All-electron theory of the coupling between laser-induced coherent phonons in bismuth

Using first principles, all-electron calculations and dynamical simulations we study the behavior of the A_1g and E_g coherent phonons induced in Bi by intense laser pulses. We determine the potential landscapes in the laser heated material and show that they exhibit phonon-softening, phonon-phonon coupling, and anharmonicities. As a consequence the E_g mode modulates the A_1g oscillations and higher harmonics of both modes appear, which explains recent isotropic reflectivity measurements. Our results offer a unified description of the different experimental observations performed so far on bismuth.