Size effects on phonon response of bismuth nanoislands to ultrafast laser excitation

Abstract

The lattice dynamics of bismuth nanoislands induced by ultrafast laser excitation is studied by ultrafast electron diffraction. The decay times of the Bragg peak intensity depend on the diffraction order. For 5-nm flat nanoislands, the decay time of the (110) diffraction order is close to the A1g optical phonon decay time. When the thickness is increased to ∼16 nm, the excitation energy is dissipated into low energy phonons, making energy transport diffusive, thus leading to a longer decay time. The temporal evolution of the (012) Bragg diffraction peak shows a two exponential decay with a second decay time of ∼40 ps emerging when the nanoislands are 5 nm thick and the laser fluence is lower than 1.5 mJ/cm2. This second decay may be attributed to a phonon bottleneck caused by size restriction.