Time-reversed ultrasonically encoded optical focusing into scattering media

Light focusing plays a central role in biomedical imaging, manipulation and therapy. In scattering media, direct light focusing becomes infeasible beyond one transport mean free path. All previous methods(1-3) used to overcome this diffusion limit lack a practical internal 'guide star'(4). Here, we propose and experimentally validate a novel concept called time-reversed ultrasonically encoded (TRUE) optical focusing to deliver light into any dynamically defined location inside a scattering medium. First, diffused coherent light is encoded by a focused ultrasonic wave to provide a virtual internal guide star. Only the encoded light is time-reversed and transmitted back to the ultrasonic focus. The time-reversed ultrasonically encoded optical focus-defined by the ultrasonic wave-is unaffected by multiple scattering of light. Such focusing is particularly desirable in biological tissue, where ultrasonic scattering is similar to 1,000 times weaker than optical scattering. Various fields, including biomedical and colloidal optics, can benefit from TRUE optical focusing.