Dynamical Studies of Phonon Polaritons by Femtosecond X-ray Diffraction
Abstract
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 LiTaO3. 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 A1 normal mode with 1-må resolution. Our experiments reveal a polariton wave with a 1.5-THz frequency and a 5-må 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.
- Publication:
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APS March Meeting Abstracts
- Pub Date:
- March 2007
- Bibcode:
- 2007APS..MAR.R1240R