Excitation of longitudinal and transverse coherent acoustic phonons in nanometer free-standing films of (001) Si

Abstract

Transmission electron diffraction is naturally sensitive to the detection of shear-type deformations in singlecrystalline structures due to the effective tilting of the lattice planes characteristic of shear, but in general is insensitive to longitudinal phonon propagation. Here, we report on the generation and detection of both transverse and longitudinal coherent acoustic phonons in 33 nm free-standing ͑001͒-oriented single crystalline Si films using femtosecond electron diffraction ͑FED͒ to monitor these laser-induced atomic displacements. The mechanism for excitation of the shear mode that leads to coupling to the longitudinal phonon is attributed to the inhomogeneous lateral profile of the optical-pump pulse and the periodic boundary condition imposed by the supporting grid structure. In this application, the constructive interference in the diffraction process makes FED particularly sensitive to the detection of coherent phonon modes and offers an atomic perspective of the dynamics involving collective motions.

The c-axis thermal conductivity of graphite film of nanometer thickness measured by time resolved X-ray diffraction

“…In addition, recent developments in structurally sensitive time resolved diffraction (TRD) techniques have made it possible to extract transient information on the atomic structure with unprecedented time resolution; in effect, adding the time dimension to the otherwise static picture offered by conventional methods. Hence, from a materials characterization viewpoint, TRD expands the range of physical constants that can be investigated as demonstrated by the following examples of time resolved electron and X-ray diffraction studies: measurement of the Gr€ uneisen parameter in Al nano-films, 3 the speed of acoustic modes in Si nano-films, 4 InAs nanowires, 5 and Bi nanofilms, 6 and the electron-phonon coupling constant in graphite. 7 A comprehensive review of time resolved electron and X-ray diffraction methods and their applications beyond materials characterization is found elsewhere (see, e.g., Refs.…”

Single shot time stamping of ultrabright radio frequency compressed electron pulses

“…23,24 Fig. 4(a) shows the silicon diffraction pattern passing through the streak plates with no streak voltage and Fig.…”

Bandgap modulation in photoexcited topological insulator Bi2Te3 via atomic displacements

“…In general, the sound velocity in a crystal is calculated as a function of the frequency of the acoustic phonon vibrations and film thickness. 49,50 In this study, the frequency of the acoustic mode (35 GHz) and film thickness (30 nm) yielded a sound velocity of 2100 m/s, which corresponds to the longitudinal-mode sound velocity (2000-2200 m/s) in Bi 2 Te 3 . 51,52 Since these acoustic phonon modulations have opposite phases in opposite planes, this effect of the acoustic phonons in the transient electron intensity can be canceled out by adding these oppositephased oscillations.…”