What is believed to be a new phenomenon - plasma-based thermally-induced optical reflection of sound (P-THORS)- overcomes the limitations of traditional THORS (e.g. need for line-of-sight, concentration dependence, and variations in efficiency at distance) by using a laser to generate a shaped plasma for the formation of highly-efficient, variable geometry, free-space acoustic reflector/barrier. These reflectors allow for the focusing and steering of acoustic waves without the need of a continuous barrier or engineered materials. This work demonstrates for the first time the generation of P-THORS barriers with highly efficient reflection efficiencies, exceeding those of traditional THORS barrier (i.e. > 70%) as well as the ability to transiently shape the barrier into various geometries to steer the acoustic wave to specific locations. P-THORS barriers are shown to reflect near 100% of incident ultrasonic signals and maintain that reflectivity for 5ms post-plasma formation. Furthermore, by shaping the plasma it was possible to selectively direct the ultrasonic and/or acoustic waves to a desired location with efficiencies of 30% or greater, depending on the geometry of the shaped plasma.
Reardon et al. (Mon,) studied this question.