Hy-Q Seminar - Darrick Chang

Hy-Q Seminar with Darrick Chang - ICFO

The ultimate limits to optical refractive index -- from quantum optics to quantum chemistry

It is well-known within quantum optics that a single, isolated atom can have an extraordinarily strong response to near-resonant light, as characterized by a scattering cross section that is much larger than its physical size. Another fundamental principle of quantum optics is that of collective enhancement, wherein an ensemble of atoms can exhibit a multiplicative enhancement of response due to atom number. However, if one takes these principles and extrapolates to the density of a real solid, it is predicted that materials can have a refractive index on the order of 10^4! This strongly goes against our empirical observations that all optical materials (at telecom/visible wavelengths) have an index of order unity.

Here, we attempt to create a unifying theory of optical response, which spans from the regime of a dilute to a dense atomic gas and finally to a real solid. We discuss the complex interplay between collective enhancement, and effects that are not included in standard quantum optics theories, including non-perturbative multiple scattering of light and the onset of quantum chemistry. We also elucidate the roles that such effects can have in limiting the refractive index and in what regimes, and what ultimate values of index might be possible with proper material design.