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Study on Fractal Invisibility Cloak

With the 14 October passing of fractal geometry founder Benoit Mandelbrot, the world’s loss of a great mind has also brought up the question “what good are fractals anyway?”

Fractals, intricate shapes built up from magnified repetitions of a simple one, have been used to make wider bandwidth, smaller, better performing antennas, a clear example of Mandelbrot’s fractal geometry in action. Fractal Antenna Systems, Inc., a Boston-area technology firm, has pioneered fractal technology. The firm’s fractal antennas and “fractal resonators” have proven to be the most successful application of fractals to date, with uses in hand-held electronics; sensors; public safety; RFID; and many others. The growing technology fields for fractal antennas and fractal resonators now encompass researchers and students in the thousands, from all continents except Antarctica.

In its latest example of the uses and advantages of fractals, Fractal Antenna Systems reports the completion of a two-year study into the mysterious world of invisibility cloaks, using fractal antennas and resonators to make it work. Invisibility cloaks, popularized in science and fantasy fiction, are devices intended to make the ‘object’, or wearer, disappear into the background. According to Nathan Cohen, Fractal Antenna Systems’ CEO, previous attempts at invisibility cloaks have not lived up to the hype attached to them: they have shadows, or are narrow in bandwidth, and require the object to already be hiding behind a mirror or screen, thus not only hiding the object from an outside observer, but hiding the outside from the object.

Notes Cohen: “Essentially, the object is hidden but also made blind to the outside by being placed behind a mirror, and certain techniques are used to re-route the light or microwaves. Such a cloak simply distorts the mirror in some controlled way. But anyone can already see this by walking into an amusement park funhouse into the altering room of mirrors. Voila! Instant invisibility cloak! In other words, altering the image/path from a mirror is not new and not a big deal. It’s a stretch to calls such devices ‘invisibility cloaks’.”

Added Cohen: “The invisibility cloak we have has unique characteristics that have not been achieved by others, because others have not used our fractal approach. Our invisibility cloak DOES allow the object to look to the outside, is wideband, and has minimal shadow and scattering. There is NO mirror of any kind. And there is no power needed to make it work. So we meet that heady requirement of making something slip into the background but also let that something ‘see out’ at the same time. This is exactly what the world sees as exciting about invisibility cloak research. Not new attempts at fun-house mirrors.”

The firm’s unique approach was first reported in March 2009. Since that time, key measurements of scattering, reproducibility, bandwidth and fidelity, as well as ability to ‘see-out’ have been done and quantified on the fractal invisibility cloak, which operates at a wide range of microwave frequencies, including those used by cell phones. “ We have an invisibility cloak, wideband at microwaves, you can see out of and melds into the background. We made careful measurements of the invisibility cloak against controls for a comparison, as any thorough experiment requires. The results are compelling and irrefutable. And, of course, exciting and fun,” said Cohen, mentioning that the results will be published and that the fractal invisibility cloak is an R and D device, not a practical application. “I am not going around wearing one to hide from microwaves, but this does show that it is feasible. It also shows a path to visual light versions, which have more serious fabrication challenges than the longer wavelength microwaves. I’d give Harry Potter 30 more years of waiting for a visual light invisibility cloak. But at least, from a technology standpoint, he can now put the order in.”


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