Strong coupling between plasmonic nanoantennae and 2D materials

• Main Author: Liu, Lin
• Other Authors: Luo Yu
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Electrical and electronic engineering
• Online Access: https://hdl.handle.net/10356/164812
• Institution: Nanyang Technological University

Strong coupling of highly localized electromagnetic fields with quantum emitters has sparked a lot of interest in both fundamental and applied research. The energy exchange rate between photons and excitons must be substantially quicker than their non-radiative decay rates in order for strong coupling to exist. At cryogenic temperatures, the exciton lifetime can be extended, whereas in high-Q resonators, the photon lifetime can be extended. Experimentally, this implies that achieving strong coupling requires the use of an ultrahigh Q resonator at a cold temperature. As a result of their highly confined mode field, plasmonic nanocavities are promising candidates for strong coupling study. This is especially true due to the large local density of states (LDOS∝Q/V_mod), which is determined by the plasmon mode quality factor Q and mode volume V_mod. Based on the plasmon-exciton systems, researchers can investigate strong coupling with a variety of quantum emitters, including dye molecules, quantum dots, J-aggregates, and two-dimensional materials. In a strongly coupled system, the coherent oscillation of the photon and exciton energies manifests itself as a distinct spectrum splitting known as the Rabi splitting Ω, which is also used to evaluate coupling strength in the strong-coupling regime. It has been established that strong plasmon-exciton coupling with 2D materials and quantum dots can achieve Rabi splitting ranging from 200 to 400 meV. When the Rabi splitting large enough leading the ratio η=g/ω_ex larger then 0.1, the system is in the ultra-strong coupling (USC) area. Here, g=Ω⁄2 is the coupling strength and ω_ex is the exciton energy. USC is a new avenue of nonlinear optics and is essential for quantum electrodynamics which is worth to explore. However, except the desire to achieve larger coupling strength, we need to be aware that the coupling between plasmon and excitons happens not just for a single exciton, but for a large number of excitons contained within the cavity mode. It is possible that the huge photonic density of states is the root cause of the large splitting. This is due to the fact that the coupling strength g_N of a cavity which includes N excitons is identified as g_N=√N (μ ⃗_e∙E ⃗_cav ), where μ ⃗_e is the transition dipole moment of the emitter, E ⃗_cav is the cavity E-field equivalent to the vacuum energy. Thus, the reported Rabi splitting may be ascribed to substantial field enhancement and/or a large number of excitons. Strong-coupling studies on nanoparticle-on-mirror (NPoM) and 2D materials are shown with huge Rabi splitting usually. Because the NPoM resonant modes are often governed by the out-of-plane electric fields, it is envisaged that the interaction with the 2D material in-plane transition dipole will be minimal. However, since the NPoM cavity mode area is often considerably larger than that of the in-plane nanoantenna, the coupling in the NPoM structure involves more excitons, leading to the large splitting seen. In quantum optics, where interactions between single photon and exciton are greatly sought, it is necessary to minimize the number of excitons engaged in strong coupling process. In addition, to demonstrate practical applications, it is highly desirable to realize active tuning of strong coupling. In this thesis, we focus on the systematic investigation of strong plasmon-exciton coupling system comprised of in-plane nanoantennae and monolayer WS2. The coupling strength of nanoantenna arrays can be efficiently modified by changing the geometry of the arrays, which includes the shape, side length, gap size, and period of the arrays. Our results demonstrate that by raising the array period to 4 µm, which is much bigger than the scattering cross-section of the proposed nanoantennae, we can obtain strong coupling of single nanoantenna with WS2 at few exciton levels. The reduction in the quantity of excitons is obtained by narrowing the gap separation of the dimer nanoantenna down to sub-10 nm and by the fact that the in-plane E-field of the planar nanoantenna is much stronger than that of the NPoM systems. Furthermore, we have studied the mechanism of active tuning in strong coupling. Through the use of thermal effects that induced by plasmon heating and other phenomena in the coupled WS2-nanoantenna system, we discovered a statistically significant relationship between the WS2 excitons number N in the system and temperature. Also, by using TPU (thermoplastic polyurethane elastomer), a self-synthesized polymer with excellent adhesion and self-healing properties, we have also been able to tune the plasmon resonance, resulting in the realization of ultra-strong coupling in the process.