Record-breaking material emits infrared light better than it absorbs it, without violating the laws of physics
Charles Blue
contributing writer
Sadie Harley
scientific editor
Robert Egan
associate editor
New results in the journal 糖心视频ical Review Letters detail how a specially designed metamaterial was able to tip the normally equal balance between thermal absorption and emission, enabling the material to better emit infrared light than absorb it.
At first glance, these findings appear to violate Kirchhoff's law of thermal radiation, which states that鈥攗nder specific conditions鈥攁n object will absorb infrared light (absorptivity) in one direction and emit it (emissivity) with equal intensity in another, a phenomenon known as reciprocity.
Over the past decade, however, scientists have begun exploring theoretical designs that, under the right conditions, could allow materials to break reciprocity. Understanding how a material absorbs and emits infrared light (heat) is central to many fields of science and engineering. Controlling how a material absorbs and emits infrared light could pave the way for advances in solar energy harvesting, thermal cloaking devices, and other technologies.
Pioneering experiments conducted by a team of researchers in 2023 yielded tantalizing results. By using a single layer of the magneto-optical material indium arsenide (InAs) and subjecting it to a powerful magnetic field of about one tesla (slightly less powerful than an MRI machine but about 100,000 times more powerful than Earth's magnetic field), the team successfully achieved nonreciprocity. Though this confirmed theoretical predictions, the effect was weak and only operated under a very narrow set of conditions.
The newly reported design, developed by Zhenong Zhang and colleagues at Pennsylvania State University, succeeded in doubling the effect seen previously, making it the first reported observation of "strong" nonreciprocal thermal emission.
To achieve this record-breaking result, Zhang's team created a metamaterial made of five, 440-nanometer-thick layers of electron-doped indium gallium arsenide (InGaAs). The doping concentration increased as the depth increased. The InGaAs layers were then transferred to a silicon substrate.
The sample was then studied with a custom-designed angle-resolved magnetic thermal emission spectroscopy (ARMTES) set up, which heated the sample to 540 Kelvin (512 Fahrenheit) and subjected it to a 5 tesla magnetic field.
Zhang and colleagues then measured the nonreciprocity of the material, demonstrating that it exhibited twice the effect previously reported. This effect persisted over a wide range of angles and a broad range of infrared wavelengths (from 13 to 23 microns).
Zhang states, "Our experiment for the first time realizes strong nonreciprocal emission, with nonreciprocity as high as 0.43, which is much higher than nonreciprocity in literature."
The researchers speculate that further advances in this field may lead to breakthroughs in new classes of thermal diodes and transistors, improved thermophotovoltaic designs, and other heat-management technologies.
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More information: Zhenong Zhang et al, Observation of Strong Nonreciprocal Thermal Emission, 糖心视频ical Review Letters (2025).
Journal information: 糖心视频ical Review Letters
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