Ultrahigh-frequency phonon resonances were excited in ultrathin films (∼5nm) by femtosecond light pulses, and their resonance frequencies were measured to determine the through-thickness elastic constants. The studied materials were Pt and Fe. The elastic stiffness increases with decreasing film thickness for both materials, whereas the normal strain showed opposite thickness behavior. Analysis of the wave propagation using the third-order elastic constants explained these trends.
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Ogi et al. (2007) studied this question.
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