Researchers developed a new method to control thermal emission at an engineered interface. Shown here as a wavy red line, the interface can be designed in any shape. Credit: Provided by Coskun Kocabas/University of Manchester and Sahin Ozdemir/Penn State
The researchers state that well-crafted interfaces offer innovative design approaches that go beyond traditional materials, with potential applications in infrared optics, sensors and other fields.
Researchers have created a method to precisely control thermal emissions, potentially transforming thermal management and camouflage technologies. This innovative approach, using topology and non-Hermitian photonics, shows significant promise for applications in satellite technology and beyond.
If a material absorbs light, it will heat up. That heat has to go somewhere, and the ability to control where and how much heat is emitted can protect or even hide devices such as satellites. Researchers from an international team, including members from Penn State, have created a new technique to regulate this heat release. They believe their method has significant potential for advancing thermal management and thermal camouflage technologies.
The team recently published their findings in science.
Led by researchers at the University of Manchester’s National Graphene Institute in England and Penn State College of Engineering in the United States, with experts from Koc University in Turkey and the Vienna University of Technology in Austria, the team demonstrated a way to build an interface that joins two surfaces with different geometric properties to localize thermal emissions from both surfaces, enabling a “perfect” thermal emitter. This means that the designed platform can emit thermal light from contained, defined emission areas with emitting units, or that the platform emits the strongest possible thermal radiation at that temperature.
“We have demonstrated a new class of thermal devices using concepts from topology – a branch of mathematics that studies the properties of geometric objects – and from non-Hermitian photonics, which is a burgeoning area of research that studies light and its interaction with matter in the presence of losses, optical gains and certain symmetries,” said corresponding author Coskun Kocabas, professor of 2D device materials at the University of Manchester.
Achievements and Challenges
The team said the work could advance thermal photonic applications to better generate, control and detect thermal emission. One application of this work could be in satellites, said co-author Sahin Ozdemir, professor of engineering science and mechanics at Penn State. Faced with significant exposure to heat and light, interfaced satellites can emit unit-emission absorbed radiation across a specially designated area designed by researchers to be extremely narrow and in whatever shape is deemed necessary.
Getting to this point, however, was not straightforward, according to Ozdemir. He explained that part of the issue is the limitation of the perfect thermal absorber-emitter at the interface, while the rest of the structures that form the platform remain “cold,” meaning those structures do not absorb or emit any form of energy.
Researchers can vary the thickness of the platinum layer, shown here as a silver circle, with one side thicker than the other. By varying the thickness, the two sides are brought to a “critical junction” point, where light absorbed by each side can be emitted together at the meeting point. This meeting point emits thermal radiation in whatever form the researchers determine. Here, it is shown as a thin red line. Credit: Provided by Coskun Kocabas/University of Manchester and Sahin Ozdemir/Penn State
“Building such a perfect shock absorber has been a huge challenge,” said Ozdemir.
It is slightly easier to build an attenuator-emitter at a desired frequency — as opposed to a perfect attenuator-emitter that can absorb and emit any frequency — by trapping light inside an optical cavity, the researchers said. The optical cavity consists of two mirrors, the first of which only partially reflects the light, while the second reflects the light completely. This configuration enables what the researchers call the “critical state of coupling,” where the incoming light partially reflected by the first mirror and the reflected light trapped between the two mirrors exactly cancel each other out. This completely suppresses reflection, so the light beam is trapped in the system, being perfectly absorbed and then emitted in the form of thermal radiation.
Innovative interface design
“We took a different approach in this work, however, by connecting two structures with different topologies, meaning they absorb and emit radiation differently,” Ozdemir said. “The structures are not at the critical junction point, so they are not considered a perfect absorber-emitter—but their interface exhibits perfect absorption and emission.”
To achieve such an interface, the researchers developed a stacked cavity with a thick layer of gold that perfectly reflects incoming light and a thin layer of platinum that can partially reflect incoming light. The platinum layer, which consists of two separate thicknesses bonded together, also acts as a broadband thermal damper. Between the two mirrors, the researchers placed a transparent dielectric, or material that insulates against electrical conduction, called parylene-C.
Researchers can adjust the thickness of the platinum layer as needed to induce the critical state of coupling at the stitched interface and block incoming light from being perfectly absorbed. They can also move the system away from critical coupling to sub- or supercritical coupling, where perfect absorption and emission cannot occur.
“By fine-tuning the thickness of the platinum layer to a critical thickness of about 2.3 nanometers, we bring the cavity to the critical coupling state 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. “Just by stitching two layers of platinum with thicknesses smaller and larger than the critical thickness on the same dielectric layer, we can create a topological interface of two cavities where perfect absorption and emission are limited. A crucial point here is that the cavities forming the interface are not in the critical state of coupling, but that the interface itself is.
The development challenges the conventional understanding of thermal emission in the field, according to co-author Stefan Rotter, a researcher at the Vienna University of Technology in Austria.
“Any 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 due to its incoherent nature.”
This work, however, showed that thermal emission can be engineered to have topological features, which can create strongly confined states of light that emit only from the topological interface between two surfaces. The researchers said they can also design interface parameters in any shape, from a narrow line to something more complicated, like the outline of the United Kingdom.
According to Kocabas, their approach to building topological systems for radiation control is easily accessible to scientists and engineers.
“This can be as simple as creating a film divided into two regions of different thickness, such that one side satisfies the subcritical junction and the other is in the supercritical junction regime, dividing the system into two topological classes different,” Kocabas. said.
The realized interface exhibits perfect thermal emission, which is protected by the reflection topology and “exhibits robustness to local perturbations and defects,” according to co-author Ali Kecebas, a postdoctoral researcher at Penn State. The team used experiments and numerical simulations to confirm the topological features of the system, as well as the non-Hermitian physics that underpins how the system works.
Reference: “Localized Thermal Emission from Topological Interfaces” by M. Said Ergoktas, Ali Kecebas, Konstantinos Despotelis, Sina Soleymani, Gokhan Bakan, Askin Kocabas, Alessandro Principi, Stefan Rotter, Sahin K. Ozdemir and Coskun Kocabas, 6 June 20 science.
DOI: 10.1126/science.ado0534
Contributors include Sina Soleymani, who earned a doctorate in engineering and mechanical science from Penn State in 2021 when the first phases of this work were completed; Konstantinos Despotelis, Gokhan Bakan and Alessandro Principi, University of Manchester; and Askin Kocabas, Koc University, Turkey.
A European Research Council Consolidator Grant, an Air Force Office Multidisciplinary University Research Initiative (MURI) Award on Programmable Systems with Non-Hermitian Quantum Dynamics, and an Air Force Office of Scientific Research Award supported this work.
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