By Amit Malewar 15 Jun, 2024
Collected at: https://www.techexplorist.com/novel-method-controlling-thermal-emission/85140/
A material’s ability to control the emission of heat can have far-reaching implications, from protecting satellites to developing thermal camouflage technologies. An international research team, including experts from Penn State, has devised an innovative method for managing thermal emissions.
Led by researchers from The University of Manchester and Penn State, along with collaborators from Koc University and Vienna University of Technology, the team has successfully localized thermal emissions from surfaces with different geometric properties, creating a “perfect” thermal emitter.
This breakthrough enables the platform to emit thermal light from designated areas with maximum efficiency, offering promising prospects for thermal management and camouflage technologies.
“We have demonstrated a new class of thermal devices using concepts from topology – a branch of mathematics studying properties of geometric objects – and from non-Hermitian photonics, which is a flourishing area of research studying light and its interaction with matter in the presence of losses, optical gain, and certain symmetries,” said corresponding author Coskun Kocabas, professor of 2D device materials at The University of Manchester.
The team’s groundbreaking work has the potential to significantly advance thermal photonic applications, allowing for improved generation, control, and detection of thermal emission. Co-author Sahin Ozdemir, professor of engineering science and mechanics at Penn State, highlighted a potential application in satellites.
With this technology, satellites could efficiently emit absorbed radiation along a specifically designated area tailored to their unique needs. However, achieving this feat was not easy, as Ozdemir explained the challenge of limiting the perfect thermal absorber-emitter to the interface while keeping the rest of the structures cold, free from absorbing or emitting any form of energy.
“Building such a perfect absorber-emitter has been a major challenge,” Ozdemir said.
The researchers have discovered a method to construct an absorber-emitter at a desired frequency using an optical cavity, which involves two mirrors. The first mirror partially reflects light, while the second completely reflects light. This setup achieves the “critical coupling condition” where incoming and reflected light cancel out exactly, resulting in perfect absorption and emission of thermal radiation.
“We took a different approach in this work, though, by bridging two structures with different topologies, meaning they absorb and emit radiation differently,” Ozdemir said. “The structures are not at the critical coupling point, so they are not considered a perfect absorber-emitter – but their interface exhibits perfect absorption and emission.”
In order to create such an interface, the team devised a cavity consisting of a thick gold layer for perfect light reflection and a thin platinum layer for partial light reflection. The platinum layer, combined with two different thicknesses, also serves as a broad-spectrum thermal absorber-emitter. Positioned between the two mirrors is a transparent dielectric known as parylene-C, which insulates against electrical conductivity.
By adjusting the thickness of the platinum layer, the researchers can achieve the critical coupling condition at the stitched interface, capturing incoming light for perfect absorption. They also have the flexibility to move the system from critical to sub- or super-critical coupling, where perfect absorption and emission are not achievable.
“By finely tuning the thickness of the platinum layer to a critical thickness of about 2.3 nanometers, we bring the cavity to the critical coupling condition where the system exhibits perfect absorption and, as a result, perfect emission,” said first author M. Said Ergoktas, a research associate in materials engineering at The University of Manchester.
“Only by stitching two platinum layers with thicknesses smaller and larger than the critical thickness over the same dielectric layer can we create a topological interface of two cavities where perfect absorption and emission are confined. A crucial point here is that the cavities forming the interface are not at critical coupling condition, but that the interface itself is.”
The researchers’ findings challenge traditional notions of thermal emission, as they demonstrate the ability to engineer thermal emission with topological characteristics. “Every hot object radiates heat in the form of incoherent, random light,” Rotter said. “Traditionally, it has been believed that thermal radiation cannot have topological properties because of its incoherent nature.”
This enables the creation of highly confined states of light that emit exclusively from the topological interface between two surfaces.
Furthermore, the researchers have shown that the parameters of the interface can be designed to any shape, from a narrow line to more complex geometries. According to Kocabas, their approach to constructing topological systems for controlling radiation is easily accessible to scientists and engineers.
“This can be as simple as creating a film divided into two regions with different thicknesses such that one side satisfies sub-critical coupling, and the other is in the super-critical coupling regime, dividing the system into two different topological classes,” Kocabas said.
The innovative interface demonstrates remarkable thermal emissivity, safeguarded by the reflection topology and “exhibiting resilience against local disruptions and flaws,” as stated by co-author Ali Kecebas, a postdoctoral scholar at Penn State. The team conducted experiments and numerical simulations to validate the system’s topological attributes and the non-Hermitian physics that underlies its functionality.
Journal reference:
- M. Said Ergoktas, Ali Kecebas, Konstantinos Despotelis, Sina Soleymani, Gokhan Bakan, Askin Kocabas, Alessandro Principi, Stefan Rotter, Sahin K. Ozdemir, Coskun Kocabas.Localized thermal emission from topological interfaces. Science, 2024; DOI: 10.1126/science.ado0534
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