Fractal Technology and the Invisibility Cloak

Dr. Nathan Cohen, CEO and founder of Fractal Antenna Systems, Inc. talks to AZoSensors about fractal technology.

Can you provide the audience with an overview of Fractal Antenna Systems Inc.?

FRACTAL is a high tech privately-held business with the mission of providing state of the art electromagnetic (EM) solutions (read: antennas and related ilk) for commercial and defence markets. It was founded in 1995 to exploit proprietary EM technologies based on fractal geometry. Our principle commercial markets are wireless & telecom; aerospace; RFID; DAS/indoor wireless; radar; remote sensing. We have a strong commitment to basic research in-house and can transition to commercial needs very rapidly. This puts us years ahead of conventional academic based innovation. It is a throwback to the vertical integration of the Bell Labs, Polaroid, and IBM era of 50 years ago.

Your new research is impressive. The firm have only recently disclosed that it managed to render a man invisible. Can you explain the functional principle to the invisible cloak?

Conventional (lens) optics is predicated on mostly refractive material and a geometric interface, to guide the electromagnetic behaviour. Transformational optics reverses the approach and uses closely spaced resonators for this ‘guiding’, replacing the refractive material and making the geometric interface secondary. It exploits the near field with evanescent waves with greater versatility in controlling the far field. Fractal shapes are key to getting wideband and high fidelity in transformational optics, as they form the resonators used to make a ‘guiding’ arrangement practical and useful.

The invisibility cloak is an excellent example of something you can make with transformational optics but not conventional optics. We used microwaves to prove it out; they slip around an obstruction, using evanescent waves between the fractal resonators, and converge again on the other side. We are able to do this at a wide bandwidth with high fidelity, and the cloak layers themselves form a ‘thin skin’ around large objects. So-called ‘perfect cloaks’, which do not use fractals, are not able to do any of this.

What were your protocols for testing a human with this instrument?

First the powers employed are so feeble - of order a milliwatt - that exposure to people is irrelevant. Our testing protocol is to place two-directional wideband antennas, in the far field. A network analyser acts as a source and receiver and we measure this ‘direct path’ gain as a spectrum. With various controls we show that various other conventional effects, such as diffraction, are not mimicking a cloak response. Then we invoke the cloak and watch the response of the direct path be restored; with a non-cloaked obstruction (a person for example) normally reducing the gain by as much as 20 dB. We essentially turn a door into a window.

What sensor technology was involved in successfully completing this test?

We use fractal resonators as fractal metamaterial. It is proprietary.

How does this new piece of technology compare to other imaging related technologies?

The fact that no one else has cloaked a person means that other methods (besides our fractal based one) are incapable of doing large scale, high fidelity, wide bandwidth implementations with those related technologies.

What were the major challenges presented to the research team during the design and development of this cloak technology?

The basic ideas took almost ten years to bubble. A 2002 DARPA project on closed spaced fractal element arrays was good for that. Searching out the history of metamaterials, such as Walser’s work, and the seminal Marconi and Franklin patent of 1919, was important, and the novel use of fractals was key. I had a deep understanding of the processes before we did a build. I visualized them and used back of the envelope physics to vet.

The major challenge is me: I have a habit of springing solutions on my team, mostly fully ‘baked’. Call it a gift or a hindrance; vision, albeit expert-based, takes some getting used to.  It’s a clear example of the distinction between ‘invention’ and ‘innovation’. Invention is a practical creation. But the worldly interface is innovation - fulfill a need; be cost effective; use extant materials and extend manufacturing practices; and so on.  My team is outstanding at innovation. Knowing an ‘invention’ is almost assuredly going to work gives the confidence needed to push the ‘innovation’. There is very little stumbling. There was no stumbling on the human invisibility cloak. When I heard from the patent office that my patent (very first on invisibility cloaks) would issue I told my team about my vision of the human scaled-up version. We had to prove something to the world and we did; we built it in days. Of course, we had built several smaller cloaks over a 5 year period. We were ready for the leap.

Are there any improvements/further developments required with this novel equipment and if so, how do you plan on approaching such developmental requirements?

Microwave is now a done-deal. The ‘mop up’ is with respect to lowering cost of materials and form factor/weight requirements. As to visible light - everyone has a fantasy about invisibility - but who has the need? You can develop the etching/imprinting technology to do infrared and even visible light. We have the players lined up for that. But that ‘need’ issue is key.  By getting the word out as to what’s possible, we are opportunity driven by those who need it. This isn’t high temp superconductivity or controlled fusion: disasters and infinite time sinks. When there are needs there will be additional cloak solutions, in our lifetimes.

What resources were used to create the invisibility cloak technology?

We asked around for DOD money but the obsession with academic funded ‘excellence’ was too pervasive. Despite the massive publicity for (at best) modest results from university research  competitors, no one else has been able to do something that’s ‘milestone’ oriented, like cloak a man. We knew we could do it, so the effort was self-funded. That enforces a discipline of economy and does not allow for indulgences and wasted time often seen in expensive fixed priced, prolonged efforts. It also forces clever approaches to save  money with in house builds of equipment, such as internally  designed anechoic chambers; and so on. We didn’t have the bucks to buy the ‘machine that goes ping’, so we didn’t miss it.

What are the main application areas for this invisibility cloak technology?

There is a lot of obstructions in the world and we intend to make them go away with invisibility cloaks. This should drive down the amount, and thus cost, of infrastructure - cloaks can be made relatively inexpensively. Microwave based wireless and telecom are key markets for example. The defence applications are rather obvious too, but I am not at liberty to discuss that.

Are you currently working on similar technology?

Yes. Fractal metamaterials are wonderful building blocks for smaller, cheaper, flatter, better, electronic and photonic components. I am especially jazzed, as a Ph.D. astrophysicist, by using them in a new form of radiative transfer.

How do you see the advancements of imaging technology shaping the world we live in?

I have been decades ahead on many devices. I’ve been trying to project 5 years ahead rather than 20 or 50. Being too far ahead is painful and penurious. Certainly printed electronics will be pervasive in imaging systems in the next decade. That’s as far as I will venture.

About Dr. Nathan Cohen

Dr. Nathan Cohen is CEO and founder of Fractal Antenna Systems,Inc. of Waltham, MA , USA. A retired professor and professional scientist, he has held positions at Cornell; Boston University; Harvard; MIT; NASA-JPL; American Stock Exchange; and many others. He holds a Ph.D. in astrophysics from Cornell University. An inventor since age 6, he has 28 issued US patents, most in imaging related technologies. He founded the field of fractal antennas and related fractal electronics. He holds the world’s first and only patent on invisibility cloaks.