Investigation of Phonon-Polariton Lifetimes and Generation of Picosecond Far-Infrared Pulses in LiNbO3

PD Thesis of Tiequn Qiu

Prof. Max. Maier, May 16th, 1997

This work, for the first time applied the coherent anti-Stokes Raman scattering (CARS) time-of-flight technique to the study of the polariton lifetime in the very low frequency region (down to 10cm-1). The propagation of polariton wave packets in the LiNbO3 crystal over a length of several cm was clearly observed. The energy velocity and lifetime of the polaritons were measured for MgO:LiNbO3 and congruent LiNbO3 at both room and liquid nitrogen temperatures. The velocity of the polaritons was found to be about 0.20c. In the frequency range from 10 to 80cm-1, polariton lifetime from about 100 to 20ps and 300 to 20ps have been measured at 300 and 77K, respectively. The experimental results have been discussed using a model where coupling of polaritons to low-frequency modes is treated. Coupling to a relaxational modes, to defect modes and phonon modes of E symmetry, have been suggested. These coupling lead to an increase of polariton damping in the low-frequency region.

In the far-infrared pulse generation experiment, the polaritons were excited close to the crystal surface in a specially designed pump geometry where the pump beams are reflected internally by the crystal surface. The polaritons propagate to the exit surface and leave the crystal as FIR light. In this way, we generated the most intense picosecond far-infrared (FIR) pulses, tunable over the widest spectral range among the published results in nonlinear optical method. The FIR pulses generated are 30 ps in duration with maximum peak power of more than 10 kW at room temperature. Up to a factor of 7 increase of the FIR pulse energy was observed by lowering the crystal temperature to 77K. The FIR radiation is continuously tunable from 10 to 200cm-1, which covers the whole far-infrared spectral region. The frequency dependence of the generated FIR energy was explained by a nonlinear optic theory taking into account the contributions from mode coupling.

We investigated the difference of FIR generation in MgO doped and congruent LiNbO3. It was found that MgO:LiNbO3 crystals are more efficient in FIR generation than congruent LiNbO3. This is due to the higher damping in congruent LiNbO3 crystals coming from the stronger coupling to the defect modes, which is in agreement with the polariton lifetime experiments.