Photocurrents in doped LiNbO3 crystals are shown to be due to a bulk photovoltaic effect with saturation voltages in excess of 1000 V (∼105 V/cm). This effect accounts for the light-induced index changes in LiNbO3. An explanation of the photovoltaic effect, based on the asymmetry of the lattice, is proposed.

Generation of dense electron-hole plasmas in silicon with intense 100-fs laser pulses is studied by timeresolved measurements of the optical reflectivity at 625 nm. For fluences F between 10 mJ/cm 2 ϽF Ͻ400 mJ/cm 2 , plasma generation is dominated by strong two-photon absorption, and possibly higher-order nonlinearities, which lead to very steep spatial carrier distributions. The maximum carrier densities at the sample surface are in excess of 10 22 cm Ϫ3 , and therefore, the reflectivity shows a mainly Drude-like freecarrier response. Within the Drude model, limits for the optical effective mass and the damping time are determined.

The study of phase-transition dynamics in solids beyond a time-averaged kinetic description requires direct measurement of the changes in the atomic configuration along the physical pathways leading to the new phase. The timescale of interest is in the range 10(-14) to 10(-12) s. Until recently, only optical techniques were capable of providing adequate time resolution, albeit with indirect sensitivity to structural arrangement. Ultrafast laser-induced changes of long-range order have recently been directly established for some materials using time-resolved X-ray diffraction. However, the measurement of the atomic displacements within the unit cell, as well as their relationship with the stability limit of a structural phase, has to date remained obscure. Here we report time-resolved X-ray diffraction measurements of the coherent atomic displacement of the lattice atoms in photoexcited bismuth close to a phase transition. Excitation of large-amplitude coherent optical phonons gives rise to a periodic modulation of the X-ray diffraction efficiency. Stronger excitation corresponding to atomic displacements exceeding 10 per cent of the nearest-neighbour distance-near the Lindemann limit-leads to a subsequent loss of long-range order, which is most probably due to melting of the material.

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