Abstract

We report on the impulsive excitation and phase-sensitive detection of terahertz phonon polaritons in LiTaO₃ with a well-defined wave vector, frequency, and direction of propagation.¹ The polaritons are excited by the difference-frequency mixing of ultrashort pulses that are generated by an amplified CPM laser (τ_p = 60 fs, λ = 625 nm, E = 5 μJ). These laser pulses are split into three. Two of the three pulses serve as excitation pulses and are focused into the sample under a given angle. This angle defines the wave vector of the polariton. The polariton frequency that corresponds to this wave vector according to the polariton dispersion is obtained from the broad bandwidth of the laser pulses. The polaritons are probed with the third pulse by means of the electro-optic effect. The damping and dispersion of the polaritons are strongly influenced by the ferroelectric phonon. This phonon leads to a strong absorption in LiTaO₃ near 6 THz (200 cm⁻¹) (Ref. 2). The strength and frequency of this phonon change when the temperature is varied. These changes are observed in the polariton dynamics.

© 1994 Optical Society of America