Penn State Researchers Break 165-Year-Old Physics Law, Paving Way for Advanced Energy and Sensing Technologies

In a groundbreaking achievement, a research team from Pennsylvania State University has shattered a 165-year-old physics principle known as Kirchhoff’s law of thermal radiation. This discovery, detailed in a paper available on the arXiv preprint server and set to be published in *Physical Review Letters* on June 23, 2025, was selected as an “Editors’ Suggestion” by the journal. The breakthrough promises significant advancements in energy harvesting, heat transfer, and infrared sensing.

Kirchhoff’s law, established in 1860 by German physicist Gustav Kirchhoff, states that a material’s ability to absorb electromagnetic radiation, such as sunlight or X-rays, at a specific wavelength and angle must equal its ability to emit radiation at the same wavelength and angle. While this law was first violated two years ago, the Penn State team, led by Assistant Professor Linxiao Zhu, achieved a far stronger break, demonstrating a contrast of 0.43 in non-reciprocity over a broad 10-micrometer wavelength band—surpassing previous measurements of 0.22, 0.12, and 0.34 with narrower bandwidths.

This significant violation of Kirchhoff’s law opens new possibilities for controlling thermal radiation, with applications in solar energy, heat management, and sensing technologies. “The capability to strongly violate Kirchhoff’s law provides a dramatically new way to control thermal radiation and can improve energy and sensing applications,” said Zhenong Zhang, co-first author and a doctoral candidate in mechanical engineering at Penn State. For example, nonreciprocal solar cells could redirect emitted energy to another cell, boosting overall efficiency to theoretical thermodynamic limits.

The team’s success stems from a novel emitter design featuring a thin film of five semiconductor layers with varying compositions. This structure, only two micrometers thick—thinner than a human hair—creates multiple resonance peaks, enabling strong nonreciprocal emission across a wide wavelength range. The film’s transferability to other surfaces further enhances its potential for integration into various devices. The breakthrough was made possible by a custom-designed angle-resolved magnetic thermal emission spectrophotometer, which allowed the team to measure thermal emission across broad angular and wavelength bands under varying temperatures and magnetic fields. “This system is key to achieving such a strong breaking of Kirchhoff’s law,” Zhu explained.

The researchers plan to further explore non-reciprocity in thermal radiation across different materials, potentially unlocking even more applications. This discovery marks a pivotal step toward revolutionizing energy conversion and thermal management technologies.