Twisted Edison: Bright, elliptically polarized incandescent light

“When producing twisted light using traditional methods such as electron or photon emission, it is difficult to produce sufficient brightness,” said Jun Lu, an adjunct researcher in chemical engineering at the University of Michigan and lead author of this week’s cover study. science.

“We came to notice that we actually had a very ancient way of generating these photons – not relying on photons and electron excitation, but rather like the light bulb that Edison developed.”

Every object that carries heat, including yourself, is constantly emitting photons (light particles) in a spectrum related to its temperature. When an object is the same temperature as its surroundings, it also absorbs an equal number of photons – this is idealized as “blackbody radiation” because black absorbs all photon frequencies.

Although the filament of a tungsten light bulb is much warmer than its surroundings, the law that defines blackbody radiation (Planck’s law) provides a good approximation to the spectrum of photons it emits. Overall, visible photons look like white light, but when you pass light through a prism, you can see a rainbow of different photons within it.

This radiation is also why you appear bright in thermal images, but even room-temperature objects are constantly emitting and receiving blackbody photons, making them also dimly visible.

Typically, the shape of an object that emits radiation doesn’t get much thought—for most purposes (as is often the case in physics), the object can be imagined as a sphere. But while the shape doesn’t affect the wavelength spectrum of different photons, it does affect a different property: their polarization.

Typically, photons from blackbody sources are randomly polarized—their waves may oscillate along any axis. The new study shows that if an emitter is twisted on a micron or nanometer scale, with each twist having a length similar to the wavelength of the emitted light, then blackbody radiation will also be distorted. The strength of a light twist, or its elliptical polarization, depends on two main factors: how close the photon’s wavelength is to the length of each twist and the electronic properties of the material, in this case nanocarbons or metals.

Twisted light is also called “chirality” because clockwise and counterclockwise rotations are mirror images of each other. The purpose of this study was to prove the premise for a more applied project the Michigan team wanted to pursue: using chiral blackbody radiation to identify objects. They envision robots and self-driving cars being able to see like mantis shrimps, distinguishing between different directions of rotation and degrees of distortion of light waves.

“Advances in blackbody radiation physics with chiral nanostructures are at the heart of this research. Such emitters are everywhere around us,” said Owen Langmuir Distinguished Professor of Chemical Sciences and Engineering, National Science Foundation Complex said Nicholas Kotov, Director of the Center for Particles and Particle Systems (COMPASS) and corresponding author of the study.

“For example, these findings could be important for self-driving cars to discern the difference between deer and humans, who emit light with similar wavelengths but different helicities, because deer fur curls differently than our fabrics.”

While brightness is the main advantage of this method of producing twisted light – up to 100 times brighter than other methods – this light encompasses a broad spectrum of wavelengths and distortions. The team has some ideas on how to solve this problem, including exploring the possibility of building lasers that rely on twisted light-emitting structures.

Kotov also wants to explore the infrared spectrum further. The peak wavelength of blackbody radiation at room temperature is approximately 10,000 nanometers or 0.01 millimeters.

“This is a region of the spectrum where there is a lot of noise, but the contrast can be enhanced by elliptical polarization,” Kotov said.

The research was supported by the National Science Foundation through COMPASS and the Office of Naval Research.

Kotov is also the Joseph B. and Florence V. Cejka Professor of Engineering, professor of polymer science and engineering, and a member of the University of Michigan’s BioInterfaces Institute. Lu is an Assistant Professor of Chemistry and Physics at the National University of Singapore.

The device was built in the COMPASS laboratory at the University of Michigan’s North Campus Research Center, and research was conducted at the Michigan Materials Characterization Center.