Epsilon-near-zero metamaterials for optoelectronic applications

Abstract

My PhD is focused on the design, fabrication and optoelectronic applications of epsilon-near-zero (ENZ) metamaterials (MMs), which have vanishing real part of the permittivity and support fascinating optical effects, including light squeezing, sub-wavelength imaging, enhanced directive emission and enhanced non-resonant optical nonlinearity. Different approaches have been developed to realise ENZ media, such as tuning the modal dispersion in narrow plasmonic channels, exploiting the natural dispersion of transparent conductive oxides and creating composite structures with metals and dielectrics. As a platform with exotic physical properties, ENZ media also exhibit enormous potentials in combination with tunability and various nanofeatures in the photonic and plasmonic regimes.

This thesis demonstrates two approaches to achieve the ENZ condition. One method is to stack metal (Ag) and dielectric (SiO2) layers periodically at sub-wavelength scales. The resulting material behaves as an effective medium with an average permittivity close to zero, and we show that this ENZ medium can enhance the emission of quantum dots. This approach generally requires nanofabrication techniques developed for flat and rigid substrates, for example, the electron beam evaporation, which are not always applicable to micro- and macroscopic devices with arbitrary shapes. To surpass these limitations, we design and experimentally demonstrate an optical freestanding and low-loss ENZ membrane in the visible range, by layering polymer (SU-8) and Ag nano-layers. Additionally, we propose a method to introduce both flexibility and electrical tunability into ENZ media by replacing the metal layer with a 2D material, graphene, in the multilayer model. The other way to obtain an ENZ response is using natural materials which operate in proximity of their plasma frequency, typically here the indium tin oxide (ITO) at the near-infrared range. The ITO thin films are deposited using radio frequency magnetron sputtering, and their permittivities are manipulated via controlling fabrication parameters. We succeed in sweeping the zero-permittivity frequency of ITO media by controlling the gas recipe and deposition temperature during the sputtering process. To obtain specific optical responses, the ENZ ITO thin films are designed to be combined with different photonic features, including nanoantenna on microsphere and nanohelix, associated with a direct fabrication approach based on electron beam induced deposition (EBID).

Furthermore, this thesis extends the research range by realising the photonic trimming of quantum emitters via various metallic nanofeatures fabricated directly using EBID method. We believe that the interaction of ENZ MMs with EBID approach offers an opportunity to create hybrid ENZ platforms for optoelectronic applications.