The proof-of-concept THz lens consists of a stack of stainless-steel plates that have been sculpted into a plano-concave geometry. [Image: Mittleman lab/Brown University]
Optical scientists and the optical industry at large are continually on the prowl for off-the-shelf components to control terahertz (THz) radiation—a frequency band that holds great promise in applications ranging from imaging to communications. A team at Brown University, USA, led by OSA Senior Member Rajind Mendis and OSA Fellow Daniel Mittleman, has unveiled a novel approach to creating those components using artificial dielectrics (Sci. Reports, doi: 10.1038/srep23023). The team has already created a proof-of-concept lens for focusing THz radiation, and believes that the same techniques could be pressed into service to create a range of other THz devices.
Stacked planar waveguides
Artificial dielectrics are man-made materials engineered to have specific properties of natural dielectrics, such as refractive index, with the specific values determined by device architecture and geometry. Such media can be developed and shaped to have a refractive index of less than one (that is, less than the index of empty space). Moreover, previous work both by the Brown scientists and others suggested that the use of artificial dielectrics comprising parallel-plate waveguides offered a promising route for controlling THz radiation, without the need for the intricately engineered subwavelength components present in other metamaterials.
The team led by Mendis and Mittleman created its THz-focusing lens using a stack of 32 stainless-steel plates—each 100 microns thick, and separated by a 1-mm airspace—that are sculpted to create a lens shape. Interestingly, because the stacked artificial dielectric has a refractive index below one, the shape of the lens is plano-concave, rather than the convex shape common in focusing lenses from natural dielectrics. The parallel-plate geometry means that the waveguides that constitute the lens act on the lowest-order transverse electric mode of the radiation, the electric field of which is oriented parallel to the plates.
Strong transmission performance
Using this device design, the team was able to focus a 2-cm-diameter beam, at a frequency of 0.17 THz, down to a spot size of 4 mm. The lens transmitted more than 80 percent of the incident radiation, a performance considerably better than that of silicon lenses and roughly equal to that of Teflon lenses. And the team’s modeling showed that the performance and properties of the artificial-dielectric lens can be tuned to specific THz wavelengths by varying the spacing between the plates. That points to a level of potential versatility that could prove useful, according to team leader Mittleman, in applications such as multispectral imaging.
Mittleman noted in a press release that the work is particularly important in the THz field because “there aren’t a lot of off the shelf components yet” in that market. “As much as anything else, this paper proves that the technology is feasible,” he said. “Now we can go and make devices that are totally new in the terahertz world.”