Quantum Optics Seminar by Nick Schilder

Polaritonic Modes in a Dense Cloud of Atoms
We analyze classical resonant light scattering by a cloud of cold   atoms with randomly distributed positions in a regime where near-field interactions between scatterers cannot be neglected. The induced dipoles form collective modes, from which only a few are polaritonic of nature. Apart from adopting a microscopic approach, it is also possible to describe the system optically by a dielectric constant. It is known that Lorentz-Lorenz (LL) formula for the calculation of an effective dielectric constant fails in these kinds of systems with strong interactions. In this work we numerically obtained the dielectric constant, which indeed deviates strongly from the LL-formula, in order to calculate the optical modes inside the effective medium. It turns out that the microscopic polaritonic modes can be identified with the optical modes of an effective medium. Having established this identification, the question raised if and under which conditions such a system enters the homogenization regime where the optical response is uniquely defined by a dielectric constant. I will also briefly touch upon this topic.