Probing Unfolded Acoustic Phonons with X Rays

Abstract

Ultrafast laser excitation of an InGaAs/InAlAs superlattice (SL) creates coherent folded acoustic phonons that subsequently leak into the bulk (InP) substrate. Upon transmission, the phonons become "unfolded" into bulk modes and acquire a wave vector much larger than that of the light. We show that time-resolved x-ray diffraction is sensitive to this large-wave vector excitation in the substrate. Comparison with dynamical diffraction simulations of propagating strain supports our interpretation.

Time-domain sampling of x-ray pulses using an ultrafast sample response

“…3 In this way, coherent optical superlattice phonons, which correspond to back-folded acoustic phonons have been excited and studied in detail. 4,5 Such systems show modulations of superlattice Bragg peaks on a picosecond (ps) timescale 6 and may eventually lead to the development of new devices such as an ultrafast x-ray switch. 7,8 On a more fundamental level, it has been shown recently that thin layer transducers can be used to excite quasi-monochromatic strain waves, which allow for studying coherent acoustic phonon dynamics.…”

Reconstructing longitudinal strain pulses using time-resolved x-ray diffraction

“…With the development of high-brightness, short pulse, hard x-ray sources in the last 25 years, TRXRD has rapidly grown to study a whole range of laser initiated structural dynamics 1,3,5,11,12,[16][17][18][19][20][21][22][23] . Like traditional optical pump-probe techniques, TRXRD typically utilizes an intense optical pump pulse to rapidly initiate structural change through the rapid heating of the material.…”

Observing backfolded and unfolded acoustic phonons by broadband optical light scattering

“…In the vicinity of the preferentially excited modes at q ¼ nG SL many modes are populated that can be observed by the white light probe in backscattering and dominate the signal. When the phonon wavepacket has passed the superlattice (after t % 2d SL =v c % 180 ps) and travels into the substrate, the backfolded branches of the phonon dispersion unfold [12,26] and the usual acoustic dispersion in bulk STO (with slightly larger slope due to the larger sound velocity of v STO ¼ 8 nm/ps compared to v c ¼ 7:6 nm/ps that can be calculated for this superlattice)…”