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

"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?

• What is the device structure? Could they just be considering drift currents rather than diffusion currents? – Jon Custer Feb 3 at 15:56
• You mean the setup being used, or the particular device where these currents are being applied to? I believe it is a Silicon structure/ layer. – user7077252 Feb 3 at 15:59
• And still, wouldn't the drift current have a direction perpendicular to the photoexcitation light? – user7077252 Feb 3 at 16:00

$$n(z, t) \approx \sigma_e |I(z, t)|;$$ where says the electron-hole pair density is direct proportional to intensity of the penetration radiation. The $$\sigma_e$$ is the absorption conversion efficient.