Study of the exciton selection rules of layer type semiconductor GaSe by thermoreflectance

By performing thermoreflectance measurements with polarized light on a face of GaSe crystal normal to the plane of the layers it has been possible to show that the direct exciton transitionis allowed and very strong when the electric field of thr light is normal to the layers, while it is 20 times weaker when the electric field is parallel to them. These findings disagree with presently available band structure calculations