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The patent-pending "Rochester Cloak" hold some serious commercial promise, in spite of its limitations

Scientists have been putting a lot of effort into developing high-tech "cloaks" -- advanced materials that direct light or sound around an object making it practically invisible.  Such studies often have involved quantum effects and/or exotic metamaterials.  But according to an exciting new study from the University of Rochester (New York), a far simpler solution might have been right before researchers' eyes.
 
Physics professor John Howell and his Ph.D student Joseph Choi have put together a work that can best be described as both beautiful and unexpected.  They demonstrated that by precisely positioning four lenses -- two of one focal length f1 and two of another focal length f2 -- a "perfect paraxial" cloaking effect could be achieved.  The pair calls their final design "The Rochester Cloak".



The finished design builds on a previous investigation into low-tech light redirection that the pair had published in a March 2014 paper in the peer-reviewed journal Optics Express (Professor Howell's 15 year-old son, who helped with the measurements, is a coauthor on that prior work).
 
In that prior work, the researchers managed to use optics to cloak an office chair, showing the capability to cloak large objects.

Cloaked Chair
A cloaked chair is seen here from the previous work by Choi and Howell.
[Image Source: Optics Express/Univ. of Rochester]

The new work, which has been detailed in a new paper submitted to Optics Express is rather different in design, but operates on the same premise -- that series of large-scale optics can cloak objects on a macroscopic scale.
 
The "low-tech" Rochester Cloak presented in the new work is a pretty incredible discovery, both from a theoretical and from a commercial standpoint.  It represents a significant advancement over both the March construct, and current more "high-tech" metamaterial cloaks.

Rochester Cloak
The Rochester Cloak in action [Image Source: Univ. of Rochester]

Professor Howell states:

There've been many high tech approaches to cloaking and the basic idea behind these is to take light and have it pass around something as if it isn’t there, often using high-tech or exotic materials.

Joseph Choi adds:

This is the first device that we know of that can do three-dimensional, continuously multidirectional cloaking, which works for transmitting rays in the visible spectrum.  This cloak bends light and sends it through the center of the device, so the on-axis region cannot be blocked or cloaked.

As Mr. Choi's comment implies, the "paraxial" part is a fancy mathematical way of saying that the cloaked region is donut-shaped.  While the cloak works most optimally with four lenses, a more flawed simple implementation can also be made with only three lenses.  
Rochester cloak
The cloak is so simple that the researchers are able to give instructions to build it at home.  They write in a press release:
  1. Purchase 2 sets of 2 lenses with different focal lengths f1 and f2 (4 lenses total, 2 with f1 focal length, and 2 with f2 focal length)
  2. Separate the first 2 lenses by the sum of their focal lengths (So f1 lens is the first lens, f2 is the 2nd lens, and they are separated by t1= f1+ f2).
  3. Do the same in Step 2 for the other two lenses.
  4. Separate the two sets by t2=2 f2 (f1+ f2) / (f1— f2) apart, so that the two f2 lenses are t2 apart.
Even in its optimal four-lens form, the cloak is imperfect as a material that crosses the center of the lens path will be exposed.  Another shortcoming is that the bigger the cloaked object, the larger the lenses and separation is needed.
 
Hence when it comes to something as big as a fighting vehicle, cloaking would require an extremely complicated design that would involve giant lenses and a donut, shaped vehicle body – this would obviously be too impractical for reality.  A full-size wearable cloak or a cloak for aerial craft both would likely be infeasible due to the weight of the lenses mounted to your body or the body of the aircraft.

Rochester Cloak
A laser trace shows the cloaked regions in the four-lens Rochester Cloak.
[Image Source: Univ. of Rochester]

With that said, the cloak does have a number of advantages over metamaterial "high-tech" cloaks.
 
First, it's macroscopic; metamaterial cloaks have mostly only been demonstrated at microscopic or nanoscopic scales.  This means that where metamaterial cloaks could one day hold greater commercial promise, this cloak could lead to instant commercial opportunities.
 
The University of Rochester has patented the cloaking scheme, which indicates that it believes the technology is ready for real world applications.  The authors suggest one such application in the University press release, which states:

While their device is not quite like Harry Potter’s invisibility cloak, Howell had some thoughts about potential applications, including using cloaking to effectively let a surgeon "look through his hands to what he is actually operating on," he said. The same principles could be applied to a truck to allow drivers to see through blind spots on their vehicles.

As these comments imply, the real commercial promise is applications where you wish you could look "through" some small part of a bigger object.
 
Aside from the commercial promise, the cloak also demonstrates optical advantages to more high-tech cloaks, as well.  Where as metamaterial cloaks often lose their effect or give visual artifacts when moving just a couple of degrees off their optimal viewing angle, the Rochester Cloak "can work for angles up to 15 degrees, or more", according to the press release.
 
Also, while many metamaterials cover only a specific part of the spectrum, the Rochester Cloak redirects virtually all common light wavelengths, making the cloaking effect equally potent in the infrared and ultraviolet spectrums.
 
Based on the results, this is likely the first optical cloak that could reliably cloak a human.  While it isn't wearable, even this capability is promising: imagine, for example, a troop carrier that appears virtually, empty, but is really carrying a full platoon.  Thus it's perhaps too early to rule out military applications.

Rochester Cloak disguising
Ph.D student Joseph Choi demonstrates his invention, the "Rochester Cloak".
[Image Source: Univ. of Rochester]

In their paper on the new cloak design, Joseph Choi and Professor Howell explain in more complexity how the cloaking actually works, using a math technique called ABCD matrices.  The pair is already improving on their design, suggesting that achromatic lenses can be used to reduce edge effects and that Fresnel lenses can be used to reduce the total length of the optical path.
 
This achievement may ultimately be overshadowed by metamaterial cloaks.  But for now the beauty and simplicity of the Rochester Cloak reigns supreme in the nascent field of optical cloaking.

Sources: University of Rochester [press release], [YouTube], [paper]





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