The Realization of Multidirectional, Free-Standing 3D Plasmonic Nanostructures via 3D Nanoprinting: A Case Study of Fractal Antennas

This study demonstrates the realization of precisely engineered 3D plasmonic nanostructures capable of manipulating and redirecting optical signals in 3D space. Lifting nanostructures into the third dimension can significantly extend current excitation/emission possibilities and signal strength, due to increased design freedom and reduced substrate influence. So far, the fabrication of metal-based 3D nanostructures poses significant challenges, attributed to either material or design. Herein, gold-based 3D nanostructures are deposited via Focused Electron Beam Induced Deposition (FEBID) and purified through etching in water vapor – a process particularly hard to control in the case of free-standing designs. By alternately irradiating the structures from opposing 90° side views, the impact of beam direction can be successfully leveraged, thereby ensuring efficient carbon removal without compromising structural fidelity. This newly introduced protocol is demonstrated for the case of fractal canopy trees, composed of free-standing nanowires with distinct lengths and diameters, oriented at defined angles to form fractal antennas of varying complexity. Plasmonic responses are evidenced both numerically and experimentally via excitation with fast electrons, showcasing broadband plasmonic activity, customizable polarizability and excellent agreement with simulation. This directional purification strategy now enables the reproducible fabrication of 3D plasmonic nanoarchitectures and is expected to significantly advance experimental capabilities and understanding in 3D nanoplasmonics.