A Time-Reversed Laser

The width, height and spacing of the cylinders depicted here dictates how the metamaterial absorbs electromagnetic energy. (Source: Duke U.)

Researchers at Duke Uni­versity have discovered that a perfect absorber of electro­magnetic waves can easily be tweaked into a sort of time-reversed laser known as a coherent perfect absorber (CPA). A laser is a device that transforms energy into coherent light, meaning the light waves are perfectly aligned with one another. Reversing the process, a time-reversed laser is a device that absorbs all of the energy from two identical electro­magnetic waves hitting it from either side in perfect synchrony. That is, the crests and troughs of their waves enter the material from either side at precisely the same time.

In 2017, Willie Padilla, professor of electrical and computer engi­neering at Duke, built the first material capable of absorbing nearly 100 percent of an electro­magnetic wave’s energy without containing even an atom of metal. The device was a meta­material, a synthetic material composed of many indi­vidual, engineered features. This parti­cular meta­material featured zirconia ceramic con­structed into a surface dimpled with cylinders like the face of a Lego brick. After compu­tationally modeling the device’s proper­ties by altering the cylinders’ size and spacing, the researchers realized that they had actually created a more funda­mental kind of CPA.

“We’ve studied this system before as a perfect absorber, but now we’ve figured out that this device can be configured to be a CPA as well,” said Padilla. “This study has shown that these seemingly different fields are actually one and the same.” The CPAs currently described in the literature all have only one mode. They work when the incoming electro­magnetic waves are either perfectly aligned or perfectly out of sync. Padilla and Kebin Fan, a research assistant professor in Padilla’s laboratory, have dis­covered that their perfect absorber is actually a CPA with two overlapping modes: it can absorb both aligned and misaligned waves.

By changing the material’s para­meters so that the two modes no longer overlap, Padilla and Fan were able to show it could easily become just like the CPAs currently in the litera­ture, but with much more versa­tility. “Typical CPAs have only one variable, the material’s thickness,” said Fan. “We have three: the cylinders’ radius, height and perio­dicity. This gives us a lot more room to tailor these modes and put them in the frequency spectrum where we want them, giving us a lot of flexibility for tailoring the CPAs.”

Now, the researchers show that their device can switch between absorbing all phases of electro­magnetic waves and only those in sync with one another merely by increasing the height of the cylinders from 1.1 milli­meters to 1.4. With this ease of tran­sition, they believe it should be possible to engineer a material that can dyna­mically switch between the two. “We haven’t done that yet,” said Padilla. “It is challenging, but it’s on our agenda.”

While there aren’t currently any devices that make use of the abilities of CPAs, Padilla and Fan have a few in mind. In principle, researchers could engineer a device that measures not just the inten­sity of incoming light like a normal camera, but also its phase. “If you’re trying to figure out the proper­ties of a material, the more measure­ments you have, the more you can under­stand about the material,” said Padilla. “And while coherent detectors do exist – we have one in our own lab, actually –they’re extremely expensive to build through other tech­nologies.” (Source: Duke U.)

Reference: J. Y. Suen et al.: A Zero‐Rank, Maximum Nullity Perfect Electromagnetic Wave Absorber, Adv. Opt. Mat., online 29 January 2019; DOI: 10.1002/adom.201801632

Link: Padilla Lab, Pratt School of Engineering, Duke University, Durham, USA

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