Strong coupling in different designs of ZnSe-based microcavity systems: polariton manipulation and potential landscape

Abstract

Exciton polaritons are quasiparticles resulting from the strong coupling of quantum well excitons with confined photons in a semiconductor microcavity. The hybrid nature of this part light and part matter quasiparticles leads to many remarkable demonstrations including polariton condensation and lasing. In last 20 years, various material systems have been used for the development of stable cavity-polariton system at room temperature. However, the realization of high quality microcavity samples that allows electrically injected polaritonic devices operating at room temperature is still a nontrivial technological challenge. Hence, more simplified device structures i.e., hybrid metal-semiconductor structures would be beneficial for the realization of practical polariton devices. In this thesis, the strong coupling effect between confined photons and quantum well excitons in different designs of ZnSe-based microcavity structures is demonstrated. First, strong light-matter interactions in a conventional microcavity structure, containing three ZnSe quantum wells, are discussed. A more simplified system e.g., hybrid metal-DBR structure (Tamm plasmon system) based on ZnSe materials is obtained. Next, the strong coupling is demonstrated in this simplified Tamm structure in the visible spectral region. For the first time evidences for the existence of a hybrid state of Tamm plasmons and microcavity exciton polaritons in a microcavity sample covered with an Ag metal layer is presented. A large confinement potential of about 13 meV for the lower polaritons is obtained when the Tamm-plasmon mode is resonantly coupled with the exciton polariton. Such a concept for the lateral confinement of the lower polariton eigenstate is very important for manipulating, shaping, and directing the flow of polaritons. In the last part of this thesis, a periodic Bragg structure with inserted ZnSe quantum wells, the so called unfolded microcavity sample is studied. The experimental evidence of strong coupling of Bragg photons to ZnSe quantum well excitons in the formation of Bragg polariton eigenstates is demonstrated. Two novel methods are proposed and experimentally realized for the modulation of the Bragg polariton resonances. The Bragg polariton modes exhibit a blue shift when the thickness of the upper layer of the Bragg structure is reduced or a thin Ag layer is deposited on top of the Bragg structure. Spectral shifts of about 12 meV and 9.5 meV of the lower Bragg polariton resonance are induced by an etch depth of about 18 nm in the upper layer and by the deposition of a 30 nm Ag layer on the Bragg structure, respectively. Such large shifts of the Bragg polariton are striking with respect to the realization of lateral potential traps for the Bragg polariton system. In addition, for the first time, a nonlinear increase of the lower Bragg polariton emission intensity is achieved when the excitation dependent measurements are performed in the hybrid Ag-Bragg structure. These results will pave the way for the development of practical polaritonic devices.