Researchers at Michigan Technological University (US) have created a promising new way to manipulate electromagnetic waves so objects appear invisible.
Using photonic crystals, this new approach to cloaking could help to overcome the challenges previous metamaterials were unable to solve, including controlling anisotropy, operating at microwave and optical wave frequencies, and decreasing losses that restrict the size of hidden objects.
“To provide theeffect of invisibility, you need to create a cloak medium with specific spatial dispersion of material parameters prescribed by the science of transformation optics,” explains Elena Semouchkina, associate professor of electrical and computer engineering at MTU. “Such medium should bend wave paths around the object and accelerate waves traveling along curved trajectories so that they arrive to the observer simultaneously with waves traveling along straight, shorter paths in air.”
Professor Semouchkina and her team perform proof-of-concept cloaking experiments at microwave frequencies
Previously,scientists have relied on metamaterials composed from metal split ring resonators. The professor and her team of graduate students are now proposing to replace metamaterials with all-dielectric photonic crystals and to provide the spatial dispersion of their refractive indices by properly varying the lattice constants of the crystals.
“Our approaches are based on our recent finding that by varying the permittivity and the size of dielectric elements, it is possible to transform the energyband diagrams of dielectric photonic crystals, so that wave propagation with superluminal phase velocities can be realized,” Semouchkina says. “By properly adjusting the lattice constants in such crystals, it is possible to control their refractive index values in a wide range. Then building the cloak medium from fragments of crystals with various lattice constants can be used for obtaining index dispersion requested by the transformation optics for the desired wave manipulating.
Semouchkina’s invisibility cloaking technology could solve some of the problems with previous approaches using metamaterials. She notes that the resonance nature of metamaterials made the invisibility phenomena inherently narrowband. Metamaterials require homogenization and experience inter-resonator coupling. In addition, losses in metal elements increase with frequency, which precludes hiding objects with dimensions larger than several wavelengths. It makes metamaterials-based invisibility devices impractical for THz and optical ranges, according to the expert.
Advantages of using dielectric photonic crystals
By contrast, “Dielectric photonic crystals can be made practically lossless, promise wider bandwidth and are free of homogenization and coupling problems,” Semouchkina says. “In addition, they allow for providing anisotropic spatial dispersion of the cloak medium, which is required for an accurate wave manipulating. This opens up the road to optics.”
The professor further notes about this breakthrough that mature technology and the existing material base for photonic crystal fabrication will extend the applicability of the proposed approaches. “Although the main focus of photonic crystal studies has been obtaining complete band gaps in their energy diagrams, various types of low-loss crystals for operation in various frequency ranges, including optics, have been described and studied experimentally,” she says.
Wave moves faster in the dielectric photonic crystal than in air. Wave passes through the cloaked object
Forseseen real-world applications for MTU’s photonic-crystals-based cloaking devices are based on two functions: making objects invisible in transmitted or reflected lightwaves and shielding or screening equipment and personnel from undesirable irradiation in a wide range of frequencies.
Photonic crystals open new paths in transformation optics
“This advance is only the beginning of utilizing photonic crystals in the area of transformation optics,” says Semouchkina in response to what excites her personally the most about this particular research advance. “Transformation optics furnishes a new tool for creating novel electromagnetic and optical devices with functionalities not known before. Coordinate transformations can be derived for compressing, expanding, bending or twisting space, thus enabling designs of invisibility cloaks, field concentrators, perfect lenses, beam-bending waveguides, etc. The functions expected from such devices promise to bring advances to various areas of human life.”
In conclusion, she says that her team’s findings open new ways for realizing devices initiated earlier by the development of metamaterials, but restricted in applications because of their drawbacks. “Incorporation of dielectric photonic crystals in the device media promises to make devices with superior functionalities real at frequencies ranging from microwaves to optics.”
Her nearest goal is now to work out the approaches for providing the anisotropy control in manipulating the waves. She and her WTU colleagues also aim to extend the developed approaches to a larger class of devices designed by using transformation optics.
The paper “Superluminal media formed by photonic crystals for transformation optics-based invisibility cloaks” by Semouchkina et al is published in a special issue of Journal of Optics on transformation optics.
Written by Sandra Henderson, Research Editor, Novus Light Technologies Today