In order to obtain an estimation of the correction factor between bulk absorption and surface absorption on the same material (GaAs), I made some calculations.
The assumptions are a bit strong:
- the signal from the photodiode detector is proportional to the phase change of the probe beam.
- the phase change of the probe beam is proportional to the optical path change.
- the optical path change is proportional to the integral of temperature along the beam path inside the material.
I calculate the temperature distribution inside a GaAs of thickness 400micron, absorption of 12ppm on a surface of 10micron (Figure1) and absorption of 12ppm inside all the bulk 400micron (Figure2).
Calculate the integral of the temperature in dz (depth), as a function of r (radius) for both bulk and surface absorption (Figure3). Optical path change.
Divide the optical path change of bulk absorption by the optical path change of surface absorption. Plot the ratio as a function of r (Figure4).
I plot the same thing in a wider range, up to 5cm or radius to see how the graph goes to zero.
In the second figure I take the second derivative of the optical path with respect to the radius. The second derivative should de proportional to the focusing power, hence, to the signal of the absorption system.
It is the same for the bulk and for the surface absorption