UC Irvine

Molecular polaritons, dressed light-molecular exciton quasiparticles, have different optical properties compared to excitons. Over the past few decades there has been extensive theoretical understanding and experimental demonstration on how the physical and chemical behaviour of exciton are modified in confined optical cavity. Qualitative and quantitative understanding of nonlinear polariton interactions are at the heart of open questions in the field. These nonlinearities manifests themselves as polariton-polaritons scattering matrices in the lowest order.

Nonlinear spectroscopic techniques, like double quantum coherence (DQC), can be used to study these nonlinearities such as vibrational anharmonicities, on-site interactions etc. A quasiparticle representation of coherent multidimensional optical signals for molecular polaritons in confined cavity (in both time and frequency domain) is developed allowing for direct observations of these polariton-polariton scattering terms that are explicitly present in the nonlinear signals.

In spirit of understanding anharmonicities, sum over state (SOS) method is employed to study modification of vibrational structure of ground electronic states for Amide-I and II motifs of N-methylacetamide due to varying cavity coupling strengths is done. Again, DQC signals are studied for illustrations.

Coherent pumping of molecular polaritons introduces several complications in extrating full structural informaion due to loss of strong coupling, non-equilibrium thermal excitations, phase-space filling factors (causing the polaritons to deviate from usual bosonic nature) and deviations from Hopfield coefficients, to name a few. Compelling remedies to the last two difficultes is proposed using theoretical framework to redefine the interacting polariton problem into a interacting deformed polaritons form. Discrepancies due to both phase-filling factors and deviations from Hopfield coefficients are encoded in a single deformation parameter $q$.

A modified DQC is derived and an unique measure for effective enhancement of this signal is presented as a function of number of polaritons. This is used to investigate collective effects due to polariton-polariton scattering. Applications are made for varying number of chromophores (for $N_p \in \{1,2,\cdots,15\}$) picked from a monomer of Light Harvesting Complex II (LHCII). It is shown that polariton-polariton scattering for non-bosonic polaritons could bring the middle polariton branches (MPBs), which are usually embedded deep within the so called ``dark-state", to light.