Optimal concentration of light in turbid materials

In our experiments we demonstrate the first method to concentrate light deep inside strongly scattering samples. We use a spatial light modulator (SLM) with thousands of individual segments to control the wave front of visible laser light. A feedback based algorithm finds the optimal configuration for which multiple scattering in the sample transforms the light into a focused spot inside the sample. The feedback is provided by a nano-sized fluorescent probe hidden in a layer of ZnO pigment. We find that the size of the focus is at least 20plusmn 4% smaller than the resolution limit of the original optical system and is concentrated to an optimal small area. A study of the intensity enhancements of both the fluorescence and excitation, performed on different probe sizes, supports the conclusion of optimal light concentration.