Optical metamaterials are structures that interact with light to challenge the laws of physics. They can exhibit a negative refractive index to be used in electromagnetic cloaks, for super-high resolution imaging, and for unusual color effects. To interact with electromagnetic waves, however, these materials have to possess dimensions comparable to the wavelengths, namely 100 nm and smaller. Such precision is enabled in state-of-the art two-dimensional (2D) nanofabrication but remains challenging in three-dimensional (3D) geometries.

The research team of Edward S. Boyden at the Massachusetts Institute of Technology has developed a unique approach to fabricate 3D patterns with nanoresolution. The process, called ImpFab for “implosion fabrication,” was reported in a recent issue of Science and relies on the following principle. A porous hydrogel, typically a polyacrylate or a polyacrylamide, is swollen in an aqueous solution containing ions or organic molecules that readily diffuse through the pores and deposit at the surface of polymeric chains. Chemical reactions can occur, such as the growth of metallic nanoparticles from ionic suspension, directly within the hydrogel. After this internal coating, the composite is shrunk down, and then further solidified by sintering to create metallic structures.

Since hydrogels can be 3D-printed at the microscale, this principle can be easily coupled with 3D printing. Using hydrogels with controllable cross-linking density, the homogeneous shrinkage occurring after dehydration results in retention of the shape, but a decrease in dimensions. As a result, 3D patterns with complex shapes and resolutions of 50 nm could be fabricated in silver. These were found to exhibit an electrical conductivity only about 10 times less than that of bulk silver despite the high porosity (see Figure).

Shweta Agarwala, a researcher at the Singapore Centre for 3D Printing and leading innovator in additive manufacturing for electronics and biotechnology, says that “currently, direct-writing of nanostructures is possible using non-contact methods like inkjet and aerosol jet, but the resolution is limited to 10 µm. Moreover, these techniques are able to print in 2D plane only. This research of using sacrificial scaffolds to pattern desired structures and shrinking them to achieve 3D nanoscale objects is fascinating.” Furthermore, Boyden emphasizes that “the contribution of the work is not just that we can achieve similar or better resolution, but rather that we have found a way to do the patterning of many different materials in a modular fashion to achieve any geometry.” Indeed, the research team provides examples of patterning with fluorescent molecules, proteins and DNA, and several metals.

Daniel Oran and Samuel G. Rodriques, the lead authors of the article, are excited by the possibilities that the method offers to create and study optical metamaterials. “There is a huge need for a robust and efficient way of generating 3D nanoscale features out of a variety of materials. We are eager to find collaborators in any domain where the benefit of arbitrary 3D geometry is paramount to asking new scientific questions or creating devices that would otherwise be impossible or impractical to fabricate,” Oran says.

Originally published in the March 2019 issue of MRS Bulletin.