Researchers demonstrate metasurfaces that control thermal radiation in novel ways

Researchers at the CUNY Graduate Center’s Advanced Science Research Center (CUNY ASRC) have experimentally demonstrated that metasurfaces (two-dimensional materials structured at the nanoscale) can precisely control the optical properties of thermal radiation generated within the metasurface itself. This pioneering work, published in Natural nanotechnologypaves the way for the creation of customized light sources with unprecedented capabilities, impacting a wide range of scientific and technological applications.

Thermal radiation, a form of electromagnetic waves generated by random fluctuations in matter due to heat, is inherently broadband and consists of many colors. The light emitted by an incandescent light bulb is a good example. It is also unpolarized and propagates in all directions due to its randomness. These characteristics often limit its usefulness in applications that require well-defined light properties. In contrast, laser light, known for its defined frequency, polarization, and direction of propagation, is well-defined, making it invaluable for many key applications in modern society.

Metasurfaces offer a more efficient solution by controlling electromagnetic waves through carefully designed nanopillar shapes arranged on their surface. By varying these structures, researchers can control the scattering of light and thus effectively “shape” light in customizable ways. So far, however, metasurfaces have only been developed to control laser light sources, and they require bulky and expensive excitation setups.

“Our ultimate goal is to develop a metasurface technology that does not require external laser sources, but allows for precise control over how its own thermal radiation is emitted and propagated,” said lead author Adam Overvig, a former postdoctoral fellow at CUNY’s ASRC Photonics Initiative and now an assistant professor at Stevens Institute of Technology. “Our work is an important step in this quest, laying the foundation for a new class of metasurfaces that do not require external laser sources, but are powered by internal incoherent oscillations of matter driven by heat.”

Unprecedented control of thermal radiation

The research team had previously published theoretical work showing that a properly designed metasurface could shape the thermal radiation it generates, giving it desirable characteristics such as defined frequencies, customized polarization, and even a desired wavefront shape capable of creating a hologram. This study predicts that, unlike conventional metasurfaces, a properly designed metasurface could both produce and control its own thermal radiation in novel ways.

In this advance, the team set out to experimentally validate these predictions and develop their novel features. The metasurface was obtained by simplifying the previously envisioned, elegant but difficult-to-realize device architecture into a single structured layer with a 2D pattern. This simplified design facilitates fabrication and practical implementation.

While conventional thermal radiation is unpolarized, an important research goal has been to enable thermal radiation with circularly polarized light, where the electric field oscillates in a rotational manner. Recent work has shown that opposite circular polarizations (rotating with left-handed and right-handed characteristics, respectively) can be split into opposite directions, but there appeared to be a fundamental limit to further control of the polarization of the emitted light.

The team’s new design transcends this limitation, enabling asymmetric circularly polarized emission toward a single direction, demonstrating full control over thermal emission.

“Customized light sources are essential to many areas of science and technology,” said Andrea Alù, Distinguished Professor and Einstein Professor of Physics at the Graduate Center of the City University of New York and founding director of the CUNY ASRC Photonics Initiative. “The ability to create compact, lightweight sources with desired spectral, polarization, and spatial characteristics is particularly compelling for applications requiring portability, such as space technology, field research in geology and biology, and military operations. This work represents an important step toward realizing these capabilities.”

The team noted that the principles applied in their current work can be extended to light-emitting diodes (LEDs), with the potential to improve another very common and cheap light source that is notoriously difficult to control.

In the future, the research team wants to combine these building blocks to achieve more complex thermal emission patterns, such as focusing the thermal emission on a specific point above the device or creating a thermal hologram. Such advances could revolutionize the design and functionality of custom light sources.