Artificial Dielectrics Leveraged to Control THz Waves
PROVIDENCE, R.I. — Future terahertz-wave devices for communications and imaging could benefit from a lens based on artificial dielectrics.
Terahertz radiation (about 100 to 10,000 GHz) is a relatively unexplored slice of the electromagnetic spectrum but it holds the promise of countless new imaging applications, as well as wireless communication networks with extremely high bandwidth. However, few off-the-shelf components are available for manipulating terahertz waves.
Artificial dielectrics are man-made media that mimic properties of naturally occurring dielectric media, or even manifest properties that cannot generally occur in nature. For example, the well-known dielectric property of refractive index, which usually has a value greater than unity, can have a value less than unity in an artificial dielectric.
Researchers have used an array of stacked plates to make a lens for terahertz radiation. The technique could set the stage for new types of components for manipulating terahertz waves. Courtesy of the Mittleman Lab/Brown University.
A team of researchers led by professor Rajind Mendis of Brown University created an lens comprising an artificial-dielectric medium made up of a parallel stack of 32 metal plates, each 100 μm thick, with a 1-mm space between each plate. The plates have semicircular notches of different sizes cut out of one edge, such that when stacked horizontally, the notches form a 3D divot on one side of the device. When a terahertz beam enters the input side of the device, slices of the beam travel through the spaces between the plates. The concave output side of the device bends the beam slices to varying degrees such that the slices are all focused on a certain point.
The convergent lens had a planoconcave geometry, in contrast to conventional dielectric lenses. The team reported results demonstrating the lens is capable of focusing a 2 -cm diameter beam to a spot size of 4 mm at the design frequency of 0.17 THz. The results also demonstrate that the overall power transmission of the lens was better than certain conventional dielectric lenses commonly used in the THz regime — 80 percent, about the same as Teflon lenses, but much higher than the 50 percent transmission achievable with silicon lenses.
Unlike Teflon and other existing lenses, however, the new terahertz lenses offer the advantage of customization; by changing the spacing between the plates, the new device can be calibrated for specific terahertz wavelengths, the researchers said.
"That can be particularly interesting if you want to image things at one frequency and not at others," said Brown professor Dan Mittleman. "One of the important things here is that this design offers you a versatility that a simple chunk of plastic with a curved surface doesn't offer."
The team also observed that under certain conditions, the lens boundary demarcated by the discontinuous plate edges resembled a smooth continuous surface, a result they say highlights the importance of this artificial-dielectric technology for the development of future terahertz-wave devices.
"Any photonic system that uses terahertz — whether it's in imaging, wireless communications or something else — will require lenses," Mittleman said.
The researchers also said that technology used to create the lens could be used to make a polarizing beam splitter for terahertz waves, which could be used to implement elementary logic gates for terahertz photonic systems, where the binary logic states are assigned to the two polarization states. This would be an essential component of a terahertz data network.
The research was published in Nature Scientific Reports (doi: 10.1038/srep23023).
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