Photonics at the macro-scale is more than 50 years old and has applications today in fields such as telecommunications, medicine, aviation, and agriculture. And yet, shrinking all the elements of traditional photonics down to microscale — to match the density of signals and processing operations inside a conventional microchip — involves completely new optical methods and materials.

 

A team of experts at the University of Delaware, including Tingyi Gu, an assistant professor of electrical and computer engineering, just recently publicized a paper in the journal Nature Communications that describes their effort to make a lens from a thin metasurface material on top of a silicon wafer. Gu says that metasurfaces have likely been made from thin metal films with nanosized structures in it. These “plasmonic” metasurfaces offered the promise of, as a Nature Photonics paper from 2017 put it, “Ultrathin, versatile, integrated optical devices and high-speed optical information processing.”

 

The challenge, Gu says, is that these “plasmonic” materials are not correctly transparent like windowpanes. Traveling just fractions of a micrometer can introduce signal loss of a few of the decibels to tens of dB. “This makes it less practical for optical communications and signal processing,” she says.

 

Her group uses a different kind of metasurface made from etched dielectric materials atop silicon wafers. Making optical elements out from dielectric metasurfaces, she says, could sidestep the signal loss problem. Her group’s paper notes that their lens introduces a signal loss of less than one dB.

 

Even a small improvement (and going from handfuls of dB down to fractions of a dB is more than small) will make a big difference, mainly because a real-world photonics chip might one day have many such components in it. And the more lossy the photonics chip, the bigger the amount of laser power needed to be pumped through the chip. More power means more heat and noise, which might ultimately limit the extent to which the chip could be miniaturized. But with her team’s dielectric metasurface lens, “We can make a device much smaller and more compact,” she says.

 

Her group's lens is made from a configuration of gratings etched in the metasurface — following a wavy pattern of vertical lines that looks a bit like the Cisco company logo. Gu’s group was able to achieve some of the familiar properties of lenses, including converging beams with a measurable focal length (8 micrometers) and object and image distance (44 and 10.1 µm). The group further used the device's lensing properties to achieve some type of optical signal Fourier Transform — and this is a property of classical, macroscopic lenses.

 

Gu says that next steps for their device include exploring new materials and to work toward a platform for on-chip signal processing. “We’re trying to see if we can come up with good designs to do tasks as complicated as what traditional electronic circuits can do,” she says. “These devices have the advantage that they can process signals at the speed of light. It doesn’t need logic signals going back and forth between transistors. … It’s going to be fast.”

 

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