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May 24, 2012

Experimental realization of an array of broadband 20% invisibility cloaks that operates in the visible frequency

A group of American researchers has created 25 000 individual "invisibility" cloaks. They are just 30 micrometres in diameter and are laid out together on a 25 millimetre gold sheet.

We report the first experimental realization of an array of broadband invisibility cloaks that operates in the visible frequency range. Such an array is capable of cloaking ~20% of an unlimited surface area. The wavelength and angular dependences of the cloak array performance have been studied.

Wider implications.
Building and studying the arrays of invisibility cloaks offers more refined experimental tools to test cloak performance. Compared to the characterization of individual cloaks, the angular performance of cloak arrays appears to be more sensitive to cloak imperfections. These findings may be useful in such related areas as acoustic and surface-wave cloaking, as well as in the potential practical applications listed above.

They could be used to slow down, or even stop, light, creating what is known as a "trapped rainbow".

The trapped rainbow could be utilised in tiny biosensors to identify biological materials based on the amount of light they absorb and then subsequently emit, which is known as fluorescence spectroscopy. Slowed-down light has a stronger interaction with molecules than light travelling at normal speeds, so it enables a more detailed analysis.

Lead author of the study, Dr Vera Smolyaninova, said: "The benefit of a biochip array is that you have a large number of small sensors, meaning you can perform many tests at once. For example, you could test for multiple genetic conditions in a person's DNA in just one go.

"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."

The 25 000 invisibility cloaks are uniformly laid out on a gold sheet, with each having a microlens that bends light around itself, effectively hiding an area in its middle. As the light squeezes through the gaps between each of the cloaks, the different components of light, or colours, are made to stop at ever narrower points, creating the rainbow.






(a) Experimental geometry of the broadband array of invisibility cloaks. A single gold-coated lens from [8] has been replaced with a gold-coated microlens array. (b) Illustration of light propagation through an array of invisibility cloaks. The cloaked areas are shown in black.

Our invisibility cloak arrays were fabricated as follows. As a first fabrication step, a commercially available microlens array4 was coated on the microlens side with a 30 nm gold film (figure 2(a)). The array was placed with the gold-coated side facing down on top of a flat glass slide coated with a 70 nm gold film. Two gold-coated surfaces were pressed against each other using a mechanical arrangement with set screws. Argon ion laser light with different wavelengths λ was coupled into the waveguide from the side. A periodic array of adiabatically tapered gaps between the gold-coated surfaces was used as a 2D array of invisibility cloaks
Figure 2. Light propagation through a rectangular cloak array formed by the gap between the gold-coated surfaces of a large microlens array (500 μm pitch, 56 mm lens radius) and a flat glass slide. (a) Microscope image of the gold-coated lens array. (b) Microscope image of 514 nm light propagation through the cloak array. Dashed line indicates the waveguide edge. Light propagation direction is indicated by the arrow. (c) Magnified image of 514 nm light propagation through the cloak array. Similar to [8], cloaked areas appear as dark circles surrounded by concentric rings.

In conclusion, we have reported the first experimental realization of an array of broadband invisibility cloaks that operates in the visible frequency range. Such an array is capable of cloaking ~20% of an unlimited surface area. We have studied the wavelength and angular dependences of the cloak array performance. While the broadband performance appears to be similar to the performance of individual cloaks in the array, the angular performance of a dense array shows signs of deterioration due to a reduction of the symmetry of the cloaking arrangement.

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