The quest to build a working “invisibility cloak” generally focuses on the use of metamaterials – artificially engineered materials with a negative refractive index that have already been used to render microscopic objects invisible in specific wavelengths of light. Now, using naturally occurring crystals rather than metamaterials, two research teams working independently have demonstrated technology that can cloak larger objects in the broad range of wavelengths visible to the human eye.
Both teams made the breakthrough using a natural crystal called calcite.
This transparent mineral boasts an optical property known as birefringence, or double-refraction. This means that when light enters the calcite, it splits into two rays of different polarizations traveling at different speeds and in different directions. This causes objects viewed through a clear piece of calcite to appear doubled.
To create their invisibility cloak, the University of Birmingham team glued two pieces of calcite with differing optical paths together and placed them on a mirror and performed demonstrations in both air and a container of liquid.
Meanwhile, the SMART Centre team used a similar method. They glued together two pieces of calcite to form a small wedge measuring 38mm x 10mm x 2mm and is placed over an intended object. Due to the light bending as it enters the cloak, objects are rendered invisible when the viewer looks from the left and right sides of the wedge, meaning it remains visible when viewed from other angles