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

Fabry-Pérot microcavities

self-hybridized polaritons

Mie modes

Strong and ultrastrong coupling

Lorentz resonances

decay rates

cavity-free polaritons

Casimir force



water droplets.

self-assembled microcavities

PJ-salen, Origohuset, Fysikgården 1
Opponent: Prof. Hui Deng, Department of Physics, University of Michigan, USA


Adriana Canales Ramos

Chalmers, Fysik, Nano- och biofysik

Polaritonic linewidth asymmetry in the strong and ultrastrong coupling regime

Nanophotonics,; Vol. 12(2023)p. 4073-4086

Artikel i vetenskaplig tidskrift

Tunable self-assembled Casimir microcavities and polaritons

Nature,; Vol. 597(2021)p. 214-219

Artikel i vetenskaplig tidskrift

Abundance of cavity-free polaritonic states in resonant materials and nanostructures

Journal of Chemical Physics,; Vol. 154(2021)

Artikel i vetenskaplig tidskrift

Perfect Absorption and Strong Coupling in Supported MoS<inf>2</inf> Multilayers

ACS Nano,; Vol. 17(2023)p. 3401-3411

Artikel i vetenskaplig tidskrift

Fano Combs in the Directional Mie Scattering of a Water Droplet

Physical Review Letters,; Vol. 130(2023)

Artikel i vetenskaplig tidskrift

Adriana Canales, Oleg V. Kotov, Betül Küçüköz, Timur O. Shegai, Self-hybridized vibrational-Mie polaritons in water droplets

Light and matter interact constantly, allowing us to study and manipulate the world around us. Usually, when light meets matter, it does not change the properties of the materials because the interactions are weak. Interestingly, it is possible to reach a strong interaction regime where light and matter share energy so fast that making the difference between them is impossible. When distinguishing light and matter is no longer possible, a new quasi-particle called polariton describes the hybrid system. External optical cavities are often used to trap photons around the materials to enhance the interaction of light with them. Careful design and fabrication of samples are required to ensure these cavities trap photons of the same energy as the excitations in the material. In this thesis, the hybrids obtained using an external cavity are called conventional polaritons.

This thesis first studies the decay rates of conventional polaritons and then proposes two ways to simplify their fabrication process. The first method uses self-assembled cavities formed by gold flakes floating in an ionic solution, creating a metallic microcavity confining visible spectrum photons. The self-assembly is given by equilibrating forces of the attractive Casimir and repulsive electrostatic forces, facilitating the microcavity fabrication process. The second approach entirely removes the external cavity. Instead, light is confined inside the material as it bounces from its edges. This work demonstrates that cavity-free polaritons exist in various materials and geometries. Two experimental demonstrations are provided, one in a 2D-atomic crystal and another in water droplets. These polaritonic water droplets occur naturally in fog and clouds, indicating that polaritons are more common than previously thought. Cavity-free polaritons offer a more straightforward and prevalent way to study their properties and applications.

Stark plasmon-exciton koppling för effektiva foton-foton interaktioner

Vetenskapsrådet (VR) (2017-04545), 2018-01-01 -- 2021-12-31.


Nanovetenskap och nanoteknik


Grundläggande vetenskaper


Atom- och molekylfysik och optik


Chalmers materialanalyslaboratorium




Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5427



PJ-salen, Origohuset, Fysikgården 1


Opponent: Prof. Hui Deng, Department of Physics, University of Michigan, USA

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