Toward Collective Chemistry under Strong Light-Matter\nCoupling

Collective strong light-matter coupling\nprovides a versatile means\nto manipulate physicochemical properties of molecules and materials.\nUnderstanding collective polaritonic dynamics is hindered by the macroscopic\nnumber of molecules interacting collectively with photonic modes.\nWe develop a many-body theory to investigate the spectroscopy and\ndynamics of a molecular ensemble embedded in an optical cavity in\nthe collective strong coupling regime. This theory is constructed\nby a pseudoparticle representation of the molecular Hamiltonian, which\nmaps the polaritonic Hamiltonian into a coupled fermion-boson model\nunder particle number constraints. The mapped model is then analyzed\nusing the nonequilibrium Green’s function theory with the self-energy\ndiagrams identified through a large <i>N</i> expansion.\nWe demonstrate that in the thermodynamic limit, the necessary condition\nto have any collective effects is to have a macroscopic cavity field.\nNumerical illustrations are shown for the driven Tavis–Cummings\nmodel, which shows an excellent agreement with exact results.