Theoretical upper limits and experimental overestimates for molecular hyperpolarizabilities: a symbiosis

A recently developed theory concerning the physical limits on optical hyperpolarizabilities is shown to be useful in the theoretical prediction of contributions of multiphoton fluorescence to experimentally determined hyperpolarizability values. Recent experimental results are compared with the theoretical limit. Without correction for the fluorescence contribution, results for the first hyperpolarizability in the forbidden region can be obtained. Proper correction of the systematic error shifts these values to the theoretical limit. Hence, experimental values above the theoretical upper limit should be suspect, but an appropriate fluorescence suppression technique is available. Reversing the argument, the observation that artificially overestimated values for the first hyperpolarizability are reduced from the forbidden region to exactly this upper limit is found to corroborate the limiting theory.