Author: Tronserve admin
Friday 30th July 2021 03:38 AM
Microsize Lens Pushes Photonics Closer to an On-Chip Future
Optical microcomputing, next-generation compact LiDAR units, and on-chip spectrometers all took one step closer to reality with the recent announcement of a new sort of optical lens. The lens isn't fabricated from glass or plastic, however. Rather, this low-loss, on-chip lens is made out of thin layers of specialized materials atop a silicon wafer. These “metasurfaces” have shown much promise in recent years as a kind of new, microscale medium for containing, transmitting, and manipulating light.
Photonics at the macro-scale exceeds 50 years old and has applications today in fields such as telecommunications, medicine, aviation, and agriculture. Nonetheless, shrinking all the elements of traditional photonics down to microscale — to match the density of signals and processing operations inside a standard microchip — involves completely new optical methods and materials.
A group of researchers at the University of Delaware, including Tingyi Gu, an assistant professor of electrical and computer engineering, freshly published a paper in the journal Nature Communications that describes their effort to formulate a lens from a thin metasurface material on top of a silicon wafer. Gu states that metasurfaces have usually 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 problem, Gu says, is that these “plasmonic” materials are not completely transparent like windowpanes. Traveling just fractions of a micrometer can introduce signal loss of a few decibels to tens of dB. “This makes it less practical for optical communications and signal processing,” she says.
Her group uses an alternate kind of metasurface made of etched dielectric materials atop silicon wafers. Making optical parts out from dielectric metasurfaces, she says, could sidestep the signal loss problem. Her group’s paper remarks 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) might make a major difference, as a result of 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 essentially 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 out of 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 a type of optical signal Fourier Transform — and this is a property of classical, macroscopic lenses.
Gu says that next steps for their device include discovering 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.”