Why is the carrier diffusion neglected as a consequence of a greater penetration depth of the photoexcitation light?

Question

"Since the diffusion lengths are considerably smaller than the penetration depth of the photoexcitation light (...), we can neglect carrier diffusion from our considerations" - THz article
From what I understand the diffusion length is the average length that the excited electrons move between the moment they are excited out of the positive ion core until the moment they fill the positive hole space they had left behind.
I imagine this distance to be horizontal in a material, in the direction of the current, and I presume that the penetration depth of the light is a vertical distance, or at a certain vertical angle from the surface, so how are these related?
Why is the carrier diffusion neglected as a consequence of the penetration depth of the photoexcitation light being greater than the diffusion length of the charge carrier?

ytlu · Answer

Near the semiconductor surface, there is electric field due to band bending, which drives the photoelectron-hole pairs diffuse in opposite directions along vertical direction.
When an article make statement like "Since the diffusion lengths are considerably smaller than the penetration depth of the photoexcitation light ", which means that they are going to use the relation:
$$
   n(z, t) approx sigma_e |I(z, t)|;
$$
where says the electron-hole pair density is direct proportional to intensity of the penetration radiation. The $sigma_e$ is the absorption conversion efficient.
In order to justify this relation, the author is exclusive the effect of diffusion effects. Very often, it is requested by referee.

Why is the carrier diffusion neglected as a consequence of a greater penetration depth of the photoexcitation light?

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