There are currently two mainstream theories when describing the generation mechanism of terahertz waves. One is an ionized photocurrent model, and the other is a four-wave mixing model. The four-wave mixing model is designed to the third-order nonlinear process in the filament. However, third-order nonlinear processes have encountered difficulties in describing the strength of terahertz waves. It has been reported in the literature that higher order nonlinear effects are more effective than third order nonlinear effects. We know that higher order nonlinearities require higher light intensity. My confusion is that the four-wave mixing model is suitable for laser energy with low laser energy, which corresponds to the pulsed laser energy when the plasma has not yet formed. Then why is the higher-order nonlinear effect for such low energy more important than the third-order nonlinear effect? My confusion stems from the b diagram in Figure 12 in the following article.
Article Broadband THz emission from gas plasmas induced by femtoseco...
For figure12 (b) In this article, it is stated that the each of the higher orders (5th and 7th) have had some arbitrary scaling applied to fit on the same figure as the 3rd order process. I believe this is the root of your confusion.
To my knowledge third-order processes are more efficient for THz generation than the higher order processes
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