Degenerate Frequency Two-Beam Coupling in Organic Media via Phase Modulation with Nanosecond Pulses

This work presents a theoretical treatment using population redistribution and the thermo-optic effect to mediate degenerate frequency two-beam coupling (TBC) in the nanosecond regime in third-order nonlinear organic solutions. We show experimentally that the energy transfer is indeed a result of TBC and can be modeled using self- and cross-phase modulation to produce the required frequency shift. As a result of the relatively long lifetimes and large phase shifts induced by population redistribution and thermo-optic effects, the coupling efficiency can be significant. For the special case when a single input beam is aligned to overlap with the Fresnel reflection of the sample/air interface, coupling efficiencies can easily exceed 50% of the incident pump energy, which can account for a severe deleterious effect in nonlinear transmission experimentation