Phase transitions and switching behaviour in thin (1.5 - 4 mum) planar cells of a ferroelectric liquid crystal (FLC) mixture are studied by optical second harmonic generation (SHG). A pronounced anisotropy of the SHG response shows a high ordering degree of the FLC molecules in the cells. We studied the electric field-induced switching of the FLC cells and their temperature dependence by means of SHG. This shows the existence of a 10-20 nm thick frozen sub-surface layer of FLC molecules,which cannot be switched by an electric field well above the bulk saturating field and at significantly high temperatures.