Metallic nanostructures exhibiting unusual or naturally unobtainable optical properties can be fabricated by various techniques. Their feature sizes and complexity have advanced greatly over the past few years and the fabrication of bulk three-dimensional (3D) systems is fast becoming a reality. However, many highly controllable fabrication techniques are limited to working with layers of planar geometries as they typically involve some combination of thin-film deposition and 'top-down' lithography utilizing electron beams or focused ion beams. This tends to result in structures that lack strongly 3D features.

Credit: AUSTRALIAN NATIONAL UNIVERSITY

Now, Isabelle Staude and colleagues from the Australian National University (Canberra) and Swinburne University of Technology (Melbourne) have demonstrated a method capable of fabricating metallic nanostructures that are more three dimensional by combining the advantages of conventional electron-beam lithography and direct laser writing (Adv. Mater. DOI: 10.1002/adma.201203564). Their method exploits the 3D writing capability of direct laser writing to form larger 3D structures like rods, and the sub-100-nm feature sizes of electron-beam lithography to pattern deposited metals. The extra dimensionality of the achieved structures enabled the researchers to confirm a magnetic response from the samples. Their approach gives a sufficiently high resolution for near-infrared and visible light applications.

Staude explained to Nature Photonics that the team was surprised how straightforward the technique is and that the very first fabrication test delivered a 3D structure. She noted, however, that it is not always easy to fabricate designed, optically functional structures to the required alignment precision. Also, to realize a reliable lift-off procedure, they found it necessary to use a thicker electron-beam resist (PMMA) than in their usual 2D protocol. However, the approach seems to be very robust and reliable.

“To be accessible with this approach, structures have to be a projection of a 2D pattern on a curved surface. For example, we cannot fabricate a helix, but we can form a tapered helix,” Staude commented. “The resolution of the gold feature sizes is determined by the resolution of the electron-beam lithography process, which is well below 50 nm. The feature sizes of the 3D polymer template are limited by the resolution of direct laser writing. Line widths below 150 nm and centre-to-centre distances of 600 nm can be routinely achieved in the employed 'IP-L' resist. The team hopes that the proposed technique will enable the fabrication of 3D chiral metamaterials and antenna structures. It may also allow metallic nanostructures to be printed directly on top of other photonic components such as optical waveguides.