Engineering metallic nanogap apertures for enhanced optical transmission

Abstract

Physics and technology of metallic nanoapertures have been of great interest in nanophotonics. In particular, enhanced optical transmission mediated by surface plasmon waves in metallic nanoapertures has been widely studied and utilized in biochemical sensing, imaging, optical trapping, nonlinear optics, metamaterials, and optoelectronics. State-of-the-art nanotechnology enables researchers to explore optical physics in complex nanostructures. However, the high cost and tedium of conventional fabrication approaches such as photolithography, electron-beam lithography, or focused-ion-beam milling have limited the utilization of metallic nanoapertures for practical applications. This dissertation explores new approaches to enable high-throughput fabrication of sub-10-nm nanogaps and apertures in metal films. In particular, we focus on a new technique called atomic layer lithography, which turns atomic layer deposition into a lithographic patterning technique and can create ultra-small coaxial nanoapertures. The resulting nanostructures allowed us to observe extraordinary optical transmission in mid-infrared regime that originates from an intriguing physical phenomenon called the epsilon-near-zero (ENZ) condition. Subsequently, we turn this nanogap structure into a high-Q-factor plasmonic resonator, called a trench nanogap resonator, by combining a nanogap and sidewall mirrors. This structure is optimized for electrical trapping of biomolecules and concurrent optical detection, which is demonstrated experimentally via dielectrophoresis-enhanced plasmonic sensing. The fabrication technique and resulting structures demonstrated in this thesis work can facilitate practical engineering of metallic nanoapertures towards harnessing the potential of plasmonics.