Applied Nanophotonics, Liquid Crystals and Biomedical Optics

Structural instabilities at nucleation and crystallization of hydrogenated amorphous silicon (a-Si:H) induced by continuous laser irradiation are investigated by means of Raman scattering. The temporal evolution shows four stages, which depend on the laser intensity as follows. (a) For relatively low intensities, the amorphous Raman spectrum exhibits quasiperiodic changes. (b) At intermediate laser levels, traces of crystalline structure appear and disappear quasiperiodically. (c) For somewhat higher irradiation levels, but below the crystallization threshold, the Raman crystallinelike peak has an oscillatory behavior for some time and after that its intensity increases and saturates. (d) For irradiation levels above the threshold, the Raman crystalline peak intensity increases monotonically and rapidly. We propose the following model, that the optical field pumps the material athermally from the amorphous disordered states towards more ordered states. This is achieved through the formation of energetically favorable small wurtzite and somewhat larger diamond submicrocrystalline clusters, whose average size depends on the pumping rate. The formation of these clusters is opposed by a restoring force from the stressed surrounding material, which accounts for the oscillations observed.