Investigation of anharmonic lattice vibrations with coherent phonon polaritons

We investigate the dielectric response of the lowest-energy TO phonon of ${\mathrm{A}}_1$ symmetry of ${\mathrm{LiNbO}}_3$ via the impulsive excitation and heterodyne detection of THz phonon polaritons. We find that the low-frequency polariton dispersion of ${\mathrm{LiNbO}}_3$ is strongly influenced by four resonances at 1.3, 2.4, 3.4, and 4.1 THz. These resonances lead to avoided crossings in the polariton dispersion and to polariton beats. The presence of these resonances provides strong evidence that the microscopic potential of this phonon is extremely anharmonic and consists of three walls.