Researchers at the Advanced Science Research Center (ASRC) at the City University of New York (CUNY) have demonstrated that metasurfaces can generate precisely controlled thermal radiation. This paves the way for a new generation of light sources for military, medical, geological, and space applications.
“Metasurfaces usually are designed to customize laser beams that come out of commercially available lasers,” said Adam Overvig, a former postdoctoral researcher at ASRC, in an email to Interesting Engineering .
“In our case, the metasurface produces thermal light at the same time as customizing it. This could greatly compact optical systems by combining both.”
Thermal radiation is the name given to electromagnetic waves generated from heat in matter. The light bulb is usually used to explain the phenomenon, where light is emitted after the filament material is heated to a certain degree.
While it was a great invention at its time, light from a bulb has its limitations. It is unpolarized and does not offer control over where it is incident, severely limiting its applications. In comparison, light from a laser is polarized, has a defined frequency, and can be well controlled in its direction of propagation. This is why a laser is broadly used for technical applications instead of a regular light bulb.
Researchers have relied on metasurfaces – two-dimensional materials built at the nanoscale to control the light from lasers. Since metasurfaces can also be arranged in an array of nanopillars across surfaces, researchers have controlled the scattering of laser light according to application requirements.
While the controlling element of this approach works at the nanoscale, the laser excitation setup is bulky and expensive. So, the researchers at ASRC have been looking for ways to replace it with something simpler and smaller.
“Our ultimate aim is enabling metasurface technology that does not require external laser sources, but can provide precise control over the way its own thermal radiation is emitted and propagates,” added Overvig, now an assistant professor at Stevens Institute of Technology, in the press release.
Converting theory into application
In 2021, the research team at ASRC released a theoretical paper that showed that a metasurface could generate thermal radiation with features such as defined frequency, customized polarization, and even a wavefront shape that could be used to create a hologram.
The study showed that a well-engineered nanostructure could generate its thermal radiation and control it. “These capabilities initially appear impossible without exotic materials, but we showed that conventional materials like glass and silicon are sufficient if designed carefully,” Overvig explained in an email to IE . But demonstrating it in the real world wasn’t easy.
“Our first design required a 3D structure requiring several fabrication steps and careful alignment. So, we did not pursue the experiment due to the difficulty. We then figured out how to achieve the same but with a single step by using a more sophisticated design,” Overwig told IE , summarizing the research progress since 2021. This was achieved by designing a single-layered, two-dimensional (2D) structure.
Although thermal radiation is unpolarized, the researchers focused on enabling thermal radiation with circularly polarized light, where the electric field oscillates in a rotational manner.
“Circular polarization is not an everyday intuitive phenomenon, though it can be important in nature,” explained Overvig to IE . “I think the mantis shrimp can “see” it, but humans can not. It is often used to study the handedness of molecules like sugar.”
Other researchers have shown that circular polarizations can be split into opposite directions, but there is a limit to how much control is possible on the polarized light. The ASRC team, however, was successful in overcoming this limitation and achieved asymmetric emission of circular polarization in a single direction, the press release said.
“The ability to create compact, lightweight sources with desired spectral, polarization, and spatial features is particularly compelling for applications requiring portability, such as space-based technology, field research in geology and biology, and military operations. This work represents a significant step towards realizing these capabilities,” added Andrea Alu, distinguished professor of physics at CUNY, in the press release.
The research could also help improve light-emitting diodes (LEDs), a cheap and common source of light that is also difficult to control.
The research findings were published in the journal Nature Nanotechnology today.
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