Pump-probe reflectivity studies of ultrashort laser-induced acousto-mechanical strains in ZnO films

• Published in: Applied physics. A, Materials science & processing Vol. 129; no. 8
• Main Authors: Kosma, Kyriaki, Kaleris, Konstantinos, Kaselouris, Evaggelos, Kaniolakis-Kaloudis, Emmanouil, Petrakis, Stylianos, Orphanos, Yannis, Gagaoudakis, Emmanouil, Binas, Vassilis, Bakarezos, Efthimios, Tatarakis, Michael, Dimitriou, Vasilis, Papadogiannis, Nektarios A.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01-08-2023; Springer Nature B.V
• Subjects: Absorption; Acoustic propagation; Acoustic waves; Acoustics; Applied physics; Characterization and Evaluation of Materials; Condensed Matter Physics; Elastic waves; Energy conversion; Finite element method; Machines; Manufacturing; Material properties; Materials science; Mathematical analysis; Mathematical models; Nanotechnology; Optical and Electronic Materials; Phonons; Photovoltaic cells; Physics; Physics and Astronomy; Processes; Reflectance; Surfaces and Interfaces; Thermal expansion; Thin Films; Wave velocity; Zinc oxide; Strain waves; Pump-probe optoacoustics; ZnO layered materials; DC sputtering technique
• ISSN: 0947-8396; 1432-0630
• DOI: 10.1007/s00339-023-06837-1

In the current work we report on the generation of acoustic strains in thin ZnO layers using optoacoustic transduction of ultrashort laser pulses into acoustic waves on an Au thin film transducer. After absorption of energy by the electron system of the metal, energy conversion, thermal expansion and mechanical deformation takes place. The generation and propagation of the induced acoustic strains are monitored in time via a degenerate pump-probe transient reflectivity optical setup at 800 nm, as opposed to most commonly used schemes that employ different wavelengths for the pump and probe beams, mostly in the vicinity of ZnO maximum absorption. The experimental results include energy relaxation times and phonon scattering frequencies and are supported by a thermal vibro-acoustic finite element model. The model is based on the combination of a revised two-temperature approach and elasticity theory, and considers anisotropic properties for the ZnO film and the computation of the elastic wave velocity for the first time. The outcomes are discussed in the context of electron–phonon coupling factors and other material properties. A good agreement between the experimental findings and the results from the numerical simulations has been established, regarding outcomes like the mean velocity of the strain waves, establishing a novel characterization method applicable to a variety of materials and structures.