Researchers confirm the inherent boundaries of electromagnetic energy absorption

March 14, 2024

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by Duke University

Transparent materials with a given thickness can absorb a theoretical fundamental limit of electromagnetic energy as determined by electrical engineers at Duke University. This new understanding will assist engineers to optimize their designs to block certain frequencies of radiation while allowing others to pass through. This can have various applications, such as stealth or wireless communications.

'A truly novel, fundamental, exact result like this is rare. Most of the physics of the known universe already have fundamental solutions or are too complex to require exact answers' says Willie Padilla, a professor of electrical and computer engineering at Duke.

The research is published in Nanophotonics.

There are many situations where certain types of light need to be absorbed, such as when designing an antenna or developing sunscreen. Maximizing this absorption often involves increasing the thickness of the material absorbing the energy. However, until now, the specific thickness necessary for this absorption in a transparent material was unknown.

Over two decades ago, Konstantin N. Rozanov of the Institute for Theoretical and Applied Electrodynamics in Moscow, Russia, calculated the maximum light over a range of wavelengths that a device of specific thickness could absorb if one side was metal-lined. This boundary on one side either reflected back or absorbed all light.

Eliminating the metal edge, and allowing the light to pass through, however, changes the game in terms of the electromagnetic spectrum.

'Many researchers have tried and failed to apply the approach Rozanov used, i.e., working in wavelength instead of frequency,' says Yang Deng, a research assistant in Padilla's lab.

A collaboration between Padilla, Deng, and Vahid Tarokh, the Rhodes Family Professor of Electrical and Computer Engineering at Duke, resulted in a new mathematical approach to solve the problem.

Tarokh was able to structure the problem in such a way as to make it solvable, demonstrating an impressive mathematical strategy.

Besides the novelty of the solution to this problem, the researchers assert that there are real-world applications. For instance, absorbers backed by metal restrict any type of electromagnetic energy passing through while some applications might require blocking certain frequencies with others passing through. For instance, mobile phones might need to block certain harmful electromagnetic radiation while allowing others like GPS or Bluetooth signals to pass through. With these parameters in place, engineers will have a more realistic picture of when optimizing their design would yield no value.

More information about this research: Willie J. Padilla et al, Fundamental absorption bandwidth to thickness limit for transparent homogeneous layers, was published in Nanophotonics in 2024.

This story was provided by Duke University.