Nonlinear Plasmonics Based on 2D Semiconductor Plasmonic Structures

Abstract

Semiconducting transition metal dichalcogenides (TMDs) are a subset of 2D materialsthat are well known for their strong optical response and large excitonic binding energy. The dominant optical response of these materials comes from their neutral (an electron in the conduction band bound to a hole in the valence band). Since the excitonic response is heavily quenched when going from monolayer to multiple layers of these materials, the interaction length between a photon and the material is limited by the atomic height of a single layer. In order to lift this restriction, surface plasmon polaritons are used to adjust the interaction direction from perpendicular to the crystal axis to in the plane of the crystal. By doing so, interaction lengths of microns or more can be achieved. In this dissertation, I will develop a hybrid exciton-surface plasmon polariton structure and investigate the third-order nonlinearities induced by a pump. To create a baseline for the nonlinaer measurements, linear tranmission were performed where the absorption of the TMD layer was measured to be 70% leading to an absorption length of 4.8 micron. By pumping the TMD layer with either photons or surface plasmons, a modulation in the transmission of the structure of ~1% and ~16% has been found for photon and plasmon pumping respectively. The effectiveness of this structure as a nonlinear plasmonic modulator was further solidied by measuring a switching time of 290 fs and a 650 fJ energy required to cause a 1% modulation in the transmission. The properties of the exciton-surface plasmon polariton structure were furthered by making use of coherent population oscillations which arise when nearly degenerate pump and probe energies cause an oscillation in the ground state population. This causes a spectrally narrow "hole" to appear in the absorption spectrum. By making use of coherent population oscillations, the pump-induced modulation of the structure has been increased to 60%. Furthermore, this narrow resonance causes a large frequency derivative to occur in the TMDs index of refraction which leads to a 1300 fold reduction in the group velocity of a surface plasmon travelling through the structure leading to the possibility for use as an optical buffer.