2D Plasmonics for Gas Sensing and Polarization Optics

Abstract

Two-dimensional (2D) materials emerged in 2004 with the isolation of graphene, a one-atom-thick honeycomb lattice of carbon atoms. The first reports on light interaction with polarizable 2D matter, i.e. 2D polaritons, came out in 2011 wereexperiments confirmed graphene's ability to sustain plasmon polaritons. Today, the 2D family is extended far beyond graphene which can host a full suite of different polaritonic modes with record-high light confinement and optical responses that can be tuned via lattice strain, optical pumping, or electrostatic gating. In this thesis we aim to exploit the optical and plasmonic responses of 2D materials to explore new system designs for selective gas sensing and ubiquitous polarization transformation. Truly robust and selective sensing of gases via remote standard optical spectroscopy, if achieved, has widespread use in key industries, including environmental, semiconductor, healthcare, and security. To date, the impeding challenge hasbeen the weak optical absorption of the gas molecules, which prevents optical read-out of gas traces at minute concentration. To overcome the weak sensitivity of optical techniques, we propose novel strategies, by utilizing plasmons in graphene to enhance light-gas interaction via promoting multiple trapping mechanisms, including surface adsorption, optical tweezing, and electrostatic bias. We discuss the relative strengths of these trapping forces and found gas adsorption in a typical nanoribbon array plasmonic setup produces measurable dips in optical extinction of magnitude 0.1 % for gas concentration of about parts per thousand level. We discuss the dynamic and nonlocal polarizability of two-dimensional electron gas with finite energy bandwidth (FBW-2DEG). This was motivated by recent developments in twisted 2D materials which exhibit isolated electronic bands and finite bandwidths. We show that a FBW allows for plasmon modes of quasi- at dispersion and large momenta to emerge. The FBW-2DEG can also potentially support low-loss plasmon modes immune to elastic or inelastic scattering-assistedLandau damping (dissipation via electron-hole pair excitation). This is of prime significance, since it allows for plasmon modes with concurrent tight spatial confinement and long propagation lengths, the two metrics critical to most branches of plasmonic science, e.g. communication, sensing, lasing, and more. The polarization of scattered light contains vital clues to nature of its light-matter interactions. The ability to control light polarization plays a key role in metrology applications such as stress analysis in glass or plastic, pharmaceutical or food ingredient analysis, biological imaging among others. The recent reports on rich near-field polaritonic-optics of twisted 2D materials with in-plane optical anisotropy, such as twisted Black Phosphorus and -MoO3, motivated us to explore the far-field polarization properties of such setups. We show that a stack of twisted anisotropic 2D materials with electrostatic control can function as arbitrary-birefringent wave-plate or arbitrary polarizer with tunable degree of non-normality. The twisted stack, thus gives access to a plethora of polarization transformers including rotators, pseudo-rotators, symmetric and ambidextrous polarizers. We explore the far-field scattering properties of anisotropic 2D materials in ribbon array configuration. Our study reveals the plasmon-enhanced linear birefringence/dichroism in these ultrathin metasurfaces, where linearly polarized incidentlight can be scattered into its orthogonal polarization or be converted into circular polarized light. We found wide modulation in both amplitude and phase of the scattered light via tuning the operating frequency or material's anisotropy.