When the nonlinear response is dominated by the Kerr effect (nonlinear refractive index change) the asymmetry is easy to understand in the limit of thin media. Essentially the Kerr effect induces a "nonlinear lens" in the material which changes the subsequent propagation of the beam. The example you show in your question would correspond to the case of a negative Kerr effect (n2

I think nonlinear refraction can be attributed to it. When the thermal process is slow, then heat accumulation is also low. In that case, this non- linearity arises solely to pure electric effects.

Have a look at the following link.

http://lmaleidykla.lt/ojs/index.php/physics/article/viewFile/3100/1927.

Regards

Dear Lam

I think the reason for asymmetric Z-scan is due to the large phase shift.

(phase shift; Dphi = k gamma Io Leff, Leff = L/no)

The experimental and the theoretical results can be confirmed in the following two papers.

The two papers show how Z-scan lines are displayed in thin and thick media.

And the analytic solutions also involve the case of small phase shifts.

In other words, it contains a symmetric case.

Regards

[1] C. H. Kwak et al, J. Opt. Soc. Am. B 16, 600(1999).

[2] S. J. Lee et al, J. Korean Phys. Soc. 67, L413(2015).

Thank you very much, Prof.Rajib Biswas and Prof.Sang Jo Lee. I agree with Professor Sang Jo Lee that the large phase shift is one of the reasons of asymmetry of the Z-scan curve, as well as thermal nonlinear, experimental error.