My modest insights on the subject are:

Ponderomotive potential

Up = F2 / 4ω2, where

ω - is a feld frequency

F - is the strength of the electromagnetic field

But I guess you are interested in the effect in a non-linear electromagnetic field in a specific environment.

It should be looking for a minimum of F.

Look at this article can be helpful.

https://link.springer.com/chapter/10.1007/978-3-642-14694-7_9.