Ultrasound-Induced Light Focusing Inside Scattering Media

Light-based methods are fundamental in biomedical and life sciences as non-invasive diagnostic and treatment tools [1]. However, the scattering nature of biological tissue and tissue-like constructs limits the maximum depth at which they can operate - typically, below one millimeter [1]. Several techniques have been developed to address this issue and guide light deeper inside tissue, including wavefront shaping and endoscopy, but they normally achieve so by sacrificing temporal resolution, or by becoming invasive. Alternatively, it has been recently shown that shaped ultrasonic waves can be used to guide light in both homogeneous [2] as well as inhomogeneous media [3]. In this case, the modulation in refractive index induced by ultrasound acts as an embedded lens or waveguide in the media helping to redirect scattered photons toward deeper sections. Still, the extent of this effect remains largely unexplored, and its benefits have only been demonstrated with transmitted light - inaccessible in most realistic scenarios. Here, we perform a detailed analysis of the focusing capabilities of ultrasound for transmitted and reflected light in different scattering media. As shown in Fig. 1a, ultrasound modulation enables to preserve a tight laser spot up to a distance of 0.9 transport mean free path (TMFP). This value represents an enhancement of a factor of 7 compared to conventional focusing using external optical elements, in good agreement with Monte Carlo simulations. When combined with a point-by-point scanning system, such focusing enables reconstructing an image with micrometric resolution inside a scattering media that would be otherwise completely hidden with conventional imaging methods - see Fig. 1b. As our results demonstrate, ultrasound offers the capability to control light deep inside inhomogeneous media, paving the way for rapidly and non-invasively imaging biological tissue.