Researchers at Rice University are using carbon nanotubes as the critical component of a robust terahertz polarizer that could accelerate the development of new security and communication devices, sensors and non-invasive medical imaging systems as well as fundamental studies of low-dimensional condensed matter systems. It is the most effective polarizer ever reported; it selectively allows 100 percent of a terahertz wave to pass or blocks 99.9 percent of it, depending on its polarization.
We demonstrate a terahertz polarizer built with stacks of aligned single-walled carbon nanotubes (SWCNTs) exhibiting ideal broadband terahertz properties: 99.9% degree of polarization and extinction ratios of 10^–3 (or 30 dB) from 0.4 to 2.2 THz. Compared to structurally tuned and fragile wire-grid systems, the performance in these polarizers is driven by the inherent anistropic absorption of SWCNTs that enables a physically robust structure. Supported by a scalable dry contact-transfer approach, these SWCNT-based polarizers are ideal for emerging terahertz applications.
The broadband polarizer handles waves from 0.5 to 2.2 terahertz, far surpassing the range of commercial polarizers that consist of fragile grids wrapped in gold or tungsten wires.
Kono said technologies that make use of the optical and electrical regions of the electromagnetic spectrum are mature and common, as in lasers and telescopes on one end and computers and microwaves on the other. But until recent years, the terahertz region in between was largely unexplored. “Over the past decade or two, people have been making impressive progress,” he said, particularly in the development of such sources of radiation as the terahertz quantum cascade laser.
“We have pretty good terahertz emitters and detectors, but we need a way to manipulate light in this range,” Kono said. “Our work is in this category, manipulating the polarization state — the direction of the electric field — of terahertz radiation.”
Terahertz waves exist at the transition between infrared and microwaves and have unique qualities. They are not harmful and penetrate fabric, wood, plastic and even clouds, but not metal or water. In combination with spectroscopy, they can be used to read what Kono called “spectral fingerprints in the terahertz range”; he said they would, for instance, be useful in a security setting to identify the chemical signatures of specific explosives.