New ‘metal’ shifts focus without tilting or moving

Polished glass has been at the center of imagery for centuries. The precise curvature allows lenses to focus light and produce sharp images, whether the object in the eye is a single cell, the page of a book, or a distant galaxy.

To change the focus to see clearly on all these scales, you must physically move the lens by tilting, shifting, or otherwise moving the lens, usually using mechanical components that extend most of the microscopes and telescopes.

Now, MIT engineers have produced a tunable “metalwork” that can focus on objects at multiple depths, without changing the physical position or shape. The lens is not made of solid glass, but of a transparent “phase-changing” material that can rearrange the atomic structure after heating and thus change the way the material moves with light.

The researchers etched the surface of the material with small, precise pattern 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 meta-surface changes accordingly. In this case, when the material is at room temperature, the meta-surface focuses light to generate a sharp image of an object at a certain distance away. After the material is heated, its atomic structure changes, and in response, the meta-surface diverts light to focus on a further removed object.

In this way, the new active “metalworkers” can adjust their focus without the need for mechanical elements. The new design, currently imaged inside the infrared band, could enable more lithe optical devices, such as miniature heat scopes for drones, ultra-compact cell phone thermal cameras and low-profile night-vision goggles.

“Our result shows that our ultra-thin adjustable lens, without moving parts, can achieve aberration-free imaging of overlapping objects placed at different depths, as opposed to traditional, bulky optical systems,” said Tian Gu, a researcher at MIT’s Materials Research Laboratory.

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

A material adjustment

The new lens is made from a phase-changing material that the team produced by adapting a material commonly used in rewritable CDs and DVDs. Called GST, it consists of germanium, antimony and tellurium, and the internal structure changes when heated with laser pulses. It allows the material to switch between transparent and opaque conditions – the mechanism that makes it possible to write, erase and rewrite data stored on CDs.

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

The team wondered if the switching capability of GSST could be adjusted to focus light on specific points, depending on the phase. The material can then serve as an active lens, without mechanical components having to shift the focus.

“When one manufactures an optical device, it is very challenging to tune its characteristics to the manufacture,” says Shalaginov. ‘That’s why it’s a holy grail for optical engineers to have this kind of platform [the metalens] to focus effectively and across a wide range.

In the hot seat

In conventional lenses, glass is precisely curved so that the incoming beam of light breaks down the lens at different angles and converges away at a certain point, known as the focal point of the lens. The lenses can then provide a sharp image of any objects at that particular distance. To image objects at a different depth, the lens must be physically moved.

Instead of relying on the fixed curvature of a material to direct light, the researchers looked at GSST-based metalwork in such a way that the focal point changes with the phase of the material.

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

“It’s a sophisticated process to build the meta-surface that varies between different functions, and requires judicious engineering of what shapes and patterns to use,” says Gu. “By 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 metalwork by placing it on a stage and illuminating it with a laser beam tuned to the infrared light band. 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 maps, commonly used to test optical systems.

The lens, in its initial, amorphous state, gave a sharp image of the first pattern. The team then heated the lens to convert the material into a crystal phase. After the transition, and with the removal of the heat source, the lens produced an equally sharp image, this time of the second, further set of bars.

“We demonstrate imaging at two different depths, without any mechanical movement,” says Shalaginov.

The experiments show that metalworkers can actively change focus without any mechanical movements. According to the researchers, metalwork could possibly be manufactured with integrated microheaters to heat the material quickly with short millisecond pulses. By changing the heating conditions, they can also tune to the intermediate states of other materials, enabling continuous focus tuning.

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

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