Over the past few decades, the repercussions of the strong light-matter interactions in solid-state systems have been an epicenter of research owing to their widespread applications in modern-day photonics and optoelectronics. In this regard, exciton-polaritons, the half-light half-matter bosonic quasiparticles arising from strong coupling between the cavity photons (light) and the excitons (matter) are of great significance owing to their hybrid nature exhibiting both the excitonic and the photonic characteristics such as the low effective mass, high spatial/temporal coherence, and nonlinearities. These features make exciton-polaritons ideal for studying numerous complex quantum phenomena including Bose-Einstein condensation, superfluidity, inversionless polariton lasing, polariton hall effect, quantum computing, and many more. From an experimental standpoint, the realization of exciton-polaritons in solid-state systems demands i) an active medium with large exciton oscillator strength and ii) an optical microcavity with a high-quality factor and a small mode volume for sustaining a reversible energy exchange between the excitons and the confined cavity photons.
Accordingly, the organic-inorganic perovskites, particularly the methylammonium lead bromide (MAPbBr₃) have emerged as a potential material system for room temperature polaritonic because of the large exciton binding energy, oscillator strength, and the ability to form self-assembled-optical-microcavities. Benefitting from these beneficial characteristics, Chapter 4 illustrates the strong exciton-photon coupling in the micro-platelet (MP) and micro-ribbon (MR) shaped MAPbBr₃ microcavities. Owing to the distant physical dimensions, the perovskite MP and MR form a simple Fabry-Perot (FP) microcavity in different directions i.e., the out-of-plane and in-plane orientation, respectively. Consequently, multimode exciton-polaritons (MEPs) were observed in the angle-resolved photoluminescence (ARPL) mappings, wherein the lower polariton branches of the MEPs were fitted via the theoretical coupled oscillator model for both the perovskite MP and the MR, yielding large vacuum Rabi splitting ~205 and 235 meV, respectively. Interestingly, the ARPL mapping of the perovskite MR geometry acquired along the FP cavity direction also manifests Young's double-slit-like interference pattern, which in conjunction with the FDTD numerical simulations directly reveals the parity and the mode order of the uncoupled cavity modes responsible for MEPs formation in the perovskite MR. Thus, our results provide a promising platform for studying the geometry-dependent strong light-matter interaction in perovskite microcavities.
While the MEPs in self-assemble perovskite microcavities are excellent probes for many-body physics, certain applications such as the monochromatic polariton lasing demand the single-mode exciton polariton. However, single-mode polariton requires thin perovskite crystals ~150 nm that no longer form a self-assembled optical microcavity within the photoluminescence active (exciton) region of the spectrum.
Therefore, an external optical cavity is mandatory to support the reversible energy interactions, essential for strong coupling. In this regard, chapter 5 demonstrates the design and fabrication of a distributed Bragg reflector (DBR) based planar microcavities, comprised of alternating TiO₂ and SiO₂ thin films. Firstly, a computational model based on the transfer matrix method is used to simulate the reflectance, transmittance, and distribution of the electric field intensity across the DBR structure. Subsequently, the fabrication was performed via the conventional electron beam evaporation technique. Interestingly, the as-grown DBR (7.5 pairs) exhibits low surface roughness (Rrms) ~0.767 nm and sharp interfaces between the neighboring TiO₂ and SiO₂ films manifesting high-quality fabrication. In addition, the high reflectance ~99.0% and the large stopband width ~190 nm in the range of 600-800 nm indicates the superior performance of the 7.5 pairs DBR. While the excellent consistency between the simulated and the experimental reflectance and transmittance spectra confirms the successful fabrication of the DBR. Finally, the high reflectivity DBRs were employed to develop a monolithic and a Tamm plasmon planar microcavities with quality factors ~215 and ~44, respectively.