What is optical camouflage

A shell made of calcite crystals can make a steel wedge invisible more easily than expected

Camouflage coat made of calcite crystal

Cambridge (USA) / London (Great Britain) - bit by bit, physicists all over the world are getting to grips with functioning stealth. They always report new advances, but mostly use complex and microstructured materials, so-called metamaterials. But stealth can also be implemented much more easily. Two British research groups are now reporting on functioning systems made from simple calcite crystals. They present their promising experiments in the journals "Physical Review Letters" and "Nature Communications".

"The greatest challenge to true invisibility lies in hiding macroscopic objects in the visible wavelength range," explain Baile Zhang and his colleagues from the Singapore-MIT Alliance for Research and Technology (SMART) Center. They have now found a solution for precisely this problem. They did without the metamaterials made up of nanostructured materials such as magnesium fluoride that were previously used for their invisibility cloak. Instead, they chose two calcite crystals (calcite), whose optical properties they determined beforehand. This material is naturally birefringent and, when coated on one side with a metal film, showed total reflection of laser light. As a result, under both green (532 nanometers) and red (650 nanometers) light, a steel wedge a few millimeters in size in a water bath under the crystal cloak completely disappeared from view.

The research group around the stealth expert John Pendry from Imperial College in London also used calcite crystals. They arranged the crystals so cleverly that polarized light in the visible spectral range was directed around the objects to be camouflaged. The result: viewed from one direction, an observer could only see the background; the actual object had disappeared from view.

These approaches are not yet ready for camouflage application. Because the stealth crystal itself is still very clearly visible so far. Nevertheless, both works show that light waves can be elegantly manipulated even without complex metamaterials. Calcite crystals are also much cheaper. The researchers can imagine the first technical applications with large areas of calcite crystals in the military sector, for example in the optical camouflage of submarines.