Antisymmetric nature of the earliest arrivals of ultrasound propagating in copy paper

Experiments with laser generation and detection of ultrasound in copy paper indicate that earliest arrivals of ultrasound are antisymmetric. A likely explanation is that the out-of-plane interferometer is relatively insensitive to the symmetric compressional pulse because of the low Poisson ratio of copy paper, but rather sensitive to its antisymmetric flexural wake associated with the random heterogeneities and gradients present in copy paper.