Polished glass has been at the center of image systems for centuries. Their precise curvature allows the lens to focus light and create sharp images, whether the object in sight is a single cell, a page on a book, or a distant galaxy.
Changing the focus to see light at all of these scales usually requires physically moving a lens, tilting, sliding, or otherwise changing the lens, usually with the help of mechanical parts that add an extra portion of microscopes and telescopes.
Now WITH ONE engineers have manufactured tunable “metalens” that can be focused on multi-depth objects without any change in physical position or shape. The lens is not made of solid glass, but of a transparent material that “changes in phase,” which, once heated, can rearrange its atomic structure and thus change the material’s relationship to light.
The researchers recorded the surface of the material, along with a few precisely designed small structures, and work together as a “metasurface” to refract or reflect light in unique ways. As the property of the material changes, the optical function of the metasurface changes. In this case, when the material is at room temperature, the metasurface focuses light to create a sharp image of an object at a certain distance. Once the material is heated, its atomic structure changes and, in response, the metasurface redirects light to focus on a distant object.
In this way, new active “metalens” can tune the focus without the need for large mechanical elements. The new design, which now features images within the infrared band, could enable faster optical devices such as miniature heat fields for drones, ultra-compact thermal cameras for mobile phones and low-profile night-vision goggles.
“Our results show that our ultrasonic tuning lens, without moving parts, can achieve an aberrant image on superimposed objects located at different depths, in line with traditional and high-volume optical systems,” says Tian Gu, Materials Research Laboratory at MIT.
We and his colleagues published the results in the journal today Nature Communications. Its 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 collaborators at the University of Massachusetts at Lowell, the University of Central Florida, and Lockheed Martin Corporation.
The new lens is made up of material that the group changes, the group created by tidying up the material that is commonly used in rewritable CDs and DVDs. The so-called GST consists of germanium, antimony, and tellurium, and its internal structure changes with heating with laser pulses. This allows the material to switch between transparent and opaque conditions – a mechanism for writing, deleting and rewriting data stored on CDs.
Earlier this year, researchers reported that another element, selenium, was being added to GST to make a new phase change material: GSST. When the new material was heated, its atomic structure changed from an amorphous and random knot of atoms to a more orderly crystalline structure. This phase change also changed the way infrared light travels through the material, affecting the refractive force, but with the least impact on transparency.
The group wondered whether GSST’s ability to change can be adapted to direct and focus light at specific points depending on the phase. The material can then serve as an active lens without the need for the mechanical parts to change focus.
“Generally when an optical device is made, it is very difficult to manufacture it after tuning its features,” says Shalaginov. “That’s why having this platform is like a holy grail for optical engineers. It allows for that [the metalens] to change the focus effectively and in a wide range “.
In a warm seat
In conventional lenses, the glass is precisely curved because the incoming light beam refracts the lens at different angles, converging at a point at a certain distance, known as the focal length of the lens. Lenses can create a sharp image of any object at that particular distance. To represent objects of different depths, the lens must be physically moved.
Instead of relying on the fixed curvature of a light-directing material, the researchers wanted to change the GSST-based metallene so that the length of the focus could change with the phase of the material.
In the new study, they fabricated a GSST layer with a thickness of 1 micron and created a “metasurface” by engraving the GSST layer on microscopic structures with various shapes that refract light in different ways.
“It’s a sophisticated process for building a metasurface that varies between different functionalities and requires rigorous engineering in what shapes and models to use,” says Gue. “Knowing how the material will behave, we can design a specific model that will focus on one point of the amorphous state and change to another point in the crystalline phase.”
They tested the new metals by placing them on a stage and illuminating them with a laser beam adapted to the infrared light band. At certain distances in front of the lens, on one side and on the other, they placed transparent objects consisting of double patterns of horizontal and vertical bars, known as resolution tables, which are usually used to test optical systems.
The lens, in its initial amorphous state, created a sharp image of the first pattern. The group then heated the lens to transform the material into a crystalline phase. After the transition, and removing the heating source, the lens created an equally sharp image, this time farther away from the second bar sets.
“We show the images at two different depths, without any mechanical movement,” says Shalaginov.
Experiments show that a metal can actively change focus without any mechanical movement. The researchers said a metal could be fabricated with integrated microwaves to quickly heat the material with short millisecond pulses. Changing the heating conditions can tune the intermediate states of other materials, allowing for constant focusing.
“It’s like cooking a steak. It starts with a raw steak, and it can go well done, or it can get weird, and anything else can be done,” says Shalaginov. “In the future this special platform will allow us to arbitrarily control the focal length of metals.”
References: Mikhail Y. Shalaginov, Sensong An, Yifei Zhang, Fan Yang, Peter Su, Vladimir Liberman, Jeffrey B. Chou, Christopher M. Roberts, Myungkoo Kang and Carlos Rios, Qingyang Du, Clayton Fowler, Anuradha Agarwal, Kathleen A. Richardson, Clara Rivero-Baleine, Hualiang Zhang, Juejun Hu and Tian Gu, February 22, 2021, Nature Communications.
DOI: 10.1038 / s41467-021-21440-9