Photon energy, four-wave mixing

The nonlinear susceptibility is evaluated using third-order perturbation theory, and resonant enhancement is readily demonstrated to occur. Four-wave mixing is a useful experimental technique to extend the energy range available to tunable dye lasers [468]. It is also of interest that processes involving excitation by three photons allow transitions between even and odd parity states to be excited, as do single-photon transitions. 

The symmetry of the polymer and sidechains for materials such as poly(di-n-hexylsilane) dictate that is identically zero for these polymers. However, recent publications have described measurements of for poly silanes to be on the order of 10" - 10 12 esu for third harmonic generation [28] and four wave mixing processes [29], Two-photon absoiption cross section, being proportional to Imx ), should therefore be strong as well for this class of polymers. We have observed this to be the case, with x(3)=3.2x10 esu at 560 nm, and the primary excitation decay paths of energy transfer, polymer chain scission, and UV fluorescence very similar to those observed with single photon exposure [21,30]. 

The limitation of this notation is obvious here, since the description of degenerate four-wave mixing (namely (4.14)) and the optical Kerr effect (OKE) is the same. Two-photon absorption is the loss process in which two photons of equal energy give rise to a dipole-forbidden (i.e. one-photon-forbidden) transition between energy levels of the material. The number of third-order effects is very large, since there are many quantum mechanical matrix elements that can contribute to... 

---