Breakthrough in Control of Thermal Radiation

Breakthrough in Control of Thermal Radiation : Metasurfaces control thermal radiation in unprecedented ways in a groundbreaking advance,  researchers at the CUNY Graduate Center Advanced Science and Research Center (CUNY ASRC) have experimentally demonstrated that metasurfaces (two-dimensional materials with nanoscale structures) can precisely control the optical properties of thermal radiation generated within the metasurface itself.  Published in Nature Nanotechnology, this groundbreaking study (“Local control of polarization and geometric phase in thermal metasurfaces”) paves the way for the development of customized light sources with unprecedented capabilities, impacting a wide range of scientific and technological applications.

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

Metasurfaces offer a solution to increase their utility by controlling electromagnetic waves through the carefully engineered shapes of nanopillars arranged across their surfaces. By varying these structures, researchers can control the scattering of light, effectively “shaping” it in customizable ways. However, so far, metasurfaces have only been developed for the purpose of controlling laser sources, requiring large and expensive excitation setups. “Our ultimate goal is to enable a metasurface technology that can precisely control how its own thermal radiation is emitted and propagated without the need for an external laser source,” said Adam Overvig, one of the paper’s lead authors and a former postdoctoral researcher at the CUNY ASRC’s Photonics Initiative and now an assistant professor at Stevens Institute of Technology.

“Our work is an important step on this path and forms the basis of a new class of metasurfaces that do not require an external laser source and are driven by incoherent vibrations inside a thermally driven material.”

Unprecedented control of thermal radiation (Breakthrough in Control of Thermal Radiation)

The research team previously published a theoretical paper (Physical Review) that showed how holograms can be created with defined frequencies, custom polarizations, and even desired wave front shapes. The study predicted that a properly designed metasurface could generate and control its own thermal radiation in a new way, unlike conventional metasurfaces.

With this breakthrough in hand, the team set out to experimentally verify these predictions and build new functions. The metasurfaces were made possible by simplifying previously envisioned elegant but difficult-to-implement device architectures into a single structured layer with a 2D pattern. This optimized design allows for easier manufacturing and practical implementation.

Traditional thermal radiation is unpolarised, but the main focus of the research has been on enabling thermal radiation with circularly polarized light, where the electric field rotates and oscillates. Recent studies have shown that opposite circular polarizations (rotating in left and right characteristics, respectively) can be split into opposite directions, but there seemed to be fundamental limitations to further control the polarization of the emitted light. The team’s new design overcomes this limitation, enabling asymmetric emission of unidirectional circularly polarized light, demonstrating complete control of thermal radiation.

“Customized light sources are essential for many scientific and technological fields,” said Andrea Alù, Distinguished Professor of Physics and Einstein Professor 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 light sources with desirable spectral, polarization, and spatial properties is particularly interesting for applications requiring portability, such as space-based technologies, geological and biological field studies, and military operations.

This work represents an important step towards realizing these possibilities.” The research team found that the principles used in the current study can also be extended to light-emitting diodes (LEDs), potentially improving another very common and inexpensive light source that is notoriously difficult to control. Going forward, the research team hopes to combine these building blocks to achieve more complex thermal radiation patterns, such as focusing thermal radiation at specific points on a device or creating thermal holograms. Such advances could revolutionize the design and functionality of custom light sources.

Source: Advanced Science Research Center, GC/CUNY