Wang, Y.M. et al. Nat. Commun. 3, 928 (2012).

The scattering of light in tissues is the bane of many researchers who would like to image biological events deep under the surface. However, scattered light also contains information about the sample that can be exploited. Wang et al. capitalize on this with a technique to extend focused fluorescence imaging to a depth of 2.5 millimeters. A focused ultrasound pulse is applied to a scattered beam of light deep within the sample, which causes a portion of the light to become frequency shifted. This frequency-shifted light is phase conjugated with high gain using a digital optical phase conjugation scheme. The phase conjugate beam is reflected back through the sample and converges at the original location of the ultrasound focus, thereby resulting in an optical focus deep within the tissue. Though the method has so far been applied only in proof-of-principle experiments, it holds promise for deep-tissue fluorescence imaging.