A new class of metallic lattices capable of directing light into numerous focal points have been designed by US scientists. These super thin lenses could find use in both consumer electronics and bioimaging.
The materials are single-layer lattices, with each site on their 33 units by 33 units grid either containing a nanostructure or not. “In its simplest form, each [lattice site] can be considered as a digital element, namely a binary 0 or 1, and then the entire lattice can be cast in the form of a long, digital piece of DNA,” explains lead researcher Teri Odom at Northwestern University in the US.
The researchers have complete control over every single lattice site during the manufacturing process, meaning an almost unlimited number of lattice designs can be made. “Since each design will direct light in a unique way, the large number of configurations means that light can be steered and concentrated anywhere in 3D,” says Odom.
These unlimited options do however cause a problem: deciding what design is optimal for a specific light focusing property can’t be done using conventional computational methods. “A single characterization would require 10291 years with the fastest supercomputers,” she explains.
Instead the team custom-built an evolutionary algorithm so they could create and modify the lattice in silico as it was developed, rather than needing to specify the lattice design at the start. “We developed an evolutionary algorithm that uses the principle of survival of the fittest from Darwinian evolution to quickly find the optimal configuration.” Only once the perfect design for each purpose was found using the computer algorithm were the metallic lattices synthesized. This work was published in Nano Letters [Huntington M. D., Lauhon L. J. & Odom T. W., Nano Lett. (2014) doi: 10.1021/nl5040573].
The team was able to make lattices that could focus light into between one and seven highly controlled focal points. “We are able to focus light into an arbitrary number of points and with nearly any spatial distribution we desire,” says Odom.
Her team also found that the focal points could be changed by changing the polarization of the light being shone into the lattice. “We have a single substrate that can show two different properties depending on polarization, which is a unique feature for metal lenses,” she adds.
These lenses could potentially find use in the next generation of small digital cameras in cell phones since the lenses are effectively flat − only 180nm thick. Imaging of live cellular processes is another potential application. “Because lattice opto-materials offer an infinite [number of] possibilities for controlling the shape of light, we anticipate that these structures will generate new imaging modes for observing biological properties in real time,” says Odom.