Metamaterials, artificial nanostructures that manipulate light, are costly and technologically challenging to fabricate. The broad, practical use of metamaterials depends on lowering their manufacturing cost and making them easier to create. Metamaterials traditionally have been made by depositing physical and chemical layers onto materials such as silicon and resin and following up the deposition process with lithography. This method is expensive and can be used only with certain materials. Consequently, the focus has shifted to creating metamaterials through the assembly of particles, rather than through the costly process of surface shaving. Researchers at POSTECH developed a method for 3D co-assembly of freestanding and freeform metamaterials using micropipette tips showcasing scattering reduction. The method surpasses limitations of existing metamaterial fabrication processes by enabling design and implementation of freeform nanophotons. Courtesy of POSTECH. To enable the cost-effective production of metamaterials in the desired shapes, a research team at Pohang University of Science and Technology (POSTECH) devised a solution-based 3D-printing process technology. The POSTECH team’s approach could significantly expand the range of materials used to make metamaterials, allowing for the unrestricted design of nanophotonic structures. The researchers shaped freeform, freestanding raspberry-like metamolecule (RMM) fibers in 3D by combining the evaporative co-assembly of silica nanoparticles and gold nanoparticles with 3D nanoprinting. First, they made the RMMs using silica and gold nanoparticles of varying sizes. Then, they stacked the RMMs on top of each other to create millimeter-size metamaterials. The researchers conducted experiments to show the light-controlling capabilities of the metamaterials that were formed by combining co-assembly techniques with 3D printing. They investigated the influence of the electric and magnetic dipole modes on the directional scattering of the RMM fibers, and demonstrated the ability to decrease the scattering of the millimeter-scale RMM fiber in the visible spectrum. The experiments showed that the magnetic response of an individual RMM can be controlled by adjusting the filling factor of the gold nanoparticles. The researchers were able to fine-tune the optical properties of the metamaterial by adjusting the ratio of silica and gold nanoparticles within the material. The team’s investigation of metamaterial production marks the first time that the optical properties of metamolecules have been verified in solution using millimeter-size RMM structures. The researchers’ approach allows results to be observed with the naked eye or through a simple microscope setup, eliminating the need for specialized equipment for verification. The new approach to making metamaterials could help resolve the challenges of fabricating freestanding metamolecule clusters with programmed geometries and multiple compositions, moving metamaterials a step closer to becoming commercially available. “This breakthrough enables the design and implementation of freeform nanophotons, surpassing the limitations of existing metamaterial fabrication processes,” professor Junsuk Rho, who led the research, said. “The versatility of this technology affords a wide range of material choices including quantum dots, catalyst particles, and polymers, making it applicable to diverse fields, from sensors to displays, in addition to metamaterial research.” The research was published in Small (www.doi.org/10.1002/smll.202303749).
