The interaction between light and matter is fundamental in perceiving and understanding the world. The interaction is typically weak, meaning light only perturbs matter without changing its properties. When photons interact strongly with material resonances caused by electronic or vibrational transitions, hybrid light-matter states called polaritons emerge. Polaritons have gained significant attention because of their potential to modify and manipulate material properties, such as conductivity, energy transport, photochemistry, and chemical reaction rates. Obtaining polaritons used to require meticulous design and fabrication of cavities, but recent efforts have aimed to simplify the process using metallic microcavities and open plasmonic cavities to expand their potential applications. Regardless of the type of cavity used, conventional polaritons rely on an external cavity, making them a rare occurrence.

This thesis aims to study and simplify the process of obtaining polaritons in theory and experiment. It begins with the extensively studied conventional polaritons obtained with metallic microcavities. The focus is on their asymmetric decay rates, a less-studied and puzzling property. Surprisingly, the asymmetry is found to be a more general effect than previously considered, occurring even in bulk polaritons. Next, instead of fabricating them, metallic microcavities are formed by the balance between Casimir and electrostatic forces in gold flakes present in the solution. These metallic microcavities then self-assemble and hybridize with the excitons in a 2D semiconductor. These microcavities can be tuned by altering the ionic concentration in the solution or through dynamic laser irradiation. Finally, the rest of the thesis is devoted to removing the necessity of an external cavity, leading to cavity-free polaritons. In this case, optical modes are sustained by the material’s geometry and hybridize without requiring an external cavity. The thesis includes experimental demonstrations of two geometries: 2D planar semiconductors sustaining exciton-polaritons and spherical water droplets sustaining vibrational polaritons. The existence of cavity-free polaritons reveals that polaritons are more common than previously thought, as even water droplets in mist are polaritonic. The findings presented here open the possibility of studying polaritonic properties in more straightforward and prevalent structures.