Transparent oxides for active nanophotonics [electronic resource]
Keywords: Applied Physics.

In this thesis, we will apply indium tin oxide (ITO), a widely used transparent conductive oxide (TCO) materials in the touchscreen and solar cell industry, to design and experimentally demonstrate active nanophotonics device. ITO behaves as a Drude metal with a plasma frequency that is controlled by its free carrier density. Attenuated total reflection (ATR) measurement can be used to excite and study the surface plasmon polaritons (SPP) on ITO film. We systematically tune the plasma frequency across the infrared (IR) range by annealing treatments in a reducing environment that produce high electron concentrations (~1021cm-3). These optical measurements are complemented by Hall measurements to obtain a comprehensive picture of the Drude response of the ITO films. It was found that a complete description of the optical properties at very high carrier densities needs to account for the nonparabolicity of the conduction band of ITO and a reduced carrier mobility. Electrical tuning of metals' optical behavior stands very challenging, because noble metals such as gold can effectively screen electrical field due to their high electron densities. ITO, as a low-electron-density metallic material, exhibits electrical-tuneable permittivities in IR regime. We experimentally show the tunability of the plasma frequency of ITO thin film by changing the sheet carrier density through electrical gating with an ionic liquid (IL). The surface plasmon resonance wavelength of ITO accumulation layer is substantially shifted from 3.9 μm to 2.4 μm with gate bias increased to 1.5 V. Quantitative analysis confirms a concomitant relation between the optical behavior and electrical properties of electron density and mobility. Generally, the SP resonances supported by noble metal nanostructures are well-explained by classical models, at least until the nanostructure size is decreased to a few nanometers, approaching the Fermi wavelength λF of the electrons. In particular, observation of quantum size effect in metallic films and its tuning with thickness has been particularly challenging. We experimentally demonstrate active tuning of quantum size effects seen in the SP resonances supported by a 20-nm-thick metallic film of ITO. The IL is used to electrically gate and partially deplete the ITO layer. The experiment shows results in a controllable and reversible blue-shift in the SP resonance wavelength above a critical voltage. A quantum mechanical model including the quantum size effect reproduces the experimental results, whereas a classical model only predicts a red-shift. We believe this work opens up new approaches to investigate quantum plasmonic phenomena and achieve tunable plasmonic devices and circuits that will operate robustly at the quantum level Lastly, we experimentally demonstrate a broadband, ultra-compact, waveguide-integrated modulator exploiting the epsilon-near-zero (ENZ) effect in ITO material. Si waveguide strip is covered with HfO2 and ITO film to form a metal-oxide-semiconductor (MOS) capacitor. Electrical gate bias is applied to accumulate electrons in ITO film to induce an ENZ layer in the spectral region near the important telecommunications wavelength of λ = 1.55µm. When the ENZ layer occurs, this modulator leverages the combination of a local electric field enhancement and increased absorption in the ITO. This leads to large changes in modal absorption upon gating. A 3 dB modulation depth is achieved in a non-resonant structure with a length under 30 µm. The results provide insight into the design of ultra-compact, nanoscale modulators for future integrated nanophotonic circuits. The ITO materials show great potential in the application of active nanophotonics. It opens up a myriad of opportunities in the fields of optical communications, integrated optical devices, advanced imaging and display systems and so on.