Real-space polariton wave packet imaging

The propagation of collective lattice vibrational wave packets that move at lightlike speeds through their crystalline hosts has been monitored throug h a real-space imaging method based on Fresnel diffraction and phase-contra st microscopy. Coherent optic phonon-polaritons generated through impulsive stimulated Raman scattering in a ferroelectric crystal are imaged with 5 m u m spatial resolution and 35 fs time resolution as they propagate out of t he excitation region and into other parts of the crystal. The "spatiotempor al'' imaging method will permit direct examination of nonlinear lattice dyn amics and will provide feedback in coherent control systems that use spatio temporal femtosecond pulse shaping to manipulate propagating excitations wi th ultrashort light pulses directed to specified sample locations at specif ied times. The method will also permit the examination of defects and fabri cated structures in bulk or thin film crystals using visible light. (C) 199 9 American Institute of Physics. [S0021-9606(99)01803-6].