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 , 2 , 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 ∼1,000 times weaker than optical scattering. Various fields, including biomedical and colloidal optics, can benefit from TRUE optical focusing. Focusing into a scattering medium is much more valuable than focusing through it. Scientists now demonstrate the dynamic focusing of light into a scattering medium by combining the ultrasonic modulation of diffused coherent light with optical phase conjugation.