This image shows how the new ultrathin carbon nanotube films optically rotate polarized light output by 90°, but only when the input light's polarization is at a specific angle with respect to the nanotube alignment direction. Image: Kono Laboratory/Rice University.Ultrathin, highly aligned carbon nanotube films, first made by Rice University physicist Junichiro Kono and his students a few years ago, have turned out to have a surprising phenomenon waiting within: an ability to make highly capable terahertz polarization rotation possible.
This rotation doesn't mean the films are spinning. Rather, polarized light from a laser or other source can now be manipulated in ways that were previously out of reach.
The unique optical rotation happens when linearly polarized pulses of light pass through the 45nm carbon nanotube film and hit the silicon surface on which it sits. The light bounces between the substrate and the film before finally reflecting back, but with its polarization turned by 90°. This only occurs, Kono said, when the input light's polarization is at a specific angle with respect to the nanotube alignment direction: the 'magic angle'.
This discovery by lead author Andrey Baydin, a postdoctoral researcher in Kono's lab, is reported in a paper in Optica. The phenomenon, which can be tuned by changing the refractive index of the substrate and the film thickness, could lead to robust, flexible devices that manipulate terahertz waves.
Kono said easy-to-fabricate, ultrathin broadband polarization rotators that stand up to high temperatures will address a fundamental challenge in the development of terahertz optical devices. The bulky devices available up to now only work with limited polarization angles, so compact devices with more capability are highly desirable.
Because terahertz radiation easily passes through materials like plastics and cardboard, these polarization rotators could be particularly useful for manufacturing, quality control and process monitoring. They could also prove handy in telecommunications systems and for security screening, because many materials have unique spectral signatures in the terahertz range.
"The discovery opens up new possibilities for waveplates," Baydin said. A waveplate alters the polarization of light that travels through it. In devices like terahertz spectrometers, which are used to analyze the molecular composition of materials, being able to adjust polarization up to a full 90° would allow for data gathering at a much finer resolution.
"We found that specifically at far-infrared wavelengths – in other words, in the terahertz frequency range – this anisotropy is nearly perfect," Baydin said. "Basically, there's no attenuation in the perpendicular polarization, and then significant attenuation in the parallel direction.
"We did not look for this. It was completely a surprise."
Baydin said theoretical analysis showed the effect is entirely due to the nature of the highly aligned nanotube films, which were vanishingly thin but about two inches in diameter. The researchers both observed and confirmed this giant polarization rotation with experiments and computer models.
"Usually, people have to use millimeter-thick quartz waveplates in order to rotate terahertz polarization," said Baydin, who joined the Kono lab in late 2019 and discovered the phenomenon soon after that. "But in our case, the film is just nanometers thick."
"Big and bulky waveplates are fine if you're just using them in a laboratory setting, but for applications, you want a compact device," Kono said. "What Andrey has found makes it possible."
This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.