Dynamical Studies of Phonon Polaritons by Femtosecond X-ray Diffraction

In solids, light propagation near resonances can be described in terms of polaritons, mixed excitations with both phonon- and photon-like character arising from the coupling of the oscillating electromagnetic field to polar lattice vibrations. Here we use time-resolved x-ray diffraction with 200-fs synchrotron pulses to directly measure the coherent lattice distortions associated with the propagation of THz phonon polaritons in ferroelectric LiTaO$_{3}$. THz radiation was generated by fs IR pulses via impulsive stimulated Raman scattering. The lattice motion component of polariton propagation was measured as time-dependent modulation of the 006 structure factor. The comparison with a time-domain simulation of THz propagation allowed extracting the absolute atomic displacements along the coordinate of the A$_{1}$ normal mode with 1-m{\AA} resolution. Our experiments reveal a polariton wave with a 1.5-THz frequency and a 5-m{\AA} peak displacement of the Ta atom with respect to the plane of the oxygen atoms. Complementing optical techniques which measure the time-dependent electrical polarization, femtosecond X-ray experiments directly monitor amplitude and phase of all structural degrees of freedom.