LONG-DISTANCE PROPAGATION AND DAMPING OF LOW-FREQUENCY PHONON POLARITONS IN LINBO3

The propagation and damping of low-frequency (10 to 80 cm(-1)), picose cond phonon-polariton pulses have been investigated in LiNbO3. The pho non polaritons propagate over a distance up to several centimeters. Th e measured frequency dependence of the absorption coefficient and the phase relaxation rate of the phonon polaritons is discussed using a mo del that takes into account the coupling of the phonon polaritons to a relaxational mode and to low-frequency excitations connected with def ect centers. The much higher polariton damping at 300 K compared to 77 K is explained by the presence of the relaxational mode, which is mis sing at 77 K, and the larger coupling strength of the low-frequency ex citations. The difference between the polariton damping of MgO doped L iNbO3 and congruent LiNbO3 is ascribed to the changes of the resonance frequencies of the defect centers.