Rewriting a century-old physics law on thermal radiation to unlock the potential of energy, sensing and more

A research team from Penn State has broken a 165-year-old law of thermal radiation with unprecedented strength, setting the stage for more efficient energy harvesting, heat transfer and infrared sensing.

Their results, currently available on the arXiv preprint server, are slated to be published in Physical Review Letters on June 23. Their paper was also selected by the journal to be featured on the journal's website as an "Editors' Suggestion."

Scientists have long observed that the capability for a material to absorb electromagnetic radiation—a wave of energy in the form of sunlight and X-rays, among others—at a given wavelength and angle must equal its capability to emit at the same wavelength and angle. This is known as a reciprocal relation, which German physicist Gustav Kirchhoff described in 1860 as Kirchhoff's law of thermal radiation.

While others first broke this law two years ago, the researchers at Penn State demonstrated a dramatic breaking of it. This stronger break is needed to achieve real-world possibilities that were previously not attainable, according to co-first author and Penn State doctoral candidate in mechanical engineering Zhenong Zhang.

"The capability to strongly violate Kirchhoff's law not only provides a dramatically new way to control thermal radiation, but also can improve fundamentally energy and sensing applications," Zhang said. "In the case of reciprocal solar cells for harvesting solar energy, for example, the solar cell needs to emit optical energy back to the sun as required by the Kirchhoff's law. This part of the energy that goes back to the sun is wasted.

"However, if we can have nonreciprocal emitters, we can send the emission toward a different direction. Then we could place another solar cell there to absorb this part of energy, increasing the overall power conversion efficiency. Such a strategy has been theoretically pointed out to enable harvesting solar energy at thermodynamic efficiency limits."

According to Linxiao Zhu, corresponding author and assistant professor of mechanical engineering at Penn State, scientists have theorized for about a decade that Kirchhoff's law of thermal radiation could be broken, although it wasn't until recently that a group directly observed a violation of Kirchhoff's law and measured a difference in the thermal radiation emitted from a heated material and its absorption.

However, according to Zhu, existing demonstrations of nonreciprocal emission and absorption typically do not achieve high contrast between emissivity and absorptivity and over broad wavelength band, which are needed for nonreciprocal-based applications.

"Previously, the experimentally demonstrated contrast between emissivity and absorptivity is typically relatively small, or over a small wavelength band," Zhu said. Non-reciprocity is measured in dimensionless parameters, meaning the confines of the system do not impact the resulting measurement, which is the difference in what was actually absorbed and actually emitted.

In a truly reciprocal system, the expected contrast between emissivity and absorptivity would be zero. "In our work, we observed the strongest contrast as 0.43, and there is also substantial contrast over a broad wavelength band of 10 micrometers. The achieved strong nonreciprocal emission points to great potential for applications."

Prior measurements by other teams were smaller and in narrower bandwidths ¾ for example, 0.22 with a bandwidth of about one micrometer, 0.12 with a bandwidth of 3.5 micrometers, and, around the time of this work, a measurement of 0.34. Zhu said his team achieved these results thanks to their emitter design.

"We designed a structure that has five semiconductor layers each with slightly different compositions," Zhu said. "Because of this material design, the infrared wavelength range where the thermal radiation has multiple resonance peaks, meaning the structure absorbs and emits thermal radiation over multiple wavelengths, so we expect to see the effect over a broad wavelength band."

The researchers also said that their emitter is a very thin film that can be transferred to other surfaces, which sets it apart from previous work and allowed for device integration.

"Our material is grown with a total thickness of about two micrometers, which is thinner than a strand of hair," said co-first author and Penn State mechanical engineering doctoral student Alireza Kalantari Dehaghi. "In our work, enabled by the material system we chose, we transferred the microscale thin film to another substrate, meaning that it could be transferred to various types of devices to increase efficiency in energy conversion, heat transfer and other applications."

The discovery is enabled by the angle-resolved magnetic thermal emission spectrophotometer they custom-designed, according to Zhu. A spectrophotometer can measure electromagnetic radiation in a wavelength-resolved fashion, meaning it can differentiate between various wavelengths to determine the radiation intensity at each wavelength.

"This system allows us to directly measure the thermal emission spectrum over a huge parameter space," Zhu said. "For example, we can measure the thermal emission over broad angular and wavelength bands, we can change the temperature of the object, and we can provide a large magnetic field to the object, which is a key to achieving this strong breaking of Kirchhoff's law in the material studied in this work."

The researchers said they plan to continue exploring non-reciprocity of thermal radiation in a variety of materials.

Pramit Ghosh, doctoral student in mechanical engineering at Penn State, is also an author on the paper.