An invisibility-cloak array made up of more than 25,000 individual minuscule cloaks has been built by a team of researchers in the US. The array is the first of its kind and operates in the visible frequency range. Individual cloaks could be used as biosensors, while the array could be used to test the performance of individual cloaks and as a way to study on-chip light manipulation.

The optical properties of materials are characterized by how they react to external electrical and magnetic fields. These properties are tailored in metamaterials – specially structured materials that possess unique optical properties that are not found naturally – so that light avoids a region of space to be cloaked. But a simpler way of achieving a similar effect is to use an "optical waveguide" – a structure, such as an optical fibre, that guides optical waves by total internal reflection.

Guiding the wave

This is what lead researcher Vera Smolyaninova and colleagues at Towson University and the University of Maryland did three years ago by placing a gold-coated lens on top of a gold-coated glass slide, where the area between the two surfaces acted as the waveguide. The double gold surfaces form a "good waveguide" explains Smolyaninova. The team found that light travelled around the space where the two surfaces touched.

These cloaks, the researchers realized, could be used to "trap" light – to slow it down, or even stop it, creating what is known as a "trapped rainbow". The trapped rainbow is seen as different wavelengths of light – and so different colours – are stopped at slightly different radii within the lens. Now, the same team has made thousands of these cloaks – with each cloak being about 30 μm in diameter – that are then laid out together on a gold sheet. Each microlens bends light around itself, effectively hiding the area it contains. The cloak array was built using a commercially available microlens array that was coated with a gold film 30 nm thick. This was placed, gold-side down, onto a gold-coated glass slide and a laser beam was directed into the array to test the performance of the cloaks at different angles.

Shadows and reflections

One of the main aims of this study, according to Smolyaninova, was to see how multiple cloaks "interfere" with each other and how the proximity of each neighbouring cloak affects the path of light across the array. The researchers found that while the cloaks worked well when light was shone along the rows of microcloaks, light incident at varied angles or slightly flawed symmetry in the array design caused shadows and scattering to appear and imperfections to become clearly visible. "As there is such an increased matter–light interaction thanks to each rainbow," explains Smolyaninova, "we can clearly see how light propagates across the array. So this could become a way of checking for cloak imperfections."

Rainbow biosensors

An interesting application for this cloak array could be in the field of biosensors that identify materials using fluorescence spectroscopy – identification based on the amount of light absorbed and then emitted by the material. "In our array, light is stopped at the boundary of each of the cloaks, meaning we observe the trapped rainbow at the edge of each cloak. This means we could do 'spectroscopy-on-a-chip' and examine fluorescence at thousands of points all in one go," says Smolyaninova.

Also, as slow light has a stronger interaction with molecules than light travelling at normal speeds, a more detailed analysis is possible. This means that it may, in theory, be possible to use this technology to build a biochip that has numerous sensors that perform tasks simultaneously. "For example, you could test for multiple genetic conditions in a person's DNA in just one go," says Smolyaninova. "You could possibly attach different dyes to different conditions and then look for them together."

What are the chances of this being used to cloak large objects in everyday life? "While it is possible to just increase the radius of our lens, it is important to remember that this is a 2D cloak. So anything cloaked by it would be invisible only in that plane," explains Smolyaninova. In the coming months the researchers will look into perfecting their method to manipulate light on a very small scale using a waveguide and then study the emerging properties of using, say, a fish-eye lens as the guide.

The research is published in New Journal of Physics.