Interaction effects in optically dense materials

The properties of optically dense materials are influenced by interactions between elementary optical excitations (oscillators). Since such interactions are absent in the dilute limit, the resulting properties are unique to optically dense materials. While linear optical experiments can probe these effects, for example the Lorentz-Lorenz resonance shift, they are often more apparent in nonlinear experiments that are sensitive to coherence. Direct gap semiconductors are typically optically dense close to the fundamental gap and have been extensively studied using ultrafast coherent spectroscopy over the last ten years. However, their coherent optical properties are very complex because of many-body interactions among the extended excitations (electron-hole pairs or excitons). Dense atomic vapors have also been studied, but typically using frequency domain techniques. We present the results of using ultrafast techniques to study both semiconductors and dense atomic vapors. This reveals the similarities and differences of the two systems, yielding insight into the characteristics of each individually.