Ab Initio Polariton Spectra of ZnTPP Molecules Collectively Coupled inside an Optical Cavity

Exciton-polaritons are quasi-particles formed by the quantum mechanical hybridization of electronic and photonic excitations. Despite extensive investigations, a fundamental understanding of molecular polariton spectra and the polariton delocalization from an \textit{ab initio} theoretical perspective remains elusive. We aim to simulate experimentally measured linear transmission spectroscopy of many Zinc(II) tetraphenylporphyrin (ZnTPP) molecules collectively coupled to a cavity from first principles. Our theoretical approach incorporates many low-lying electronic excitations in ZnTPP molecules, as well as collective light-matter couplings between ZnTPP and the quantized radiation modes, both of which are shown to be the key to accurately recovering the experimental spectra. We further analyzed to what extent the polariton and dark states are delocalized over many molecules, for the first time, using fully \textit{ab initio} descriptions of the molecules. We finally investigate the linewidth as a function of detuning, providing new theoretical insights into the experimentally observed motional narrowing behavior. Our work presents first-of-its-kind theoretical studies on molecular polariton spectra, offering a new perspective on molecular polariton formation in realistic \textit{ab initio} molecular systems whose rich, many-state nature provides spectral features enabled by the high density of electronic states beyond simple quantum optics models.