Thesis title: Optical response of hybrid light-matter modes
This thesis presents a comprehensive investigation of low-energy THz hybrid light-matter collective modes in solids - excitations that emerge from the coupling of light to dipolar-active oscillations. We first discuss a classical framework, that proves effective for the characterization of the linear optical response of simple modes in isotropic materials. However, with the goal of describing more complex structures, such as anisotropic layered compounds, or exotic phenomena in the nonlinear regime induced by intense THz pulses, we then develop a path-integral many-body framework to be employed in the remainder of the thesis. This approach is an efficient and powerful tool that provides insights into the microscopic properties of the hybrid excitations, while maintaining analytical simplicity and transparency,
Focusing on layered superconductors, we study Josephson plasmons arising from the mixing between light and superconducting phase fluctuations. We demonstrate that the conventional picture of longitudinal-transverse decoupling breaks down in anisotropic systems, and we provide indirect evidence through the appearance of an unexpected finite-frequency absorptive peak in the linear response. We further investigate Josephson plasma modes in bilayer superconductors, where an additional peak emerges due to the interlayer capacitive coupling, and one low-energy plasmon is found to be invisible in density-coupled measurements, revealing its mixed longitudinal-transverse character.
We then turn to the pump-probe response of infrared-active phonons in isotropic and uniaxial crystals. We propose a theoretical and experimental setup, based on three-wave-mixing protocols such as THz pump - optical probe, that could efficiently map the dispersion of transverse phonon-polaritons in non-centrosymmetric systems. Applying this framework to the uniaxial ferroelectric lithium niobate as a prototypical case study, we reconstruct the dispersion of the low-energy E-symmetry mode. Finally, we extend the pump-probe formalism to two-dimensional spectroscopy, a novel nonlinear technique recently applied in solid compounds. This protocol offers the potential to reveal new nonlinear excitation pathways and to disentangle couplings between modes. We discuss the technique within our many-body formalism to show how it can be employed to reveal the dominant nonlinear process of a material, and thus infer its microscopic properties.
Overall, this thesis advances the understanding of hybrid light-matter excitations, while offering new theoretical tools to be employed in future studies on more complex phenomena, such as phonon-plasmon couplings, THz emission of phonons in ferroelectric materials, or light-matter coupling in chiral systems.