Phonon-polariton damping by low-frequency excitations in lithium tantalate investigated by spontaneous and stimulated Raman scattering

We investigated the damping of phonon polaritons in congruent lithium tantalate (LiTaO 3 ) single crystals by measuring spontaneous Raman spectra of the low-frequency wing of the A 1 phonon line, the dependence of the linewidth of the stimulated Raman scattering gain curves on polariton frequency and the spatial damping of propagating phonon-polariton pulses by nonlocal time-delayed coherent anti-Stokes Raman scattering. The damping and the dispersion curve of the lowest-frequency A 1 phonon polaritons were determined from 10 to 200 cm −1 at room temperature and at 77 K. The observed frequency dependence of the damping is discussed in the context of low-frequency excitations and a Debye relaxational mode.