Big Promise for Largest Metalens

Lenses have been focusing light for our imaging, magnifying, and fire-starting needs for a good 2,700 years now. The technology hasn’t changed much in all that time: a shaped and polished piece of glass or crystal redirects light waves using the power of refraction. 

But the past few years have seen the rise of an entirely new method of focusing light. The metalens uses an array of nano-towers of varying sizes—and Fermat’s principle that light follows the path of minimum time—to redirect light waves. Metalenses hold the promise of a world filled with cheap, light, durable imaging. There’s just one problem: The lithography used to make them has kept them necessarily small. Recently, a team of Harvard University researchers found a means for making metalenses large enough for practical purposes. 

Build Your Network: Become an ASME Member

Metalenses are typically created with electron beam lithography, a slow process better suited to prototypes than mass production. Recently, though, Federico Capasso, a physicist at Harvard University, turned to deep-ultraviolet projection lithography to create the billions of nano pillars that make up a metalens. It’s the same tool used to make semiconductor chips and, therefore, produces only the tiniest products. That’s why metalenses have been till now not much bigger than a piece of glitter—20 to 30 millimeters wide at best. 

Now Joon-Suh Park, a former graduate student at SEAS and current postdoctoral fellow in Capasso’s team, has figured out how to use CMOS (complementary metal–oxide–semiconductor) foundry technology to stitch together lenses made with deep-ultraviolet projection lithography. 

“These tools are very, very accurate,” Park said. “The tool I used has a resolution alignment limit of about 25 nanometers—that was within our error tolerance range.” 

25 nm is much smaller than the optical wavelengths they were redirecting.

The result is a lens with a 10-centimeter diameter—big enough to have already captured images of the moon, the sun, the North America Nebula, and the April 2024 total solar eclipse. Its nano pillars are each less than a micron thick and 1.5 microns tall—double the height of the pillars in previous metalenses. 

Such a lens has some serious advantages. For one thing, because the light is focused according to the height of the nanopillars, there’s no need for the thickness of traditional lenses, or multiple lens, like those found in telescopes, microscopes, and conventional cameras to correct and flatten images. That means the metalens will be much lighter—ideal for space applications and earthbound ones, too. 

The lens also withstands extreme temperature variations, another advantage for space. Park and his colleagues put their lenses on a hotplate, brought it up to 200 °C, and then plunged it in liquid nitrogen to bring the temperature to -200 °C, all within five seconds. The lens came out unscathed. Similarly, it survived vibration tests in an “ultrasonic bath” without any visible damage. 

The metalenses are also capable of doing some valuable tricks. 

“We can also include polarization dependent focusing,” Park said. “Which means we can have one image of polarization sent to one sensor and the other to another sensor and have the polarization information extracted from what we’re seeing with a single meta optic, instead of having multiple lenses and multiple trains of optics systems.”

The entire lens—the substrate and nano-pillars—are produced from a single slab of fused silica. That’s what makes it so robust. In fact, metalenses may be tough enough to stand up to the strength of a high-powered laser, which would be useful in applications involved with redirecting beamed energy. 

“You want as large of a lens as possible to reduce the power density,” Park said, “because you don’t want anyone burning in between.”

You Might Also Like: Optical Metamaterials Evolve Using Algorithms

Smaller metalenses are already primed to enter the market. Metalenz, a company started by Capasso and his former graduate student Rob Devlin (who is now its CEO), has designed metalenses for smart phones and a host of other consumer electronics applications, manufactured by one the world’s largest semiconductor foundries, United Microelectronics Corporation (UMC).

Park’s 10-centimeter metalenses are a first effort, and there’s still room for improvement. Right now, the light they collect is monochromatic, and they’re much less efficient than traditional lenses. But Park has plans for improving both. 

“We will integrate structures and what we call dispersion engineering, so we can do broadband, high efficiency meta-surfaces,” Park said. 

But whatever level of efficiency they reach, and however broad the wavelengths they collect, metalenses are already poised to change the world of optics thanks to the method they’re made with and the material they’re made of. Until now, the lens, camera, sensor, and chip have all been separate items that need to be engineered to work together. 

“Now the same foundry can make the electronics, the actual chip, the sensor, and the actual metalens,” Capasso said. “This is huge, a total game changer—no exaggeration.”

Michael Abrams is a technology writer in Westfield, N.J.