Design and realization of nanogap metamaterials in the visible regime for electric field confinement

Abstract

Confinement of light has been investigated thoroughly because it is a direct way to enhance light and matter interaction. Local enhancement of electromagnetic wave has been applied to triggering interesting nanopho-tonic behavior and to realizing high performance optical devices. Especially, visible light confinement is im-portant because it has various potential usage in optical devices and energy devices. Among promising nanostructures, nanogap structures have gained much attention for their strong light confinement potentials. Using conventional lithography techniques including deposition with varying angles, nanosphere lithography, focused ion beam lithography and self-assembled monolayers lithography, it was hard to achieve uniform and controllable nanogap arrays in large scale. Besides, nanogap structures so far have had operating wave-length longer than visible light due to the fact that the resonance wavelength is mainly determined by the overall dimensions of the unit structure, which remained larger than a hundered nanometer in most cases. In this work, uniform and large-area nanogap array operating in visible range is fabricated by combining the block copolymer lithography, atomic layer deposition and template stripping methods. The fabricated struc-ture can effectively confine visible light within a volume of $1.57×10^{-5} \lambda^3$, where $\lambda$ is resonance wavelength of proposed nanostructure. In addition, strong electric field enhancement was predicted inside the insulator nanogap and it was verified using surface-enhanced Raman spectroscopy (SERS) signal with graphene oxide as a Raman analyte. The structure’s strong plasmonic resonance was observed in the transmission spectrum and it correlated with numerical simulations obtained using finite-difference time-domain (FDTD) method. The structure has not only a small effective mode volume, compared with other nanoresonators operating in the visible range, but also possesses atomically flat surface and uniformity.