Measurement of a phonon-polariton dispersion curve by varying the excitation wavelength

Coupling between optical phonons and light leads to the formation of phonon polaritons, resulting in important changes to the material dispersion. Commonly, the phonon-polariton dispersion curve has been determined by transient-grating impulsive stimulated Raman-scattering measurements, but reported results for the prototypical dispersion curve of LiNbO3 have led to some controversy. We present a procedure to measure the phonon-polariton dispersion curve by varying the pump wavelength with fixed probe wavelength that allows measurement over a broad range of wave vectors. We determine the lowest-frequency E-mode phonon-polariton dispersion curve for LiNbO3 with measured wave vectors ranging from 1 x 10(3)cm(-)(1) to 19 x 10(3)cm(-)(1), which is the broadest known optically measured phonon-polariton dispersion curve for LiNbO3. The broad range offers improved understanding of the extracted material parameters. We also note some challenges that occur in interpreting the data at short pump wavelengths; these effects help explain controversial differences between previous transient-grating measurements of LiNbO3 dispersion curves.