Polished glass has been at the center of imaging systems for centuries. Its precise curvature allows the lenses to focus light and produce sharp images, whether the object in view is a single cell, a book page, or a distant galaxy.
Changing focus to see clearly at all of these scales generally requires physically moving a lens, tilting, sliding, or otherwise moving it, usually with the help of mechanical parts that are added to most microscopes and telescopes.
Now MIT engineers have made adjustable “metalenses” 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 “phase-change” material that, after heating, can rearrange its atomic structure and thus change the way the material interacts with light.
The researchers etched the surface of the material with tiny structures with precise patterns that work together as a “metasurface” to refract or reflect light in unique ways. As the material property changes, the optical function of the metasurface varies accordingly. In this case, when the material is at room temperature, the metasurface focuses the light to produce a sharp image of an object at a distance. After the material is heated, its atomic structure changes and, in response, the metasurface redirects the light to focus on a more distant object.
In this way, the new active “metalenses” can sharpen their focus without the need for bulky mechanical elements. The novel design, which currently takes images within the infrared band, can enable more agile optical devices such as miniature thermal sights for drones, ultra-compact thermal cameras for mobile phones and low-profile night vision goggles.
“Our result shows that our ultra-thin tunable lens, with no moving parts, can achieve aberration-free images of overlapping objects placed at different depths, rivaling traditional and bulky optical systems,” says Tian Gu, research scientist at the Materials Research Laboratory of MIT.
Gu and his colleagues published their results today in the journal Communications from nature. 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 the MIT Lincoln Laboratory; and collaborators from the University of Massachusetts at Lowell, the University of Central Florida, and Lockheed Martin Corporation.
A material retouch
The new lens is made from a phase-change material that the team manufactured by fitting a material commonly used on rewritable CDs and DVDs. Called GST, it is made up of germanium, antimony, and tellurium, and its internal structure changes when heated with laser pulses. This allows the material to switch between transparent and opaque states, the mechanism that allows data stored on CDs to be written, erased and rewritten.
Earlier this year, researchers reported adding another element, selenium, to GST to make a new phase-change material: GSST. When they heated the new material, its atomic structure changed from a random, amorphous tangle of atoms to a more ordered crystalline structure. This phase change also changed the way infrared light traveled through the material, affecting refractive power but with minimal impact on transparency.
The team wondered if GSST’s switching capability could be adapted to direct and focus light at specific points depending on its phase. The material could then serve as an active lens, without the need for mechanical parts to change its focus.
“In general, when you manufacture an optical device, it is very difficult to adjust its characteristics after manufacturing,” says Shalaginov. “That is why having this type of platform is like a holy grail for optical engineers, allowing [the metalens] to shift focus efficiently and over a wide range. “
In the hot seat
In conventional lenses, the glass is precisely curved so that the incoming light beam is refracted from the lens at various angles, converging at a point at a certain distance, known as the lens focal length. Lenses can produce a sharp image of any object at that particular distance. To image objects at a different depth, the lens must physically move.
Rather than relying on a material’s fixed curvature to direct light, the researchers sought to modify GSST-based metalenses so that the focal length changes with the material’s phase.
In their new study, they fabricated a 1-micron thick layer of GSST and created a “metasurface” by etching the GSST layer into microscopic structures of various shapes that refract light in different ways.
“It’s a sophisticated process to build the meta surface that switches between different functionalities, and it requires judicious engineering of what kind of shapes and patterns to use,” Gu says. “By knowing how the material will behave, we can design a specific pattern that will focus on one point in the amorphous state and change to another point in the crystalline phase.”
They tested the new metalenses by placing them on a stage and illuminating them with a laser beam tuned to the infrared light band. At certain distances in front of the lens, they placed transparent objects made up of double-sided horizontal and vertical bar patterns, known as resolution graphs, which are typically used to test optical systems.
The lens, in its initial amorphous state, produced a sharp image of the first pattern. The team then heated the lens to transform the material into a crystalline phase. After the transition, and with the heat source removed, the lens produced an equally sharp image, this time from the second farthest set of bars.
“We show images at two different depths, without any mechanical movement,” says Shalaginov.
Experiments show that a metalens can actively change focus without any mechanical movement. The researchers say that a metalens could potentially be made with built-in microheaters to rapidly heat the material with short pulses of milliseconds. By varying the heating conditions, they can also be tuned to the intermediate states of other materials, allowing continuous focal adjustment.
“It’s like cooking a steak: you start with a raw steak and it can be well done, or it can be half raw and anything in between,” says Shalaginov. “In the future, this unique platform will allow us to arbitrarily control the focal length of the metalenses.”