ESE PhD Thesis Defense – “Low-Dimensional Excitons for Electro-Optics”

For nearly two decades, low-dimensional media have promised to produce atomically-thin optical devices. However, making optical devices in the deep-subwavelength regime requires techniques to confine light for efficient light-matter interactions. One approach is to pattern a metasurface to create a cavity mode, but the effectiveness of this mode can be enhanced by a resonance within a medium. Phonons, vibrations of atomic nuclei, have been used in this regard in the mid-infrared, but the main approach in the visible and near-infrared (NIR) ranges has been plasmons in either metals or doped insulators. Plasmons rely on free-carrier to produce a strong optical response at a specific wavelength. However, plasmons require a large concentration of free-carriers which makes them insensitive to electrostatic doping. Instead, excitons, electron-hole pairs near the band gap of a semiconductor, also interact strongly with light while also being highly sensitive to electrostatic doping. Therefore, excitons are excellent candidates for high-performance, ultrathin electro-optical devices. My research aims to investigate the electrostatic tunability of excitons in low-dimensional materials and use them to produce electro-optical modulators. In this thesis, I (i) experimentally demonstrate full 2π phase modulation using exciton-polaritons in a transition metal dichalcogenide (TMDC)/insulator superlattice; (ii) investigate the tunability of excitonic optical anisotropy in wafer-scale films of aligned single-walled carbon nanotubes (SWCNTs); and (iii) study the properties of excitonic hyperbolicity in chirality-pure SWCNTs and boron nitride nanotubes (BNNTs). Overall, my thesis research demonstrates the potential of excitons for ultrathin electro-optical devices.