I made a simulation of the temperature distribution on a GaAs sample of 0.4mm in thinckness and 5% of absorption.

First with the absorption distributed on all the substrate (bulk) and then only on the coating (surface).

Plot the map of the temperature, as a function of the depth and of the radius, for both cases (bulk and surface).

Used the temperature distribution in the optical simulation with a 1310nm wavelength probe.

Simulated a scan along the sample for both cases and found the same value of the AC/DC PD signal.