LAUSR

We consider the absolute-value limits of the refractive index in atomic systems. There has been growing interest in finding materials with an unusual index of refraction, but there are natural limitations on the index that can be attained by controlling basic multi-level systems with external fields. We explain how the ensemble density in a medium is an important practical limitation and use this to calculate a baseline value for the refractive index of a two-level system. We show how the additional transitions and fields in three- and four-level systems give greater control over the refractive index and consider how coherence effects and four-wave mixing can affect it in the linear regime. We find that the only parameters, besides the medium optical depth/density, that can effectively change the value of the attainable index are the frequencies of the involved transitions.