Directly patterned substrate-free plasmonic “nanograter” structures with unusual Fano resonances

The application of three-dimensional (3D) plasmonic nanostructures as metamaterials (MMs), nano-antennas, and other devices faces challenges in producing metallic nanostructures with easily definable orientations, sophisticated shapes, and smooth surfaces that are operational in the optical regime and beyond. Here, we demonstrate that complex 3D nanostructures can be readily achieved with focused-ion-beam irradiation-induced folding and examine the optical characteristics of plasmonic “nanograter” structures that are composed of free-standing Au films. These 3D nanostructures exhibit interesting 3D hybridization in current flows and exhibit unusual and well-scalable Fano resonances at wavelengths ranging from 1.6 to 6.4 µm. Upon the introduction of liquids of various refractive indices to the structures, a strong dependence of the Fano resonance is observed, with spectral sensitivities of 1400 nm and 2040 nm per refractive index unit under figures of merit of 35.0 and 12.5, respectively, for low-order and high-order resonance in the near-infrared region. This work indicates the exciting, increasing relevance of similarly constructed 3D free-standing nanostructures in the research and development of photonics and MMs. Nanostructures with grater-like structures exhibit Fano resonances with unusual properties, making them promising for plasmonic sensing. Researchers from China and the UK fabricated these structures over a large surface area by using a focused ion beam to induce nanopatterning and folding of self-supporting thin gold films. This fabrication technique can readily produce complex three-dimensional structural elements and does not require a substrate. The Fano resonances of the grater-like structures were highly sensitive to the refractive index of the surrounding medium (over 2,040 nanometres per refractive index unit in the near-infrared region), and hence the structures could potentially be used for high-performance plasmonic sensing. Furthermore, the structures may also possess novel mechanical responses and thus may find applications in optomechanics, pressure sensing and temperature monitoring on nanoscale dimensions.