Nonlinear Absorption and Refraction in Multilevel Organic Molecular System

The nonlinear absorption and refraction in a multilevel organic molecular system is described by using the density matrix theory. The total absorptive coefficient of the system in the low-density case is equal to a linear sum of contributions from each energy level. Similarly, the total refractive index is equal to a linear sum of contributions from each energy level plus the refractive index of the vacuum. The absorption coefficient or refractive index due to each level is proportional to the population of that level, where the constant of proportionality is called the absorption cross-section or the refraction volume, respectively. The relation between the absorption cross-section and the refraction volume for each level is also given.