Problems with absorption coefficient
 

I am still trying to simulate a solid semi-transparent material which is melted by a laser beam using the discrete ordinates model.

The problem is: I need to adjust the absorption coefficient of the material.
But whenever i pick a value of the absorption coefficient which is very low, the energy of the laser beam doesn't enter the computational domain.
With a high absorption coefficient, the energy of the beam only heats the material close to the surface.

What i want is that the material close to the surface has a very low absorption coefficient, but nevertheless allows the energy of the beam to enter the computational domain. Further away from the surface, the absorption should be higher to allow heating of the material.

I already tried to define the absorption coefficient in this manner with a DEFINE_PROPERTY UDF, but without success.

If anyone is familiar with this issue, i would be very grateful for his or her help.