Abstract

Infrared-reflectivity measurements showed that the low-frequency dielectric response of LiNbO₃ is dominated by the lowest-energy A, phonon with a resonance frequency of 7.5 THz.¹ Unfortunately, this technique suffers from bad signal-to-noise ratios for frequencies below 5 THz so that little is known about the response in this frequency regime. Recently, it was shown that the low-frequency dielectric response of nonlinear crystals can also be investigated with coherent phonon polaritons.^2,3 We use this technique to investigate the low-frequency dielectric response of LiNbO₃. The polaritons are impulsively excited and phase-sensitively detected using laser pulses with a pulse duration of 60 fs, a central wavelength of 625 nm and an energy per pulse of 5 μJ. These laser pulses are split into three. Two of the pulses generate phonon polaritons by difference-frequency mixing. The wave vector of the polaritons can be varied by tuning the angle between the two excitation pulses. The polaritons are phase-sensitively detected by the time-delayed third pulse.

© 1994 IEEE