MIT engineers have created a customizable metal lens that can focus on objects at different depths without changing its physical position or shape. The lens is not made of solid glass, but of a transparent phase-changing material that, after heating, can rearrange its atomic structure and thereby change the way the material interacts with light.
Polished glass has been at the heart of imaging systems for centuries. Their precise curvature allows the lenses to focus light and create crisp images, whether the object in sight is a single cell, a page in a book, or even a distant galaxy.
Changing focus to see clearly at all of these scales usually requires physically moving the lens by tilting, sliding, or otherwise moving, usually with the help of the mechanical parts that make up most microscopes and telescopes.
The researchers engraved the surface of the material with tiny, 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. As 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 metal lens can adjust 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 can provide aberration-free imaging of overlapping objects at different depths, competing with traditional bulky optical systems.”Tian Gu, Researcher, Materials Research Laboratory, Massachusetts Institute of Technology
The new lens is made from a phase change material that the team made by customizing a material commonly found in CD and DVD rewritable discs. 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 reported adding another element, selenium, to 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 the GSST’s switching capacity could be adjusted to direct and focus light at specific points based 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 being made, it is very difficult to adjust its characteristics after manufacturing. That is why having such a platform is the holy grail for optical engineers, allowing metal lenses to effectively switch focus over a wide range. “Tian Gu, Researcher, Materials Research Laboratory, Massachusetts Institute of Technology
In conventional lenses, the glass is precisely curved so that the 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.
Rather than relying on a fixed curvature of the material for direct light, the researchers tried to modify 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 it on stage and illuminating it with a laser beam tuned to infrared light. At certain distances in front of the lens, they placed transparent volumes.