Use of optical anisotropy for study of ultrafast phase transformations at solid surfaces

A new method of crystalline-order detection in highly absorbing anisotropic crystals is worked out and is demonstrated experimentally on a monocrystal Zn. The method is based on partial transformation of incident p-polarized electromagnetic wave into s-polarized reflected wave due to optical anisotropy. The method is applicable to anisotropic metals (for example, Zn, Ti, Cd) and makes it possible to follow changes of crystalline structure in thin (10-100 nm) surface layers. It must be emphasized, that the method permits the detection of changes of the long-range order, whereas most of the conventional methods provide information on changes of the short-range order, which need not be changed on melting and amorphization for certain crystals. Using picosecond laser pump pulse (time duration approximate to 1 ps) and streak camera "Agat", surface melting and evaporation of Zn are studied. By means of measurement of time dependencies of s- and p-components of a reflected probe pulse (time duration approximate to 500 ps) the dynamics of melting and evaporation of a surface layer was studied at various flows of energy laser pump pulse. The characteristic time of disappearance of the long-range order is < 3 ps. The crystal structure is restored through 100-300 ps after action of a pump pulse. The theoretical analysis of experimental results was performed. Estimations, based on the proposed model, are in satisfactory agreement with the experimental results. Pump-probe experiments with time resolution higher than 3 ps are in progress.