Fundamental Limits of Invisibility Cloaks

This is an example of a metasurface, which can create negative refraction. (Source: Birck Nanotech. Center, Purdue U.)

This is an example of a metasurface, which can create negative refraction. (Source: Birck Nanotech. Center, Purdue U.)

Researchers in the Cockrell School of Engi­neering at The University of Texas at Austin have been able to quantify fundamental physical limitations on the performance of cloaking devices, a technology that allows objects to become invisible or unde­tectable to electro­magnetic waves including radio waves, micro­waves, infrared and visible light. The researchers’ theory confirms that it is possible to use cloaks to perfectly hide an object for a specific wavelength, but hiding an object from an illu­mination containing different wavelengths becomes more challenging as the size of the object increases.

Andrea Alù, an electrical and computer engineering professor and a leading researcher in the area of cloaking techno­logy, along with graduate student Francesco Monticone, created a quanti­tative framework that now establishes boundaries on the bandwidth capabilities of electro­magnetic cloaks for objects of different sizes and composition. As a result, researchers can calculate the expected optimal performance of invisi­bility devices before designing and developing a specific cloak for an object of interest.

Cloaks are made from arti­ficial materials, called meta­materials, that have special properties enabling a better control of the incoming wave, and can make an object invisible or transparent. The newly established boundaries apply to cloaks made of passive metamaterials – those that do not draw energy from an external power source. Understanding the bandwidth and size limi­tations of cloaking is important to assess the potential of cloaking devices for real-world appli­cations such as communi­cation antennas, biomedical devices and military radars, Alù said. The researchers’ framework shows that the performance of a passive cloak is largely determined by the size of the object to be hidden compared with the wavelength of the incoming wave, and it quantifies how, for shorter wavelengths, cloaking gets drastically more difficult.

For example, it is possible to cloak a medium-size antenna from radio waves over rela­tively broad bandwidths for clearer communications, but it is essen­tially impossible to cloak large objects, such as a human body or a military tank, from visible light waves, which are much shorter than radio waves. “We have shown that it will not be possible to drastically suppress the light scat­tering of a tank or an airplane for visible frequencies with currently available techniques based on passive materials,” Monticone said. “But for objects comparable in size to the wavelength that excites them, the derived bounds show that you can do something useful, the restric­tions become looser, and we can quantify them.”

The graph shows the trade-off between how much an object can be made transparent and the color span over which this phenomenon can be achieved. (Source: Cockrell School of Eng.)

The graph shows the trade-off between how much an object can be made transparent and the color span over which this phenomenon can be achieved. (Source: Cockrell School of Eng.)

In addition to providing a practical guide for research on cloaking devices, the researchers believe that the proposed framework can help dispel some of the myths that have been developed around cloaking and its potential to make large objects invisible. “The question is, ‘Can we make a passive cloak that makes human-scale objects invisible?’ ” Alù said. “It turns out that there are stringent constraints in coating an object with a passive material and making it look as if the object were not there, for an arbitrary incoming wave and obser­vation point.”

Now that bandwidth limits on cloaking are available, researchers can focus on developing practical appli­cations with this technology that get close to these limits. “If we want to go beyond the performance of passive cloaks, there are other options,” Monticone said. “Our group and others have been exploring active and nonlinear cloaking techniques, for which these limits do not apply. Alter­natively, we can aim for looser forms of invisibility, as in cloaking devices that introduce phase delays as light is trans­mitted through, camou­flaging techniques, or other optical tricks that give the impression of trans­parency, without actually reducing the overall scattering of light.”

Alù’s lab is working on the design of active cloaks that use meta­materials plugged to an external energy source to achieve broader transparency bandwidths. “Even with active cloaks, Einstein’s theory of relativity fundamentally limits the ultimate performance for invisi­bility,” Alù said. “Yet, with new concepts and designs, such as active and nonlinear meta­materials, it is possible to move forward in the quest for trans­parency and invisi­bility.” (Source: U Texas)

Reference: F. Monticone & A. Alù, Invisibility exposed: physical bounds on passive cloaking, Optica 3, 718, DOI: 10.1364/OPTICA.3.000718

Link: Dept. of Electrical and Computer Engineering, The University of Texas at Austin, USA

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