A new cloaking device that breaks through some of the limitations of previous devices, using inexpensive and widely available materials has been developed at the University of Rochester.
“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,”
said John Howell, University of Rochester physics professor.
Bypassing specialized components and materials, the researchers developed a blend of four standard lenses that keeps the object hidden as the viewer moves up to several degrees away from the optimal viewing position. Called a multidirectional “perfect paraxial” cloak,
“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,”
said graduate student Joseph Choi.
Cloaking The Cloaking Device
Numerous cloaking designs work well as long as you look at an object straight on, but if you move the angle of your viewpoint even a little, the object becomes visible. Previous cloaking devices also often make the background shift radically, making it obvious a cloaking device is in use.
To both cloak an object and leave the background untouched, the researchers established the lens type and magnification power needed, plus the exact distance to separate the four lenses.
To test their device, a cloaked object was placed in front of a grid background.
As they looked through the lenses and changed their viewing angle by moving from side to side, the grid shifted accordingly as if the cloaking device was not there. There was no discontinuity in the grid lines behind the cloaked object, compared to the background, and the grid size, due to magnification, matched.
Howell and Choi’s configuration improves on other cloaking devices, however, it’s not perfect.
“This cloak bends light and sends it through the center of the device, so the on-axis region cannot be blocked or cloaked,”
In other words, the cloaked area is shaped like a doughnut. The cloak also has edge effects, although they can be reduced when sufficiently large lenses are used.
Howell and Choi demonstrate in their paper mathematics for this type of cloaking that can work for angles up to 15 degrees, or more. They use a technique called ABCD matrices which describes how light bends when going through lenses, mirrors, or other optical elements.
- Joseph S. Choi and John C. Howell, Paraxial ray optics cloaking, Opt. Express 22, 29465-29478 (2014)
- N. Landy and D. R. Smith, A full-parameter unidirectional metamaterial cloak for microwaves, Nat. Mater. 12, 25–28 (2013).