Exploring the Connection between Coherent Acoustic Phonons and Bright-Field Contrast in Ultrafast Electron Microscopy
- 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)Copyright Statement
- Country of Publication:
- United States
- Language:
- English
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