Nonlinear Acoustics at GHz Frequencies in a Viscoelastic Fragile Glass   Former
  Using a picosecond pump-probe ultrasonic technique, we study the propagation of high-amplitude, laser-generated longitudinal coherent acoustic pulses in the viscoelastic fragile glass former DC704. We observe an increase of almost ten percent in acoustic pulse propagation speed of its leading shock front at the highest optical pump fluence which is a result of the supersonic nature of nonlinear propagation in the viscous medium. From our measurement we deduce the nonlinear acoustic parameter of the glass former in the GHz frequency range across the glass transition temperature. 
Christoph Klieber, Vitalyi E. Gusev, Thomas Pezeril, Keith A. Nelson
cond-mat.mtrl-sci