Coherent longitudinal acoustic phonons in pyrite

We have studied the coherent longitudinal acoustic (LA) phonon in a single crystal of pyrite (FeS2) by femtosecond pump and probe spectroscopy. The coherent LA phonon signal is observed as a very clear oscillatory structure in the reflection type pump and probe measurement. The amplitude, decay rate, frequency and phase of the signal show large changes as the wavelength of the laser is varied. By applying a wave-packet model to the analysis of the coherent LA phonon signal, it is found that the refractive index, the extinction coefficient and the longitudinal sound velocity are obtained from the coherent LA phonon signal without need for any other material parameters. From the experimental result, we precisely determine these values for the pyrite sample. The wavelength dependence of the change of the complex dielectric constant is also obtained.