Fundamentals of THermally-induced Optical Reflection of Sound (THORS)

Abstract

A recently discovered phenomenon known as THermally-induced Optical Reflection of Sound (THORS), provides the ability to generate acoustic reflective barriers by exciting media in the path of an infrared (IR) laser beam, causing abrupt changes in compressibility (and acoustic impedance) between the excited and surrounding media. Initial studies showed that THORS barriers are capable of reflecting incident acoustic waves at efficiencies of 25% or greater. Additionally, THORS waveguiding channels, can reduce attenuation of sound waves from 1/r to 1/r0.6 with distance. Characterization of THORS in ethanol-based studies using a CO2 laser showed the effects that several functional and physical parameters have on reflection/suppression efficiency. Studies showed that a minimum optical modulation frequency of 600 Hz is necessary to achieve efficient acoustic reflection/suppression. Acoustic frequencies (1-20 kHz) had no influence on the reflection/suppression efficiency of the barrier. Furthermore, barrier reflection/suppression efficiencies were unaffected by localized thermal gradients up to 53 °C or the sound wave angle of incidence relative to the THORS barrier. As a proof-of-concept, studies showed that acoustic waves can be steered 90-degree around a corner of an acoustic dampening material. Additionally, THORS waveguiding channels were demonstrated to reduce the signal-to-background signal of sound waves internal to the channel, by externally reflecting soundwaves outside the channel. For the first time, the generation of THORS barriers in ambient air is achieved, using water vapor serving as the absorbing species, allowing for the translation of THORS to environments outside of a sealed environmental chamber for potential real-world application. Studies using a Carbon Monoxide (CO) laser showed that THORS barriers can reflect/suppress ultrasonic and acoustic waves at efficiencies as great as 70%. The temporal dynamics of THORS barrier generation is also studied by incrementally changing the delay between incident ultrasonic pulses and laser excitation pulses. Furthermore, photoacoustic enhancement was demonstrated for the first time with THORS channels, generated with axicon lenses. Finally, the molecular dynamics of THORS barriers were studied via Raman imaging. By imaging atmospheric nitrogen in the path of the CO laser, the formation of a THORS barrier is observed as it is temporally and spatially distributed. Observed THORS barrier intensities were temporally congruent with ultrasonic temporal dynamic studies. Additionally, barrier widths of maximum efficiency barriers were similar in size to the laser beam at 4.6 ± 2.5 mm. This thesis discusses the fundamental principles and characteristic features that govern the formation of a THORS barrier for the optical reflection and waveguiding of sound. The studied characteristics that define the efficiency and performance of THORS barriers substantiate the phenomenon and implicate its strong potential for applications in fields such as photoacoustic sensing, sub-surface tissue imaging, and acoustic stealth technologies.