calculate the exact properties of input microwaves that would result in perfect power transfer to the absorber.

They found that for correctly chosen input microwaves, the labyrinth absorbed an unprecedented 99.999% of the power they sent into it. This showed explicitly that coherent perfect absorption could be achieved even without a laser run backward in time.

Scientists then took a step forward towards transferring power wirelessly. They repeated the experiment in a cavity, a plate of brass several feet in each direction with an oddly shaped hole in the middle. The shape of the hole was designed so that the microwaves would bounce around it in an unpredictable, chaotic way.

They placed a power absorber inside the cavity and sent microwaves to bounce around the open space inside. They were able to find the right input microwave conditions for coherent perfect absorption with 99.996% efficiency.

This was when scientists demonstrated coherent perfect absorption in their disordered microwave labyrinth. However, their experiment was not quite as general as the new work from Anlage and colleagues.

In the previous work, the microwaves entering the labyrinth would still be untangled by a hypothetical reversal of time. This might seem like a subtle distinction, but the authors say showing that coherent perfect absorption doesn’t require any kind of order in the environment promises applicability virtually anywhere.

Lei Chen, a graduate student in electrical and computer engineering at UMD, said, “If we have an object to which we want to deliver power, we will first use our equipment to measure some properties of the system. Based on those properties, we can get unique microwave signals for this kind of system. And it will be perfectly absorbed by the object. For every unique object, the signals will be different and specially designed.”

Although this technique shows great promise, much remains to be done before the advent of wireless and plug-less offices. The perfect absorber depends crucially on the power being tuned just right for the absorber.

Journal Reference: 
  1. Lei Chen et al. Perfect absorption in complex scattering systems with or without hidden symmetries, Nature Communications (2020). DOI: 10.1038/s41467-020-19645-5 
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