Quantum-mechanical description of dielectric response in the condensed phase

A quantum-mechanical description is presented for the dielectric response of an optically active two-level system that is strongly coupled to a continuum of bath states. In the case where the coupling to the continuum leads to strong damping of the optical polarization, we find that it is essential to consider the thermal occupation and the lower boundary energy of the quantum-mechanical bath states to get a correct description of the dielectric response.