  In turbid materials it is impossible to concentrate light into a focus with
conventional optics. Recently it has been shown that the intensity on a dyed
probe inside a turbid material can be enhanced by spatially shaping the wave
front of light before it enters a turbid medium. Here we show that this
enhancement is due to concentration of light energy to a spot much smaller than
a wavelength. We focus light on a dyed probe sphere that is hidden under an
opaque layer. The light is optimally concentrated to a focus which does not
exceed the smallest focal area physically possible by more than 68%. A
comparison between the intensity enhancements of both the emission and
excitation light supports the conclusion of optimal light concentration.
