Scientists find a way around a 160-year-old law of physics to create programmable heat

There are certain laws of physics that heat must follow.

Take Kirchhoff's law of thermal radiation, for example, which applies the idea of reciprocity to heat, and dictates that a surface's ability to absorb heat at a specific angle and wavelength must also match its ability to emit heat at the same angle and wavelength.

It's a rule that makes thermal energy difficult to control in ways we might like to, and although workarounds have been found before, they're inefficient and volatile.

In a new study published in Laser & Photonics Reviews and carried out by an international team of researchers, we may have a way to separate absorption and emission to achieve programmable heat.

The innovation is based on the manipulation of light using a magnetic field: the direction of the heat emission can therefore be controlled, the manipulation can be switched on and off, and the system can even remember its state while powered off.

"We made heat radiation behave in a smarter way," says physicist Shunsuke Murai, from Osaka Metropolitan University in Japan.

"Achieving these capabilities in a working model could enable a new generation of efficient infrared emitters, thermal energy devices, sensors, and photonic memory technologies."

The device the researchers imagined is called a metagrating. It combines a magneto-optical material that adjusts the behavior of absorbed heat when hit by a magnetic field, and a phase-change material that acts as a memory bank.

If you're a physical media fan, you might be interested to know that the phase-change material here – which can switch between amorphous (disordered) and crystalline (ordered) states – is Ge₂⁢Sb₂⁢Te₅, an alloy of germanium, antimony, and tellurium also used in rewritable CDs and DVDs.

That 'grating' is important too. Tiny, carefully designed ridges would trap and channel the incoming light, making it more manageable than previous systems, and more viable as a practical solution.

By adjusting the angle of the light, the strength of the magnetic field, and the physical dimensions of the grating, the researchers could 'program' the desired heat absorption behavior – without the same reciprocal heat emissions.

"By simply adjusting the incident angle, the operating wavelength of the nonreciprocal absorber can be continuously tuned across a broad spectral range," write the researchers.

That flexibility and versatility mean the programmable device would have a wide range of potential applications. This is all theoretical physics and math at the moment, though – albeit well-calculated and modeled – and the next stage is to actually build a prototype.

"This work establishes a rigorous physical framework for active non-reciprocal thermal control, paving the way for next-generation chip-scale thermal photonics," write the researchers in their published paper.

The published study describes decoupling heat emission from heat absorption as a "critical frontier in modern thermal photonics", though it's worth pointing out that the research focused mainly on absorption – the emission part was largely assumed rather than explored in detail.

Requiring an external magnetic field to control the properties of the device and its material adds a few extra wrinkles too, though there's no reason why this should present a significant stumbling block to further development.

There are of course a vast number of systems and technologies that make use of light and heat and their manipulation, and these findings could be useful across many of them. It's another reminder that the laws of physics are there to be broken.

Related: Startling Exception Discovered to 200-Year-Old Law of Physics

"Our ultimate goal is to develop compact devices that can actively control heat radiation, much like electronic circuits control the flow of electricity," says physicist Koichi Okamoto, from Osaka Metropolitan University.

"Such devices could be used in smarter infrared sensors, more efficient energy systems, and new types of photonic memory that store information using light and heat instead of electrical charges."

The research has been published in Laser & Photonics Reviews.

This article was fact-checked by Michael Irving and edited by Rebecca Dyer. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.

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