Self-Assembled Nano-Lotus Pod Metasurface for Light Trapping

Abstract

Concentration of electromagnetic waves in deep-subwavelength volumes has been widely investigated as a direct way of enhancing light-matter interactions. However, a homogeneous array of subwavelength nanogaps suitable for visible light localization and enhancement is difficult to realize due to the limitations of conventional lithography techniques. Here, a uniform array of ultrasmall plasmonic resonators with precisely controlled nanogaps ("nano-lotus pods") is presented for the visible light confinement and realized without any photo- or beam-based lithography steps. The unit motif of this metasurface with a physical volume of 52 x 60 x 40 nm(3) is designed to resonantly trap visible light into an effective mode volume of 1.57 x 10(-5) lambda(3)(0). Each nano-lotus pod can be considered as a curved metal-insulator-metal waveguide which exposes both of its end faces and thus hot spots with the strongest electric fields on the outermost flat surface. To realize this unique nanostructure, a template-stripping method is employed in conjunction with block copolymer self-assembly and atomic layer deposition which guarantee a homogeneous array over large areas. It is experimentally demonstrated that the proposed metasurface can be used as a highly uniform and flat substrate for surface-enhanced Raman spectroscopy of various analytes, especially a stiff two-dimensional material.