Metalens Uses Heat Rather Than Motion to Shift Focus

JOEL WILLIAMS, ASSOCIATE EDITOR
joel.williams@photonics.com

A new metalens design developed by researchers at MIT, which is able to change its focus without tilting or moving forward or backward, may find use in miniature heat scopes for drones, ultracompact thermal cameras for cellphones, and low-profile night-vision goggles, its developers said. The lens technology uses heat to focus in on different objects, operating in the infrared band.

In current architectures, glass must move for a lens to be focused onto an object or at a point.

To create the lens, researchers looked to previous advances with GST (germanium-antimony-tellurium), a material commonly used in rewriteable CDs and DVDs and favored for its ability to shift between transparent and opaque states when heated by laser light. In their previous work, the MIT researchers had incorporated another element — selenium. The new material, GSST, is optically transparent and refracts light in the infrared band. It has an amorphous, chaotically ordered structure at room temperature. Once heated with laser light, it changes to an organized crystalline structure.

As the material’s structure changes, so do its optical properties. Refracting power changes, though transparency remains largely the same.

The metalens, made from GSST, uses carefully patterned structures to refract infrared light. When heated, its structure changes, which enables it to change its focus. Courtesy of Tian Gu et al.

With that work in mind, the researchers investigated its potential in metalens designs. To create a lens, the material is etched with miniscule, precisely patterned structures that combine to refract or reflect light. The researchers fabricated the lens with a 1-µm-thick layer of GSST.

“It’s a sophisticated process to build the metasurface that switches between different functionalities, and requires judicious engineering of what kind of shapes and patterns to use,” said Tian Gu, a researcher in MIT’s Materials Research Laboratory. “By knowing how the material will behave, we can design a specific pattern which will focus at one point in the amorphous state, and change to another point in the crystalline phase.”

The researchers tested the metalens by placing it on a stage and then illuminating it with an infrared laser. At certain distances in front of the lens, they placed resolution charts — transparent objects composed of double-sided patterns of horizontal and vertical bars — typically used to test optical systems.

In the initial amorphous state, the lens produced a sharp image of the first pattern. The researchers then heated the lens to shift the material to its crystalline phase. With the transition finished and the heat source removed, the lens produced an equally sharp image of the second, further set of bars.

“Our result shows that our ultrathin tunable lens, without moving parts, can achieve aberration-free imaging of overlapping objects positioned at different depths, rivaling traditional, bulky optical systems,” Gu said.

Those bulky optical systems typically require several lenses to produce an image free from aberration. When using manual focus on a typical SLR style camera, a ring on the barrel of the lens system is turned, which mechanically alters the positions of the internal lensing systems back and forth to bring objects of varying distances into focus. For metalenses with fixed optical properties, changing focus would necessitate a similar system.

“The work is significant because it enables tuning of optics without mechanical moving parts and at the same time maintaining diffraction limited performance for the first time,” postdoctoral associate Mikhail Shalaginov told Photonics Media. “The latter is important as it ensures that the image sharpness is not compromised during the tuning process.”

Applications and next steps

According to the researchers, a metalens could potentially be fabricated with integrated microheaters, expeditiously heating the material with short millisecond pulses. By varying the heating conditions, they could also tune to the material’s intermediate states, enabling continuous focal tuning.

“We can use integrated microheaters to control the phase change reversibly,” Gu told Photonics Media. “For amorphization, a short and large pulse is sent to bring the material to above melting point, and then quench to maintain the amorphous state. For crystallization, a longer and smaller pulse is applied to keep the material at below the melting point.”

The material is nonvolatile; it does not need to be maintained at a certain temperature to hold focus, and the focusing process itself is very quick.

“For amorphization it is a few microseconds, and for crystallization we can probably go down to approximately one millisecond based on our current material,” Juejun Hu, principal investigator in the Photonic Materials lab and associate professor of materials science and engineering, told Photonics Media. “It is possible to tune the composition so the crystallization is faster/slower. The switching speed is probably adequate for the vast majority of applications as it is much faster than mechanically moving lenses.”

Looking ahead, the researchers are working to develop the technology for use in practical applications. Currently they have their sights set on a parafocal zoom lens based on this technology.

“Once realized, we can significantly simplify zoom lens architecture, and again, without moving parts,” Hu said.

The researchers also expressed interest in realizing compact optical modules that can be integrated with standard instruments such as infrared imagers or microscopes.

The research was published in Nature Communications (www.doi.org/10.1038/s41467-021-21440-9).