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 through a real-space imaging method based on Fresnel diffraction and phase-contrast microscopy. Coherent optic phonon-polaritons generated through impulsive stimulated Raman scattering in a ferroelectric crystal are imaged with 5 mu m spatial resolution and 35 fs time resolution as they propagate out of the excitation region and into other parts of the crystal. The "spatiotemporal'' imaging method will permit direct examination of nonlinear lattice dynamics and will provide feedback in coherent control systems that use spatiotemporal femtosecond pulse shaping to manipulate propagating excitations with ultrashort light pulses directed to specified sample locations at specified times. The method will also permit the examination of defects and fabricated structures in bulk or thin film crystals using visible light. (C) 1999 American Institute of Physics. [S0021-9606(99)01803-6].