Ultrafast Frequency Dynamics of Coherent Phonons in Te Under High Density Photoexcitation

We report on the ultrafast dynamics of large amplitude A1 coherent optical phonons in Te after excitation with femtosecond laser pulses. Using time-resolved pump-probe dielectric tensor measurements on a single crystal Te sample we can monitor the excitation of the A1 phonon mode due to photoexcited carrier density redistribution. By performing a short-term Fourier transform of the observed dielectric tensor oscillations we can track the evolution of their local frequency and determine the time-resolved frequency response of the photoexcited A1 phonon mode.

The purpose of the experiment is to determine if mode anharmonicity (due to the large amplitude of the induced oscillations) or phonon mode softening (due to perturbation of the harmonic potential by the photoexcited carriers) dominates the frequency dynamics of the coherent phonons. Reflectivity-based studies of coherent phonons in Bi reveal a linear relationship between the local frequency and the square amplitude of the oscillations, which is expected for an anharmonic potential. Softening is also observed but is significantly weaker than anharmonicity. In contrast, we do not observe any anharmonic behavior in our data, but only mode softening. Therefore, in agreement with theoretical predictions, mode softening is more important in Te than mode anharmonicity.