Effect of ion irradiation on coherent phonon dynamics in bismuth

We investigate the effects of crystalline disorder on the dynamics of coherent optical phonons in bismuth using optical heterodyne detection (OHD). The excitation and relaxation of A(1g) coherent phonons are studied with femtosecond time resolution for a bismuth single crystal, as-deposited microcrystalline film, and ion-irradiated microcrystalline film. The relaxation lime of the coherent phonon in bismuth decreases with increasing disorder, from 4.75 ps for the single crystal to 2.16 ps for the ion-irradiated microcrystalline film. The decrease in relaxation time is attributed to the dampening of phonons due to grain boundaries and point defects, and correlates with the spatial localization of phonons.