A Northwestern University research team has developed tiny optical elements from metal nanoparticles and a polymer that one day could replace traditional refractive lenses to realize portable imaging systems and optoelectronic devices. The flat and versatile lens, a type of metalens, has a thickness 100 times smaller than the width of a human hair.
The properties of metalenses depend on the rationally designed arrangement of nanoscale units. Metalenses have emerged as an attractive option for flat lenses but are currently limited by their static, as-fabricated properties and their complex and expensive fabrication. For imaging operations such as zooming and focusing, most metalenses cannot adjust their focal spots without physical motion. One major reason is that the building blocks of these lenses are made of hard materials that cannot change shape once fabricated. It is difficult in any material system to adjust nanoscale-sized features on demand to obtain tunable focusing.
The researchers demonstrated a versatile imaging platform based on fully reconfigurable metalenses made from silver nanoparticles. During a single imaging session, their metalens device can evolve from a single-focus lens to a multi-focal lens that can form more than one image at any programmable 3D position. They built their lenses out of an array of cylindrical silver nanoparticles and a layer of polymer patterned into blocks on top of the metal array.
By simply controlling the arrangement of the polymer patterns, the nanoparticle array could direct visible light to any targeted focal points without needing to change the nanoparticle structures. This scalable method enables different lens structures to be made in one step of erasing and writing, with no noticeable degradation in nanoscale features after multiple erase-and-write cycles. The technique can reshape any pre-formed polymer pattern into any desirable pattern using soft masks made from elastomers.