This paper deals with the fundamental limit of speed of the photorefractive effect at a given power level. Such a limit is derived by counting the number of photons needed for the formation of a photorefractive index grating.
Pochi Yeh, Pochi Yeh,
"Invited Paper Fundamental Limit Of The Speed Of Photorefractive Effect", Proc. SPIE 0825, Spatial Light Modulators and Applications II, (19 February 1988); doi: 10.1117/12.941990; https://doi.org/10.1117/12.941990