Polished glass has always been the center of the imaging system. Their precise curvature allows the lens to focus light and produce clear images, whether the object being observed is a single cell, the page of a book or a galaxy in the distance.
Changing the focus to be clearly visible at all these ratios usually requires physically moving the lens by tilting, sliding, or moving the lens in other ways, usually with the help of a large number of mechanical parts required by microscopes and telescopes.
Now, engineers at the Massachusetts Institute of Technology have created an adjustable “metal element”
The researchers etched the surface of the material with tiny, precisely patterned structures that work together as “metasurfaces”, refracting or reflecting light in unique ways. As the material properties change, the optical function of the metasurface also changes. In this case, when the material is at room temperature, the metasurface condenses light to generate a clear image of the object at a certain distance. After the material is heated, its atomic structure changes, and in response, the metasurface redirects the light to focus on further objects.
In this way, the new active “metal element” can adjust its focus without the need for bulky mechanical elements. The current novel design for imaging in the infrared band can enable more flexible optical devices, such as micro heated mirrors for drones, ultra-compact thermal imaging cameras for mobile phones, and low-key night vision goggles.
A researcher from the Massachusetts Institute of Technology’s Materials Research Laboratory said: “Our results show that our ultra-thin tunable lens has no moving parts and can achieve aberration-free imaging of overlapping objects at different depths, which is comparable to traditional bulky The optical system is comparable.”
Gu and his colleagues published their results in the magazine today Nature Communications. His co-authors include Hu Juejun, Mikhail Shalaginov, Zhang Yifei, Zhang Fan, Su Peter, Carlos Rios, Du Qingyang and Anuradha Agarwal of the Massachusetts Institute of Technology. Vladimir Liberman, Jeffrey Chou and Christopher Roberts of MIT Lincoln Laboratory; collaborators of MIT Lowell University, University of Central Florida and Lockheed Martin.
The new lens is made of phase change material, and the team made it by adjusting materials commonly used in rewritable CDs and DVDs. It is called GST, and it contains germanium, antimony and tellurium, and its internal structure changes when heated by a laser pulse. This allows the material to switch between a transparent state and an opaque state. This mechanism enables the data stored in the CD to be written, erased and rewritten.
Earlier this year, researchers reported that another element, selenium, was added to GST to create a new phase change material: GSST. When they heated the new material, its atomic structure changed from an amorphous, disordered atomic entanglement to a more ordered crystal structure. This phase shift also changes the way that infrared light passes through the material, affecting the refractive power, but has little effect on transparency.
The team wanted to know if the switch function of GSST could be adjusted to direct and focus light on specific points based on its phase. In this way, the material can be used as an active lens without the need for mechanical parts to shift its focus.
Shalaginov said: “Usually, when manufacturing optical equipment, it is very difficult to adjust its characteristics after manufacturing.” “This is why having this platform is like the holy grail of optical engineers. [the metalens] In order to effectively switch the focus in a large range. “
In hot seat
In a traditional lens, the glass is precisely curved, so the incident light beam will refract and leave the lens at various angles and converge at a certain distance. This is the focal length of the lens. The lens can then produce a clear image of any object at that specific distance. To image objects of different depths, the lens must be physically moved.
Researchers do not want to rely on the fixed curvature of the material to guide the light, but want to modify the GSST-based metal element by changing the focal length with the phase of the material.
In their new research, they fabricated a 1 micron thick GSST layer and created a “meta surface” by etching the GSST layer into various shapes of microstructures (refracting light in different ways).
Gu said: “The complex process of constructing metasurfaces is very complicated. You need to switch between different functions, and you need to design wisely which shapes and patterns to use.” “By understanding how materials behave, we can design a specific The pattern will concentrate on one point in the amorphous state and transform into another point in the crystalline phase.”
They tested the new metal powder by placing it on the stage and testing it with a laser beam tuned to the infrared band. At a certain distance in front of the lens, they placed a transparent object consisting of a double-sided pattern of horizontal and vertical bars (called a resolution meter), usually used to test optical systems.
The lens produces a clear image of the first pattern in its initial amorphous state. Then, the research team heated the lens to transform the material into a crystalline phase. After the transition, with the heating source removed, the lens produced the same clear image, this time with the second set of more distant stripes.
Shalaginov said: “We demonstrated imaging at two different depths without any mechanical movement.”
Experiments show that metal elements can actively change the focus without any mechanical movement. Researchers say that it is possible to use integrated micro heaters to make metal elements to quickly heat materials with short millisecond pulses. By changing the heating conditions, they can also be tuned to the intermediate state of other materials to achieve continuous focus adjustment.
Shalaginov said: “It’s like cooking steak-one starts with raw steak and can be done very well, or can be medium rare, and anything else in between.” “In the future, this unique platform It will allow us to arbitrarily control the focal length of the metal lens.”
Convex to concave: More metasurface ripples will produce a wide range of lenses
Mikhail Y. Shalaginov et al. Reconfigurable all-dielectric metal with diffraction limited performance, Nature Communications (2021). DOI: 10.1038 / s41467-021-21440-9
Courtesy of MIT
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Citation: The new “metalens” shifts focus without tilting or moving (February 22, 2021), retrieved from https://phys.org/news/2021-02-metalens-shifts-focus-tilting.html February 22, 2021
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