Exploring the Connection between Coherent Acoustic Phonons and Bright-Field Contrast in Ultrafast Electron Microscopy

Journal Article · Mon Aug 05 00:00:00 EDT 2019 · Microscopy and Microanalysis

DOI:https://doi.org/10.1017/S1431927619010766· OSTI ID:1595006

Du, Daniel X. ^[1]; Flannigan, David J. ^[1]

---

1. Univ. of Minnesota, Minneapolis, MN (United States)

Intense ultrafast photoexcitation of archetypal semiconducting materials, such as Ge and GaAs, ends in the rapid generation of large charge-carrier densities that propagate outward from the high-fluence region at initially hypersonic velocities. This produces a number of interesting effects that become interwoven with the underlying lattice; including the development of acoustic-type propagating oscillatory modes, excitation of coherent propagating strain waves, and coalescence of phase behaviors of the acoustic phonons and charge carriers via wave-particle drag. Using bright-field imaging in an ultrafast electron microscope, we directly imaged the behavior of photoexcited, nanoscale coherent strain waves in single-crystal Ge. In addition to single preferential wavevectors and initially hypersonic phase velocities (up to 35 nm/ps), we were able to directly resolve single-phonon constant velocities and an overall time-varying phase-velocity dispersion to the bulk speed of sound over approximately one nanosecond. Comparison to dispersion behaviors expected for symmetric and asymmetric Lamb-type modes showed good agreement with experiments, suggesting the preferential excitation of a single, symmetric first-order mode. Yet, quantitative comparison to the expected and measured contrast strengths associated with the coherent dynamics is needed in order to further elucidate the precise microscopic mechanisms.

• Research Organization: Univ. of Minnesota, Minneapolis, MN (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES)
• Grant/Contract Number: SC0018204
• OSTI ID: 1595006
• Journal Information: Microscopy and Microanalysis, Vol. 25, Issue S2; ISSN 1431-9276
• Publisher: Microscopy Society of America (MSA)
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY