Bright, twisted light can be produced thanks to nanostructured filaments with twisted geometry, according to scientists at the University of Michigan.
Planck’s law ignores but does not prohibit black-body radiation (BBR) from being circularly polarized. BBR from nanostructured filaments with twisted geometry from nanocarbon or metal has strong ellipticity from 500 to 3000 nanometers. The submicrometer-scale chirality of these filaments satisfies the dimensionality requirements imposed by fluctuation-dissipation theorem and requires symmetry breaking in absorptivity and emissivity according to Kirchhoff’s law. The resulting BBR shows emission anisotropy and brightness exceeding those of conventional chiral photon emitters by factors of 10 to 100. Image credit: Lu et al., doi: 10.1126/science.adq4068.
“It’s hard to generate enough brightness when producing twisted light with traditional ways like electron or photon luminescence,” said Dr. Jun Lu, a researcher at the University of Michigan.
“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.”
“Every object with any heat to it, including yourself, is constantly sending out photons in a spectrum tied to its temperature.”
“When the object is the same temperature as its surroundings, it is also absorbing an equivalent amount of photons — this is idealized as blackbody radiation because the color black absorbs all photon frequencies.”
While a tungsten lightbulb’s filament is much warmer than its surroundings, the law defining blackbody radiation — Planck’s law — offers a good approximation of the spectrum of photons it sends out.
All together, the visible photons look like white light, but when you pass the light through a prism, you can see the rainbow of different photons within it.
This radiation is also why you show up brightly in a thermal image, but even room-temperature objects are constantly emitting and receiving blackbody photons, making them dimly visible as well.
Typically, the shape of the object emitting the radiation doesn’t get much consideration — for most purposes, the object can be imagined as a sphere.
But while shape doesn’t affect the spectrum of wavelengths of the different photons, it can affect a different property: their polarization.
Usually photons from a blackbody source are randomly polarized — their waves may oscillate along any axis.
The new study revealed that if the emitter was twisted at the micro or nanoscale, with the length of each twist similar to the wavelength of the emitted light, the blackbody radiation would be twisted too.
The strength of the twisting in the light, or its elliptical polarization, depended on two main factors: how close the wavelength of the photon was to the length of each twist and the electronic properties of the material — nanocarbon or metal, in this case.
Twisted light is also called ‘chiral’ because the clockwise and counterclockwise rotations are mirror images of one another.
The study was undertaken to demonstrate the premise of a more applied project that the Michigan team would like to pursue: using chiral blackbody radiation to identify objects.
They envision robots and self-driving cars that can see like mantis shrimp, differentiating among light waves with different directions of twirl and degrees of twistedness.
“The advancements in physics of blackbody radiation by chiral nanostructures is central to this study. Such emitters are everywhere around us,” said University of Michigan’s Professor Nicholas Kotov.
“These findings, for example, could be important for an autonomous vehicle to tell the difference between a deer and a human, which emit light with similar wavelengths but different helicity because deer fur has a different curl from our fabric.”
While brightness is the main advantage of this method for producing twisted light — up to 100 times brighter than other approaches — the light includes a broad spectrum of both wavelengths and twists.
The authors have ideas about how to address this, including exploring the possibility of building a laser that relies on twisted light-emitting structures.
They want to explore further into the infrared spectrum. The peak wavelength of blackbody radiation at room temperature is roughly 10,000 nanometers or 0.01 millimeters.
“This is an area of the spectrum with a lot of noise, but it may be possible to enhance contrast through their elliptical polarization,” Professor Kotov said.
The team’s work was published in the journal Science.
_____
Jun Lu et al. 2024. Bright, circularly polarized black-body radiation from twisted nanocarbon filaments. Science 386 (6728): 1400-1404; doi: 10.1126/science.adq406