Visibly invisible

Fabricated calcite crystals could be all that is needed to make a so-called "invisibility cloak", a material that renders an otherwise opaque object transparent to visible light. Earlier efforts have been functional only on the microscopic scale or in the infra-red or radio wavelengths and used nanofabricated composites and metamaterials.

 

Invisibility has been a perennial topic in science fiction and science fantasy throughout the genre and scientists have recently begun to take steps towards fulfilling this fantasy. Metamaterials have been developed, for instance, that allow electromagnetic radiation to flow, like a stream around a half-submerged boulder, past an object as if it were not there and so give the illusion of the object being invisible. However, making an object invisible to our eyes rather than infra-red or radio detectors had until now remained impossible.

 

Now, two independent teams, one in the UK, the other based in the US and Singapore, have both discovered that calcite can act as a visible-light cloaking material because of its natural birefringent properties. Shuang Zhang of the University of Birmingham and John Pendry of Imperial College London have demonstrated using red and green laser beams and incoherent white light that a fabricated block of calcite can hide objects dozens of millimeters high in air [Zhang et al. Nature Commun. (2011) 2, 176 online, doi:10.1038/ncomms1176].

 

They show that the cloak is capable of hiding three-dimensional objects three to four orders of magnitude larger than optical wavelengths and does this by avoiding major issues usually associated with cloaking devices, such as size, bandwidth and image distortion. The effect is seen in the transformation of a deformed mirror into what appears to be a flat mirror as viewed from any angle.

 

Baile Zhang and colleagues at the Singapore-MIT Alliance for Research and Technology (SMART) Centre and at the Massachusetts Institute of Technology, USA, found they could shield a steel wedge a maximum of 2 mm high from polarized red, green or blue light under water using two pieces of fabricated calcite [Zhang et al. Phys. Rev. Lett. (2011) 106, 033901, doi:10.1103/PhysRevLett.106.033901]. The cloak works only in a two-dimensional plane

 

Both approaches now pave the way for future practical cloaking devices. The next step will be to somehow engineer the cloaking material to be functional simultaneously at all wavelengths and perhaps then with normal, as opposed to polarized, visible light.