In their latest series of experiments, the Duke team demonstrated that a metamaterial construct they developed could create holograms—like the images seen on credit or bank cards—in the infrared range of light, something that had not been done before.
The Duke engineers point out that while this advance was achieved in a specific wavelength of light, the principles used to design and create the metamaterial in their experiments should apply in controlling light in most frequencies.
Artistic rendering of a section of metamaterial hologram demonstrating the various metamaterial elements used in this work. he hologram consists of three layers of gold elements in a SiO2 matrix over a Ge substrate.
As a result of advances in nanotechnology and the burgeoning capabilities for fabricating materials with controlled nanoscale geometries, the traditional notion of what constitutes an optical device continues to evolve. The fusion of maturing low-cost lithographic techniques with newer optical design strategies has enabled the introduction of artificially structured metamaterials in place of conventional materials for improving optical components as well as realizing new optical functionality. Here we demonstrate multilayer, lithographically patterned, subwavelength, metal elements, whose distribution forms a computer-generated phase hologram in the infrared region (10.6 μm). Metal inclusions exhibit extremely large scattering and can be implemented in metamaterials that exhibit a wide range of effective medium response, including anomalously large or negative refractive index; optical magnetism; and controlled anisotropy. This large palette of metamaterial responses can be leveraged to achieve greater control over the propagation of light, leading to more compact, efficient and versatile optical components.
“In the past, our ability to create optical devices has been limited by the properties of natural materials,” said Stéphane Larouche, research scientist in electrical and computer engineering at Duke’s Pratt School of Engineering. “Now, with the advent of metamaterials, we can almost do whatever we want to do with light.
“In addition to holograms, the approach we developed easily extends to a broad range of optical devices,” Larouche said. “If realized, full three-dimensional capabilities open the door to new devices combining a wide range of properties. Our experiments provide a glimpse of the opportunities available for advanced optical devices based on metamaterials that can support quite complex material properties.”
The results of Larouche’s experiments, which were conducted in the laboratory of senior researcher David R. Smith, a professor of electrical and computer engineering, appeared in an advanced online publication of the journal Nature Materials. The research was supported by the Army Research Office’s Multidisciplinary University Research Initiative (MURI).
The metamaterial device fashioned by the Duke team doesn’t look anything like a lens, though its ability to control the direction of rays passing through it surpasses that of a conventional lens. While traditional lenses are made of clear substances—like glass or plastic—with highly polished surfaces, the new device looks more like a miniature set of tan Venetian blinds.
These metamaterials are constructed on thin slabs of the same material used to make computer chips. Metal elements are etched upon these slabs to form a lattice-like pattern. The metal elements can be arranged in limitless ways, depending on the properties desired.
“There is unquestionable potential for far more advanced and functional optical devices if greater control can be obtained over the underlying materials,” Larouche said. “The ability to design and fabricate the components of these metamaterial constructs has reached the point where we can now build even more sophisticated designs.
“We believe that just about any optical device can be made more efficient and effective using these new approaches,” he said.
The hologram designed by the Gerchberg-Saxton algorithm could not be reproduced perfectly due to many limitations of our process. To determine the importance of the different factors limiting the performance of the hologram, a series of simulations were run where those effects were added one by one.
The phase-only hologram produced by the Gerchberg-Saxton algorithm transmits all the incident energy, and has an efficiency of 56%, the rest of the energy being distributed as noise in the image plane, higher order components, and guided modes. The 3-layer hologram also transmits all the incident energy, but has an efficiency of only 21% because of its sub-optimal phase contrast; 44% of the energy goes in the zeroth order beam, visible in the top left corer of the image.