Coherent phonons in bismuth film observed by ultrafast electron diffraction

A Bugayev,H E Elsayed-Ali,A Esmail,M Abdel-Fattah

doi:10.1063/1.3574888

Abstract

The generation of coherent phonons in polycrystalline bismuth film excited with femtosecond laser pulse is observed by ultrafast time-resolved electron diffraction. The dynamics of the diffracted intensities from the (110), (202), and (024) lattice planes show pronounced oscillations at 130–150 GHz. The origin of these coherent acoustic phonons is discussed in view of optical phonon decay into two acoustic phonons. Different drop times in the intensity of the diffraction orders are observed and interpreted as anisotropy in the energy transfer rate of coherent optical phonons.

Highlights

The interaction of femtosecond laser pulses with semimetals and semiconductors produces electronic excitations coupled with lattice vibrations through the deformation potential and stimulated Raman scattering.[1,2] If the frequencies of Raman active phonons in the laser-excited material are smaller than the inverse duration of the laser pulse, the laser-induced change in the equilibrium positions of the nuclei results in generation of coherent optical and acoustical phonons.[3]
Time-resolved electron diffraction was used to study the dynamics of coherent acoustic phonons in a single-crystalline Bi film (30–50 nm thickness) on a picosecond time scale.[14]
We report on the characteristics of coherent phonons as observed in modulation of temporal behavior of the diffracted intensities from the (110), (202), and (024) lattice planes of polycrystalline free-standing 22±2 nm thick Bi film

Summary

Coherent phonons in bismuth film observed by ultrafast electron diffraction

We report on the characteristics of coherent phonons as observed in modulation of temporal behavior of the diffracted intensities from the (110), (202), and (024) lattice planes of polycrystalline free-standing 22±2 nm thick Bi film. An electron gun, described in our previous publications,[15,16] was used to produce electron pulses by photoemission from silver thin film photocathode excited by frequency-tripled femtosecond (110 fs duration, 800 nm wavelength, at 1 kHz repetition rate) laser pulses. The gun operated at acceleration voltage 35 keV, producing electron pulses of ∼1.5 ps duration at the photocathode excitation level used. The Bi film was exposed to the fundamental 800 nm wavelength at a pulse energy density ∼2 mJ/cm[2] that corresponds to excited carrier’s density ∼1021 cm-3. The electron diffraction pattern of the studied Bi film, without excitation, and its radial intensity distribution

Ibg Ibg

Imin are the maximum and minimum intensities of

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