Turns out, 'invisibility cloaks' won't work on muggles

If you were longing for the Harry-Potter invisibility cloak, then it's time you pop your bubble as a recent study has some heart-breaking revelations.

The University of Texas researchers 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.

Researchers Andrea Alù and 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, which 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 (a typical radio-wave antenna, for example, or the tip of some optical microscopy tools), the derived bounds show that you can do something useful, the restrictions become looser, and we can quantify them."

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 study appears in the journal Optica.