I learned using COMSOL Multiphysics 5.1. I made a model of the sample in 2D with axisimmetric revolution around the pump beam axis. I built a thin rectangle for the absorbing coating (10 micron) and a thick rectangle for the substrate (3mm). The mesh is fine (1micron) near the heat source and in the coating, and coarse (0.5mm) far from the center (see the picture mesh.png). The heat source is a gaussian beam of size 70micron and power 1W. The absorption ratio is 12ppm. Chopper frequency is 430Hz but the waveform is a sine, because I'm going to compare this  solution with the analitic solution which has a sine waveform instead of a square one. I attach the plot of the temperature of the point (r,z)=(0,0) as a function of time for 100 ms. And an animation of the temperature distribution. with the color scale in kelvin. I cannot upload the file and I don't understand why. It is a gif file and it's less than 10Mb. this is the link to my dropbox: https://www.dropbox.com/s/gd28tb1zc7gxdob/movie.gif?dl=0