Why is the Refractive Index of Optical Materials so Small?
July 20th, 2021 DARRICK CHANG Theoretical Quantum-Nano Photonics

It is interesting to observe that all optical materials we know of have a refractive index of order unity at telecom or visible wavelengths. Yet we seem to lack any deep understanding of this seemingly universal phenomenon. Furthermore, this observation is difficult to reconcile with the well-known fact that a single isolated atom can have a giant response to near-resonant light, as characterized by a scattering cross-section much larger than the physical size of the atom. Indeed, according to conventional textbook formulas, this would lead to an index of order 104 for collections of atoms at solid densities. Here, we take a bottom-up approach to this question, investigating how the refractive index of an atomic medium evolves as a function of density, starting from the well-understood regime of dilute, isolated atoms. We explore the roles that different mechanisms, such as non-perturbative multiple scattering of light and quantum chemistry, can have in limiting the index, what the ultimate limits of index might be, and why our conventional textbook descriptions are insufficient.

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