Researchers demonstrate metasurfaces that control thermal radiation in an unprecedented way

Researchers at the Advanced Science Research Center at the CUNY Graduate Center (CUNY ASRC) have experimentally demonstrated that metasurfaces (two-dimensional nanostructured materials) can precisely control the optical properties of thermal radiation generated within the metasurface itself. This groundbreaking work, published in Nature Nanotechnologypaves the way for the development of customized light sources with unprecedented capabilities and has implications for a wide range of scientific and technological applications.

Thermal radiation – a form of electromagnetic waves created by heat-induced random fluctuations in matter – is broadband in nature and consists of many colors. A good example is the light emitted by a light bulb. It is also unpolarized and spreads in all directions due to its randomness. These properties often limit its usefulness in applications that require well-defined properties of light. In contrast, laser light, known for its defined frequency, polarization and direction of propagation, is well-defined, making it invaluable for many important applications in modern society.

Metasurfaces offer a solution for greater utility by controlling electromagnetic waves through carefully engineered shapes of nanopillars arrayed across their surfaces. By varying these structures, researchers can control light scattering and effectively “shape” light in a customizable way. However, so far, metasurfaces have only been developed to control laser light sources and require bulky, expensive excitation setups.

“Our ultimate goal is to enable a metasurface technology that does not require external laser sources but instead provides precise control over the way its own thermal radiation is emitted and propagated,” said one of the paper’s lead authors, Adam Overvig, a former postdoctoral fellow at the CUNY ASRC’s Photonics Initiative and currently an assistant professor at the Stevens Institute of Technology. “Our work is an important step along this path and lays the foundation for a new class of metasurfaces that do not require external laser sources but are powered by internal incoherent matter vibrations driven by heat.”

Unprecedented control over 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 properties such as defined frequencies, user-defined polarization, and even a desired wavefront shape that can create a hologram. This study predicted that, unlike conventional metasurfaces, an appropriately engineered metasurface could generate and control its own thermal radiation in novel ways.

With the current breakthrough, the team aimed to experimentally confirm these predictions and build on their new functionalities. The metasurface was achieved by simplifying the previously planned device architecture, which was elegant but difficult to realize, to a single structured layer with a 2D pattern. This optimized design makes it easier to manufacture and put into practice.

While conventional thermal radiation is unpolarized, a major research focus has been on enabling thermal radiation using circularly polarized light, where the electric field oscillates in a rotational manner. Recent work had shown that opposite circular polarizations (rotating with left- and right-handed features, respectively) could be split into opposite directions, but there seemed to be a fundamental limit to further control of the polarization of the emitted light.

The team’s new design overcomes this limitation by enabling asymmetric emission of circular polarization in a single direction, thus providing complete control over heat emission.

“Individual light sources are essential to a number of scientific and technological fields,” said Andrea Alù, Distinguished Professor and Einstein Professor of Physics at the City University of New York Graduate Center and founding director of the CUNY ASRC Photonics Initiative. “The ability to create compact, lightweight sources with desired spectral, polarization, and spatial properties is particularly attractive for applications that require portability, such as space-based technology, field research in geology and biology, and military operations. This work represents a significant step toward realizing these possibilities.”

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

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