Why is optical refractive index so small?

The refractive index of a material directly determines the minimum size that we can construct optical devices and the smallest length scales that light can resolve. Despite the game-changing implications that an ultrahigh index material would have, it is interesting to observe that all optical materials that we know of have an index that is of order unity. Surprisingly, a deep understanding of the mechanisms that lead to this universal behavior seems to be lacking. Moreover, this observation is difficult to reconcile with the fact that a single isolated atom is known to have a giant optical response, as characterized by a resonant scattering cross section that far exceeds its physical size. We have developed a theory of the maximum index of a disordered atomic ensemble. Interestingly, despite the giant response of an isolated atom, we find that the maximum index does not indefinitely grow with increasing density but rather reaches a limiting value of n ≈ 1.7. This limit arises from the highly nonperturbative nature of multiple light scattering and near-field optical interactions in such a system. Our work is a promising first step to understand the limits of refractive index from a bottom-up, atomic physics perspective. Furthermore, identifying the limiting mechanisms should also pave the way to developing circumventing strategies, in order to realize an ultrahigh index material and open up vast new technological possibilities with light.