Direct time-resolved measurement of anharmonic lattice vibrations in ferroelectric crystals

Coherent lattice vibrations are driven beyond the harmonic limit through impulsive stimulated Raman scattering excitation with intense femtosecond pulses. High overtones of the fundamental lattice vibrational frequency and wave vector are observed in the time-dependent responses of the ferroelectric crystals lithium tantalate and lithium niobate. The results provide controlled experimental access to anharmonic regions of lattice potential energy surfaces, including the ''soft'' mode or ''collective reaction coordinate'' paths along which ions move from their initial lattice positions toward the positions they occupy in new lattice structures, i.e., new domain orientations or crystalline phases. The observations represent a significant extension of coherent optical control over material behavior, and a step toward ''collective mode-selective chemistry'' through which cooperative structural rearrangements might be induced. (C) 1997 American Institute of Physics.