The bulb’s Edison-style filament is twisted at the microscale. When the length of each twist matches the wavelength of the light emitted by the filament, the lightwaves twirl as they move through space. [Image: Brenda Ahearn/Michigan Engineering]
The development of at-source circularly polarized light (CPL) emitters has potential applications for emergent telecommunications, encrypted networks, quantum optical computing and other advanced technologies. However, closely spaced vibronic levels prevent CPL devices for near-infrared (NIR) wavelengths from achieving high brightness and polarization anisotropy.
Now, researchers at the University of Michigan, USA, have reportedly discovered that twisted carbon nanotube yarns or tungsten wires can emit circularly polarized thermal radiation up to 100 times brighter than traditional CPL emitters (Science, doi: 10.1126/science.adq4068). Interestingly, the technology is similar to light bulbs first demonstrated by Thomas Edison nearly 150 years ago.
“It's hard to generate enough brightness when producing twisted light with traditional ways like electron or photon luminescence,” said study author Jun Lu in a press release accompanying the research. “We gradually noticed that we actually have a very old way to generate these photons—not relying on photon and electron excitations, but like the bulb Edison developed.”
Twisted light takes a turn
While CPL emitters for visible wavelengths have made progress in terms of brightness, the close proximity of vibronic states typical for NIR transitions greatly accelerates excitation decay and poses a significantly greater challenge. Lu and his colleagues realized that black-body radiation could offer an alternative approach to generating brighter CPL that spans a broader spectrum.
“The bundles of carbon nanotubes or tungsten microfibers are strongly twisted to produce twisted filaments with distinct helical geometry. After that, we pass current through them, which results in heating,” said senior author Nicholas A. Kotov. “The thermal emission of the heated filaments is strongly circularly polarized because the chiral geometry of the filaments results in the chirality of the emitted light.”
Jun Lu examines the twisted filament glowing within the bulb. He, along with his team, demonstrated for the first time that a twisted filament could produce twirling light waves. [Image: Brenda Ahearn/Michigan Engineering]
The resulting black-body radiation shows emission anisotropy and brightness exceeding traditional CPL emitters by factors of 10 to 100. In addition, the helical structure of the filaments enables precise spectral tuning of the chiral emission, with CPL spanning the visible, NIR and mid-infrared ranges.
Remote detection
Ultimately, the researchers aim to demonstrate the use of chiral black-body radiation to identify objects, inspired by the otherworldly vision of the mantis shrimp. This unique marine crustacean is the only animal known to be able to detect CPL; the authors envision robots and self-driving cars one day having the same ability.
“The leading edge of ultrasonic vehicles can be red-hot, and circular polarization of the light can give us the information about its internal geometry, invisible from the surface,” said Kotov. “We want to investigate in greater depth the effects of chiral geometry on black-body radiation and observe these effects deeper in IR range so that we could use it for object identification.”