• energy transfer
• molecular polaritons
• organic molecules
• polariton patterning
• strong light-matter coupling

In recent decades, extensive research has been dedicated to exploring and advancing microcavities that facilitate strong light-matter coupling. Within these structures, the interaction between photons and excitons surpasses the intrinsic damping mechanisms of the system, resulting in the formation of hybrid radiation-matter states, known as cavity polaritons [1]. Polaritons exhibit distinctive properties due to their dual photonic and excitonic nature: the photonic component grants them a low effective mass and high propagation speeds, while the excitonic contribution enables strong nonlinear interactions and external field tunability [2]. These attributes have paved the way for a diverse range of polaritonic applications, including polariton condensates, polariton transistors and switches, energy transfer enhancement, and emerging quantum technologies [2].
Among the diverse materials investigated for strong coupling applications, organic semiconductors have garnered significant attention due to their advantageous properties. In particular, organic excitons, also known as Frenkel excitons, exhibit strong localization around individual excitation sites [1], thus possess large binding energies (in the range of 0.1–1 eV), and high oscillator strengths. These characteristics make it possible to observe macroscopic quantum phenomena even at room temperature, making organic materials promising candidates for realizing advanced polaritonic systems [3].
A particularly promising application of organic polaritons arises from their delocalized nature, which can be exploited for triggering long-range energy transfer. This phenomenon is essential for enhancing power conversion efficiencies in organic photovoltaic and optoelectronic devices, which are affected by poor exciton transport properties. Both theoretical and experimental studies have confirmed that polariton-mediated energy transfer can occur over distances that would otherwise be unattainable through conventional excitonic transport.
Another significant advancement in organic polaritonics involves the spatial confinement of polaritons to precisely control their energy distribution and density. By engineering the polariton energy landscape, it is possible to create potential wells that confine polaritons within predefined regions, enabling applications in topological polaritonics, neuromorphic computing, and quantum simulation.
However, such systems generally remain static, as they are determined by the specific cavity configuration. Nevertheless, the ability to switch polariton states on or off via external stimuli [4] could unlock new and exciting possibilities for optimizing energy flow in photonic systems. One strategy for achieving this level of dynamic control is to embed photochromic molecules within the microcavity. These molecules experience reversible photoisomerization upon exposure to specific wavelengths of light, resulting in different absorption profiles for each isomeric form. By creating a microcavity where only one isomer interacts with the photonic mode, the number of active emitters and, thus, the coupling strength can be controlled optically. This approach allows for the modulation of polariton characteristics and the localized management of energy transfer.
This thesis explores the integration of photochromic materials into Fabry-Pérot microcavities to achieve two primary objectives: (1) demonstrating optically controlled long-range energy transfer and (2) enabling localized polariton patterning through spatially resolved photoswitching.
Two types of photochromic microcavities were fabricated to investigate these phenomena. The first design consisted of a polymeric bilayer microcavity incorporating a donor-acceptor system, where a spiropyran-based photochromic donor was embedded within a poly(methyl methacrylate) (PMMA) matrix, and an acceptor dye (a cyanine derivative (BRK)) was dispersed in a poly(vinyl alcohol) (PVA) layer. Exposure to UV light triggers the photoisomerization of the SP molecules, effectively controlling the concentration of donor molecules and inducing a variation in the light-matter coupling constant. As this concentration increases, polariton states emerge, initiating the energy transfer process from the SP molecules to the BRK molecules. Furthermore, exposure to visible light restores the energy levels to their initial uncoupled state, effectively deactivating the polariton-facilitated energy transfer process. Through a theoretical model the experimental results have been quantitatively analyzed revealing a nearly six-fold increase of the relative contribution to the total emission by the acceptor species when the multilayer was embedded inside the resonant cavity compared to bare film. These results pave the way for the development of effective gating systems for light-mediated energy transport, which introduces new opportunities for applications in light harvesting, light-emitting devices, and photovoltaic cells. The second type of microcavity consisted of a single PMMA-spiropyran layer confined between metallic or hybrid metallic-dielectric mirrors. A confocal microscope equipped with a multi-wavelength excitation laser system was employed to induce and analyze polariton patterns with micrometer-scale spatial precision. Specifically, circular regions with diameters ranging from 1.2 µm to 10 µm were generated, where the emission peak shifted toward lower energies compared to unexposed areas, in accordance with the polariton formation process. The ability of the photoisomerization process to be reversed allows for the conversion from the MC to SP state upon visible light irradiation, enabling precise spatial control over polariton suppression. Utilizing the confocal microscope’s focused laser scanning function, complex two-dimensional designs with various geometrical configurations were inscribed. These results demonstrate the potential for reversible spatial patterning, opening new avenues for the development of adaptable photonic architectures and optically controlled quantum technologies.

References
[1] Agranovich, V.M. et al. 2003. Thin Films and Nanostructures: Electronic Excitations in Organic Based Nanostructures. Elsevier Academic Press. Vol 31.
[2] Sanvitto, D. et al. The road towards polaritonic devices. Nature Mater 15, 1061–1073 (2016).
[3] Jiang, Z. et al. 2022. Exciton-Polaritons and Their Bose–Einstein Condensates in
Organic Semiconductor Microcavities. Advanced Materials. 34: 2106095.
[4] Thomas, P. A. et. al. 2021. All-optical control of phase singularities using strong light-matter coupling. Nature Communications. 13: 1809.