Metalenses promise potential for a paradigm shift of conventional optical devices. They comprise tiny, antenna-like surface patterns that can focus light to magnify distant objects in the same way as traditional curved glass lenses, but they have the advantage of being flat. The aperture sizes of metalenses are usually bound within hundreds of micrometers by the commonly used fabrication methods, limiting their usage on practical optical devices like telescopes. Researchers have now, for the first time, demonstrated a high-efficiency, single-lens, refractive metalens telescope. Their metalens works in the near-infrared region with nearly diffraction-limited focal spot sizes and a high peak focusing efficiency of 80.84%. Based on the metalens, they built a single-lens telescope and acquired images of the lunar surface, revealing its geographical structures.
The demonstration of the metalens telescope proves the exciting potential lying in the metasurfaces and could bring new possibilities for areas involving large optical systems, including geosciences, planetary observation, and astrophysical science. Image credit: Zhang et al., doi: 10.1021/acs.nanolett.2c03561.
“Traditional camera or telescope lenses have a curved surface of varying thickness, where you have a bump in the middle and thinner edges, which causes the lens to be bulky and heavy,” said Dr. Xingjie Ni, a researcher at the Pennsylvania State University.
“Metalenses use nano-structures on the lens instead of curvature to contour light, which allows them to lay flat.”
“That is one of the reasons, modern cellphone camera lenses protrude from the body of the phone: the thickness of the lenses take up space, though they appear flat since they are hidden behind a glass window.”
Metalenses are typically made using electron beam lithography, which involves scanning a focused beam of electrons onto a piece of glass, or other transparent substrate, to create antenna-like patterns point by point.
However, the scanning process of the electron beam limits the size of the lens that can be created, as scanning each point is time-consuming and has low throughput.
To create a bigger lens, Dr. Ni and colleagues adapted a fabrication method known as deep ultraviolet (DUV) photolithography, which is commonly used to produce computer chips.
“DUV photolithography is a high-throughput and high-yield process that can produce many computer chips within seconds,” Dr. Ni said.
“We found this to be a good fabrication method for metalenses because it allows for much larger pattern sizes while still maintaining small details, which allows the lens to work effectively.”
Zhang et al. captured images of the lunar surface using their large-aperture metalens telescope. Image credit: Xingjie Ni.
The researchers modified the method with their own novel procedure, called rotating wafer and stitching.
They divided the wafer, on which the metalens was fabricated, into four quadrants, which were further divided into 22 by 22 mm regions — smaller than a standard postage stamp.
Using a DUV lithography machine, they projected a pattern onto one quadrant through projection lenses, which they then rotated by 90 degrees and projected again.
They repeated the rotation until all four quadrants were patterned.
“The process is cost-effective because the masks containing the pattern data for each quadrant can be reused due to the rotation symmetry of the metalens,” Dr. Ni said.
“This reduces the manufacturing and environmental costs of the method.”
As the size of the metalens increased, the digital files required to process the patterns became significantly larger, which would take a long time for the DUV lithography machine to process.
To overcome this issue, the scientists compressed the files using data approximations and by referencing non-unique data.
“We utilized every possible method to reduce the file size,” Dr. Ni said.
“We identified identical data points and referenced existing ones, gradually reducing the data until we had a usable file to send to the machine for creating the metalens.”
Using the new fabrication method, the scientists developed a single-lens telescope and captured clear images of the lunar surface — achieving greater resolution of objects and much farther imaging distance than previous metalenses.
Before the technology can be applied to modern cameras, however, they must address the issue of chromatic aberration, which causes image distortion and blurriness when different colors of light, which bend in different directions, enter a lens.
“We are exploring smaller and more sophisticated designs in the visible range, and will compensate for various optical aberrations, including chromatic aberration,” Dr. Ni said.
The team’s work was published in the journal Nano Letters.
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Lidan Zhang et al. 2023. High-Efficiency, 80 mm Aperture Metalens Telescope. Nano Lett 23 (1): 51-57; doi: 10.1021/acs.nanolett.2c03561
Source : Breaking Science News