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Broadband Terahertz Time-Domain Spectroscopy of Complex Phonon-Polariton Dispersion Relation

The low-frequency phonon-polariton of a ferroelectric congruent lithium niobate crystal was studied. The lowest frequency IR active A1(z) and E(x) modes were measured by the broadband polarized Terahertz Time-Domain Spectroscopy (THz-TDS) using the pulse coherent THz radiation from 0.2 to 4.5 THz. The complex dispersion relations were determined using the real and imaginary parts of a polariton wavevector calculated from the complex extraordinary and ordinary refractive indices for the A1(z) and E(x) symmetry phonon-polaritons, respectively. The remarkable change of the dispersion relation was observed in the vicinity of TO and LO mode frequencies for both real and imaginary parts of a polariton wave vector. The observed complex dispersion relations of two symmetries are in agreement within the experimental uncertainty with the dispersion curves fitted by the damped harmonic oscillator model.