Journal
OPTICS EXPRESS
Volume 23, Issue 13, Pages 17191-17204Publisher
OPTICAL SOC AMER
DOI: 10.1364/OE.23.017191
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Funding
- Operational Program Education and Lifelong Learning, Action Archimedes III (sub-action 19: Innovative optoacoustic device for 3d spatiotemporal micro-characterization of composite materials based on ultrafast laser pulses)
- European Union (European Social Fund)
- Greek national funds (National Strategic Reference Framework)
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The role of thin-film metal transducers in ultrafast laser-generated longitudinal acoustic phonons in Si (100) monocrystal substrates is investigated. For this purpose degenerate femtosecond pump-probe transient reflectivity measurements are performed probing the Brillouin scattering of laser photons from phonons. The influence of the metallic electron-phonon coupling factor, acoustical impedance and film thickness is examined. An optical transfer matrix method for thin films is applied to extract the net acoustic strain relative strength for the various transducer cases, taking into account the experimental probing efficiency. In addition, a theoretical thermo-mechanical approach based on the combination of a revised two-temperature model and elasticity theory is applied and supports the experimental findings. The results show highly efficient generation of acoustic phonons in Si when Ti transducers are used. This demonstrates the crucial role of the transducer's high electron-phonon coupling constant and high compressive yield strength, as well as strong acoustical impedance matching with the semiconductor substrate. (C) 2015 Optical Society of America
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