Polished glass has been at the heart of imaging systems for centuries.Their precise curvature
Change focus to see clearly in all of thesescale, usually requires physical movement of the lens by tilting, sliding, or otherwise moving, usually with the help of mechanical parts that make up most microscopes and telescopes.
Researchers engraved the surface of the materialtiny, precisely patterned structures that work together as a metasurface, refracting or reflecting light in a unique way. When material properties change, the optical function of the metasurface changes accordingly. In this case, when the material is at room temperature, the metasurface focuses the light to create a clear image of the object at a certain distance. When the material is heated, its atomic structure changes, and in response, the metasurface redirects light to focus on a more distant object.
Thus, the new active metalens canadjust focus without the need for bulky mechanical elements. The new design, which currently allows for infrared imaging, could allow for more flexible optical devices such as miniature thermal imaging cameras for drones, ultra-compact thermal imaging cameras for mobile phones and low-profile night vision goggles.
"Our result shows that our ultra-thin tunable lens with no moving parts canprovide aberration-free imaging of overlapping objects located at different depths, competing with traditional bulky optical systems."
Tian Gu is a research scientist in the Materials Research Laboratory at the Massachusetts Institute of Technology.
The new lens is made from a material with variablephase, which the team produced by tweaking the material commonly used in rewritable CDs and DVDs. Called GST, it is composed of germanium, antimony and tellurium, and its internal structure changes when heated by laser pulses. This allows the material to switch between transparent and opaque states - a mechanism that allows you to write, erase, and overwrite data stored on CDs.
Earlier this year, researchers reportedthe addition of another element, selenium, to the GST to create a new phase-changing material called GSST. As they heated the new material, its atomic structure shifted from an amorphous, disordered 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 it was possible to customizeswitching capacity GSST for directing and focusing light at specific points, depending on its phase. In this case, the material can serve as an active lens without the need for mechanical parts to shift its focus.
"In general, when an optical device is made, it is very difficult to adjust its characteristics after manufacture.That's why having such a platform is the holy grail for optical engineers, which allows you toMetalens are effective at switching focus over a large range."
Tian Gu is a research scientist in the Materials Research Laboratory at the Massachusetts Institute of Technology.
In ordinary lenses, the glass is precisely curved, so thatthe incident light beam is refracted from the lens at different angles, converging at a point at a certain distance, known as the focal length of the lens. The lenses can then create a clear image of any object at a specific distance. To display objects with different depths, the lens must be physically moved.
Instead of relying on a fixedthe curvature of the material for direct light, the researchers tried to change the GSST-based metalens so that the focal length changes depending on the phase of the material. In their new study, they fabricated a 1 micron thick GSST layer and created a metasurface by etching the GSST layer onto microscopic structures of various shapes that refract light in different ways.
They tested the new metalens by placing itonto the stage and illuminated with a laser beam tuned to the infrared range of light. At certain distances in front of the lens, they placed transparent objects consisting of double-sided horizontal and vertical stripe patterns known as resolution diagrams, which are commonly used to test optical systems.
The lens in the initial amorphous state gave a sharpimage of the first pattern. The group then heats up the lens to transform the material into a crystalline phase. After the transition and with the remote heating source, the lens produced an equally sharp image, this time of a second, more distant set of stripes.
Experiments show that metalens canactively change focus without any mechanical movement. The researchers say the metal lens could potentially be made with built-in micro-heaters to quickly heat the material in short millisecond pulses. By changing the heating conditions, they can also tune in to intermediate states of other materials, providing continuous focus adjustment.
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