A three-dimensional theoretical model for a new measurement of surface
photo-displacement in semiconductor materials is presented. In the th
eoretical model, two additional photothermal effects, i.e., the air-th
ermal lens effect above the sample surface and the photoreflectance ef
fect of the sample, are taken into account. It is shown that these add
itional effects can appreciably influence the diffraction pattern and
the detected photothermal signal.