Strong Light-Matter Coupling: the Road from Conventional to Cavity-Free Polaritons

Doctoral thesis, 2023

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 excitonpolaritons
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.