Photoacoustics for Direct Light-Guiding Inside Transparent and Scattering Media

The rapid and precise delivery of light at targeted positions on a sample is essential for any light-based application, including laser materials processing or imaging. However, current systems for light control rely on bulky and costly optical components placed outside the sample. This configuration can be limited by the sample's geometry, poor flexibility in the illumination, and shallow penetration depth, particularly for scattering samples. Here a novel approach is proposed for guiding light within optically transparent and scattering media while obviating any external components. By simply employing an absorptive material and a pulsed laser, this method leverages the photoacoustic generation of a localized pressure wave to induce refractive index gradients within a medium. These gradients act as non-invasive optical waveguides, that allow for light focusing and guiding across several millimeters at sub-microsecond time scales. The principle and implementation of this method are described, the light-guiding effects through various tissue phantoms are simulated and measured, and micrometric laser marking of a photo-absorbing layer is demonstrated inside a 7-mm-thick scattering phantom. The possibility to operate with endogenous absorbing materials and low pressures makes photoacoustic-enabled light guiding a promising step toward rapid light delivery at conditions not feasible today.