Fabrication of suspended, three-dimensional chiral plasmonic nanostructures with single-step electron-beam lithography

Abstract

Recent years have witnessed explosive development of chiral plasmonics due to the fact that chiral plasmonic nanostructures give rise to broadband and scalable chiroptical effects orders of magnitude larger than naturally occurring materials. Various chiral plasmonic nanostructures have been demonstrated based on top-down and bottom-up fabrication techniques. However, three-dimensional (3D) chiral plasmonic nanostructure fabrication still remains challenging in many aspects. Here, we demonstrate suspended 3D chiral plasmonic nanostructures fabricated with only one-step electron-beam lithography. Our approach is unique since no alignment is required in the fabrication processes and the top and the bottom structures are self-aligned. Our 3D chiral plasmonic nanostructure consists of a suspended ultrathin silicon nitride membrane with perfectly-aligned L-shape and disk-shape gold nanostructures on its two respective sides. Such suspended chiral plasmonic nanostructures possess strong chiroptical properties at optical frequencies, which can be engineered by simply changing the disk size on one side of the membrane. The origin of the chiroptical properties is also analyzed using the plasmon hybridization model. Experimental results are in good agreement with the finite-difference time-domain simulations. Such suspended chiral plasmonic nanostructures could be highly applicable for chirality analysis of biomolecules, drugs, and chemicals.