“We have come up with a new solution to the problem of invisibility based on the use of dielectric (nonconducting) materials,” says Xiang Zhang, team leader and principal investigator with Berkeley Labs. “Our optical cloak not only suggests that true invisibility materials are within reach, it also represents a major step towards transformation optics, opening the door to manipulating light at will for the creation of powerful new microscopes and faster computers.”
Zhang and his team have previously implemented invisibility devices using metamaterials, which are a composite of metals and dielectrics that have extraordinary optical characteristics. The researchers developed one type of metamaterial by growing silver nanowires within porous aluminum oxide and another by alternating layers of silver and magnesium fluoride in a fishnet pattern. Using these metallic metamaterials, the team at Berkeley Labs proved that light can be bent backwards, a phenomenon previously unseen in nature.
At microwave frequencies, the metallic metamaterials have been successful in accomplishing invisibility. However, in order to attain genuine invisibility, the cloak needs to function at optical frequencies, which is proving to be a challenge since the metal elements absorb excessive amounts of light.
Zhang further iterates, “Even with the advances that have been made in optical metamaterials, scaling sub-wavelength metallic elements and placing them in an arbitrarily designed spatial manner remains a challenge at optical frequencies.”
To overcome the optical issue, the team constructed the cloak from dielectric materials that are transparent at optical frequencies. The cloak was displayed in a 250 nanometer thick rectangular slab of silicon, which was applied to aid as an optical waveguide. This, in order to confine light in the vertical dimension, while simultaneously allowing it to propagate freely in the other two dimensions.
Next, a pattern of holes with diameters of 110 nanometers is prepared on the slab, adapting it into a metamaterial that permits light to act like water flowing around a rock. In the conducted experiments, the cloak enclosed an area of approximately 3.8 microns by 400 nanometers and accurately exhibited invisibility at variable angles of incident light.
Currently, the cloak can only function in light with a wavelength between 1,400 and 1,800 nanometers. This specific range lies within the near-infrared portion of the electromagnetic spectrum, which makes it impossible to see by the human eye. Nevertheless, Zhang reveals that the cloak is fairly effortless to fabricate and should be upwardly scalable, adding that he believes it can be altered to function in visible light, what will make it visible to the naked eye.
“In this experiment, we have demonstrated a proof of concept for optical cloaking that works well in two dimensions,” says Zhang. “Our next goal is to realize a cloak for all three dimensions, extending the transformation optics into potential applications.”
TFOT has previously written about self-assembling optics, developed at the University of California, Berkeley – these are, in essence, nanoscale particles that can self-assemble into various optical devices. You are also welcome to check out our article about how optical textiles monitor MRI patients during their scans, as well as a coverage of water repellent fabrics, which were developed at the University of Zurich.
A copy of the Nature Materials paper “An Optical Cloak Made of Dielectrics” by Zhang, et al., can be obtained here. For more information about the research, please visit Zhang’s website at UC Berkeley. To learn more about the invisibility cloak, you are welcome to read this UC Berkeley press release.