Determine the physical origin of the central darkening feature

Investigate and determine the physical mechanism responsible for the darkening observed near the geometric center ahead of the bulk shock in the water–sapphire sample assembly, including whether it arises from interface-wave-induced opacity in the sapphire, substrate deformation, or changes in the optical properties of the water.

Background

The streak-camera measurements show a faster-moving disturbance ahead of the bulk shock in water that produces early darkening around the geometric center of the laser ring. Its measured propagation speed is consistent with the Rayleigh-wave speed of sapphire, suggesting that the feature may be associated with an interface wave. However, the paper also identifies substrate deformation and changes in the optical properties of water as alternative explanations. Because the physical origin remains unresolved, the mechanism is left for investigation in follow-up work.