The new “metalens” shifts the focus without tilting or shifting

The design can enable miniature zoom lenses for drones, mobile phones or night vision goggles

Polished glass has been at the heart of the image processing system for centuries. Their precise curvature allows the lenses to focus light and produce sharp images, whether the object being seen is a single cell, a page of a book, or a distant galaxy.

Changing the focus to see clearly on all of these scales usually requires physically moving the lens by tilting, sliding, or otherwise moving the lens, usually with the help of mechanical parts that add the bulk of the microscope and telescope.

Now, MIT engineers have created an adaptable “metalens” that can focus on objects at multiple depths, without changes in their physical position or shape. The lens is not made of solid glass, but of a transparent material that “changes phase”, which after heating can rearrange its atomic structure and thus change the way the material interacts with light.

Researchers have eroded the surface of the material with tiny, precisely sampled structures that work together as a “meta-surface” to refract or reflect light in unique ways. As the property of the material changes, the optical function of the metasurface varies accordingly. In this case, when the material is at room temperature, the metasurface focuses light giving a sharp image of the object at a certain distance. Once the material heats up, its atomic structure changes, and in response, the metasurface redirects light to focus on a more distant object.

In this way, the new active “metalens” can adjust its focus without the need for bulky mechanical elements. The new design, which is currently imaging in the infrared, can enable more agile optical devices, such as miniature heat ranges for drones, ultra-compact thermal cameras for mobile phones and low-profile night vision goggles.

“Our result shows that our ultra-thin adjustable lens, without moving parts, can capture overlapping objects located at different depths without aberration, competing with traditional, bulky optical systems,” says Tian Gu, a scientist at MIT’s Materials Research Laboratory.

Gu and his colleagues published their results today in the journal Nature Communications. Its co-authors are Juejun Hu, Mikhail Shalaginov, Yifei Zhang, Fan Yang, Peter Su, Carlos Rios, Qingyang Du and Anuradha Agarwal from MIT; Vladimir Liberman, Jeffrey Chou and Christopher Roberts of the MIT Lincoln Laboratory; and associates at the University of Massachusetts at Lowell, the University of Central Florida, and Lockheed Martin Corporation.

Material adjustment

The new lens is made of phase-shifting material, which the team made by tweaking the material commonly used on rewritable CDs and DVDs. Called GST, it contains germanium, antimony and tellurium, and its internal structure changes by heating with laser pulses. This allows the material to switch between transparent and opaque state – a mechanism that allows you to write, erase and rewrite data stored on CDs.

Earlier this year, researchers reported adding another element, selenium, to GST to make a new phase-changing material: GSST. When the new material was heated, its atomic structure shifted from an amorphous random web of atoms to a more ordered, crystalline structure. This phase shift also changed the way infrared light passes through the material, affecting refractive power but with minimal impact on transparency.

The team wondered if GSST’s switching ability could be adjusted to direct and focus light at specific points, depending on the phase. The material could then serve as an active lens, without the need for mechanical parts to shift the focus.

“In general, when someone makes an optical device, it is very difficult to adjust its characteristics after production,” says Shalaginov. “That’s why it’s like having a platform like the Holy Grail for optical engineers, which makes it possible [the metalens] for efficient and large focus shifting. “

In a hot seat

In conventional lenses, the glass is precisely curved so that the incoming beam of light is refracted from the lens at different angles, converging at a certain distance known as the focal length of the lens. Lenses can then create a sharp image of any object at that particular distance. To shoot objects at different depths, the lens must be physically moved.

Instead of relying on the fixed curvature of the light-directing material, the researchers sought to modify the GSST-based metalens in such a way that the focal length changes with the phase of the material.

In their new study, they produced a 1 micron-thick GSST layer and created a “metasurface” by carving a GSST layer into microscopic structures of different shapes that refract light in different ways.

“It’s a sophisticated process for building a meta-surface that switches between different functionalities and requires prudent engineering of what shapes and patterns are used,” Gu says. “Knowing how the material will behave, we can design a specific pattern that will focus at one point in the amorphous state and change to another point in the crystal phase.”

They tested the new metalens by setting it on stage and illuminating it with a laser beam tuned to an infrared belt of light. At certain distances in front of the lens, they placed transparent objects consisting of double-sided patterns of horizontal and vertical bars, known as resolution diagrams, which are commonly used to test optical systems.

The lens in its initial amorphous state gave a sharp image of the first sample. The team then heated the lens to transform the material into a crystalline phase. After the transition and after removing the heating source, the lens created an equally sharp image, this time of a second, further row of rods.

“We show the recording at two different depths, without any mechanical movements,” says Shalaginov.

Experiments show that metalens can actively change focus without any mechanical movements. The researchers say the metalens could potentially be produced with integrated microheaters to heat the material quickly with short millisecond pulses. By changing the heating conditions, they can also adapt to the middle conditions of the other material, allowing continuous focal adjustment.

“It’s like cooking a steak – it starts with a raw steak and can go up to a well-done one, or it could be done medium infrequently, and anything else in between,” Shalaginov says. “In the future, this unique platform will allow us to arbitrarily control the focal length of the metal.”

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