Laser-induced phonon-phonon interaction in bismuth

We demonstrate that the coupling between laser-induced coherent phonons in bismuth leads to the appearance of mixing signals in the isotropic reflectivity. As a consequence modes that cannot usually be detected by means of the isotropic reflectivity show up. We further demonstrate that this interaction is strongly dependent on the laser fluence and is for that reason only observable when sufficiently intense laser pulses are used. In addition, we demonstrate that the coupling between phonons of the same symmetry leads to the appearance of higher harmonics in the isotropic reflectivity.