title: Viscoelastic properties and efficient acoustic damping in confined polymer nano-layers at GHz frequencies.
abstract: We investigate the viscoelastic properties of confined molecular nano-layers by time resolved optical pump-probe measurements. Access to the elastic properties is provided by the damping time of acoustic eigenmodes of thin metal films deposited on the molecular nano-layers which show a strong dependence on the molecular layer thickness and on the acoustic eigen-mode frequencies. An analytical model including the viscoelastic properties of the molecular layer allows us to obtain the longitudinal sound velocity as well as the acoustic absorption coefficient of the layer. Our experiments and theoretical analysis indicate for the first time that the molecular nano-layers are much more viscous than elastic in the investigated frequency range from 50 to 120 GHz and thus show pronounced acoustic absorption. The longitudinal acoustic wavenumber has nearly equal real and imaginary parts, both increasing proportional to the square root of the frequency. Thus, both acoustic velocity and acoustic absorption are proportional to the square root of frequency and the propagation of compressional/dilatational acoustic waves in the investigated nano-layers is of the diffusional type, similar to the propagation of shear waves in viscous liquids and thermal waves in solids.
authors.affiliation: Department of Physics and Center for Applied Photonics, University of Konstanz, Germany.
authors.forename: Mike
authors.initials: M
authors.lastname: Hettich
authors.affiliation: Department of Physics and Center for Applied Photonics, University of Konstanz, Germany.
authors.forename: Karl
authors.initials: K
authors.lastname: Jacob
authors.affiliation: Department of Physics and Center for Applied Photonics, University of Konstanz, Germany.
authors.forename: Oliver
authors.initials: O
authors.lastname: Ristow
authors.affiliation: Department of Physics and Center for Applied Photonics, University of Konstanz, Germany.
authors.forename: Martin
authors.initials: M
authors.lastname: Schubert
authors.affiliation: Department of Physics and Center for Applied Photonics, University of Konstanz, Germany.|Instituto Balseiro &Centro Atomico Bariloche (CNEA), and CONICET, Argentina.
authors.forename: Axel
authors.initials: A
authors.lastname: Bruchhausen
authors.affiliation: LUNAM Universités, CNRS, Université du Maine, LAUM UMR-CNRS 6613, Av.O. Messiaen, 72085 Le Mans, France.
authors.forename: Vitalyi
authors.initials: V
authors.lastname: Gusev
authors.affiliation: Department of Physics and Center for Applied Photonics, University of Konstanz, Germany.|Institute of Technical Physics, German Aerospace Center, Pfaffenwaldring 38-40, 70568 Stuttgart, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Dekorsy