Optical Properties of Micro/Nano Structured Metal-Insulator-Metal Nanocavities

Plasmonic metamaterials provide a great deal of optical tunability and interesting phenomena in terms of light-matter interaction. Metal-Insulator-Metal (MIM) nanocavities are one of the most versatile systems for nano light confinement and waveguiding. These cavities exhibit Epsilon-Near-Zero (ENZ) modes, which can be used in conjunction with emitters to improve their optical properties. MIM nanocavities have been extensively studied, but when the cavities are confined in one more direction, the behavior changes. The purpose of this study is to investigate the effect of lateral micro/nano structuring on the optical properties of these cavities. The ENZ cavity resonance present in planar cavities persist even when the lateral dimensions are reduced to the micrometer range. When the diameters of the cavities are further reduced to hundreds of nanometers, electric (ED) and magnetic dipole (MD) modes come into picture. These modes are present even in micron size structures, but they fall in infrared region. Recently all-dielectric nanostructures have gained popularity to manifest these dipole modes, as they eliminate the dissipative losses present in metallic nanostructures and support toroidal dipoles (TD) when the geometry is optimized. The destructive interference of any two of these dipole modes, which cancels the far-field radiation, results in a non-radiative mode known as anapole. However, this necessitates the use of materials with a high refractive index (n), typically n ≥ 3. In low index materials like Alumina (Al2O3), the MIM configuration provides the necessary field confinement and can excite these modes. The existence of ENZ cavity modes in MIM micropillars, as well as dipole and anapole-like modes in MIM nanopillars, is demonstrated experimentally. Photolithography (for micropillars) and electron-beam lithography (for nanopillars) were used to fabricate, and ellipsometry to characterize the samples. To facilitate cost-effective, large area fabrication, Talbot lithography technique was exploited to get smaller feature sizes (<500nm) using standard photolithography setup. In addition, a lithography-free, dry-synthesis technique was explored to produce nanophorous metal films and their plasmonic properties were examined.