The nonlinear-optical response of a quantum system as a function of its energy spectrum

In the present work we use the Monte Carlo method to study the nonlinear optical response classified by energy spacing of the system, aiming to understand certain unresolved questions including the gap between experimental values of the off-resonant hyperpolarizabilities of molecules and the fundamental limit. The results suggest an explanation for the origin of the factor of 20-30 gap between the best molecules and the fundamental limits and also confirm the validity of three-level ansatz, which states that when the first and second hyperpolarizabilities of a quantum system are at the limit, only three states contribute to the nonlinear response.