Spatiotemporal Visualization of the Formation and Decay of Thermally Induced Optical Reflection of Sound (THORS) Barriers in Ambient Air

This paper provides the first temporally resolved visualization of the formation and decay profiles of Thermally induced reflection of sound (THORS) barriers in ambient air, revealing the spatiotemporal characteristics of these novel acoustic barriers. In this work, a 532 nm neodymium-doped yttrium aluminum garnet (Nd:YAG) laser coupled with an intensified charge-coupled device (ICCD) is used to Raman image N₂ in ambient air, thereby allowing for the visualization of the spatial dynamics of the air density variations at these THORS barriers. Studies were conducted at various ambient temperatures and with air turbulence across the beam path revealing no change in barrier size or shape under typical environmental disturbance conditions. Raman images of a barrier formed by a repetitively pulsed CO laser reveal an abrupt barrier density change between the optically depleted region and the surrounding air, with the slope of the imaged barrier density increasing rapidly during the first 20 ms of barrier formation, indicative of the predicted increase in barrier abruptness associated with enhanced THORS efficiency. As seen in previous temporal studies of THORS barrier efficiencies, these images reveal that multiple laser pulses at an optimized optical frequency are capable of achieving maximum continuous suppression efficiencies through molecular depletion in the optically excited region. These imaging studies revealed that the maximum barrier efficiency required a minimum of eight laser pulses to achieve the desired barrier density change and depletion, agreeing with previous temporal studies that showed in maximum suppression efficiency after 16 ms with one ms excitation laser pulses. Furthermore, visualization of the barrier size revealed that thermal redistribution of the photothermally excited molecules resulted in a THORS barrier approximately 50% larger than the excitation beam width and that this barrier remains constant for as long as 15 ms after the final laser pulse and at laser powers between 50 and 250 W.