US researchers determine the limits of energy absorption in transparent materials

For years, scientists have relied on a simple strategy of increasing the material’s thickness to increase light absorption. Straight and logical. But what if the material was transparent and allowed light to pass? What was the thickness needed for such a material? The answer had remained a mystery.

The Russian solution that worked briefly

More than twenty years ago, Konstantin N. Rozanov, a researcher at the Institute for Theoretical and Applied Electrodynamics in Moscow, Russia, introduced a constraint in the problem that proved useful in solving the puzzle.

Rozanov lined the transparent material with a piece of metal on one side. This helped him figure out the range of wavelengths that would get absorbed in the material since the metal lining either reflected or absorbed the light that came its way.

This allowed researchers to arrive at a mathematical solution to the problem. The issue, however, was that researchers had no clue when the metal edge was removed and the light could pass through.

More importantly, Rozanov had a clever trick that made the solution work. He worked with wavelengths instead of frequency. The metal approach failed to solve the problem when researchers worked with frequency.

Duke researchers step up

Willie Padilla, an electrical and computer engineering professor at Duke University, sought to solve this problem to make better devices. He teamed up with Vahid Tarokh, an electrical and computer engineering professor at Duke. Tarokh’s expertise is working on new formulations and getting the most out of datasets.

Without going into the nitty-gritty of the underlying math, Tarokh managed to see through Rozanov’s trick and pulled out a proverbial rabbit from a mathematical hat. “Hindsight is 20/20, but even mathematicians call these creative strategies’ tricks,’ said Padilla about Tarokh’s work.

In addition to delivering a long-awaited solution, Padilla and Tarokh’s work has some practical applications. An absorbing material with a metal backing does not allow any electromagnetic energy to pass through. However, for certain applications, one would prefer certain frequencies to get through while most are blocked.

For instance, the dangers of electromagnetic radiation from cellular phones are well known. The mathematical solution now allows these to be blocked while still allowing signals from GPS or Bluetooth to pass through. Alternatively, stealth applications can also be developed for military use.

To make life easier for researchers, the math might help teams determine when fundamental limits have been reached, and further optimization efforts will not yield results.

“Much of the physics of the known universe already have fundamental solutions or are too complex to get an exact answer,” added Padilla in the press release. “In any field, finding a truly novel, fundamental, exact result like this is rare.”

The research findings were published in the journal Nanophotonics.