Thermally-induced optical reflection of sound (THORS) for photoacoustic sensing

Abstract

The ability to precisely control and manipulate acoustic waves can be highly limiting in applications and environments where placement of physical barriers for acoustic steering cannot be employed (e.g. tissues, air, etc.) In this work, we describe the ability to generate acoustic waveguides via thermally-induced optical reflection of sound (THORS) for the manipulation of acoustic waves in free space (i.e., air). Abrupt, density barriers are formed by photothermally depleting the sample in a laser beam’s path via photothermal processes, resulting in sharp differences in compressibility and significant acoustic reflection (greater than 30%). Optical waveguiding of sound can be achieved by generating THORS channels with a cylindrical (ring shaped) laser beam. By containing the acoustic waves inside a THORS cylindrical channel, a dramatically reduced acoustic decay profile of 1/r⁰˙⁶with distance is achieved. Additionally, we describe the effects that optical modulation frequency of the THORS channel has on the efficiency of acoustic waveguiding. We also show how external acoustic waves, incident to a THORS channel are suppressed, increasing the signal-to-background ratio of the internally waveguided acoustic signals. Optical waveguiding of acoustic waves offers a new paradigm in the manipulation of sound over extended distances, providing potentially significant improvements to photoacoustic sensing, secure communications, and many other applications.