In this paper , we analyze the nonlinear response of a three - level a model system , using a perturbation theory based on the liouville equation . of motion and considering the lorentz local field effect
2.
The terahertz emission , which is a second - order response , and the four - wave mixing signal , a third response , are given the expressions related to the lorentz local field factor l . we found a four - wave mixing signal in the direction 2k2 - k1 for the negative time delay , and the frequency of the four - wave mixing signal has a shift related to the lorentz local field factor l
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