Optimal concentration of light in turbid materials

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 dye-doped probe inside a turbid material can be enhanced by spatially shaping the wavefront 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 dye-doped probe sphere that is hidden by an opaque layer. The light (λ ¼ 532nm) is optimally concentrated to a focal area smaller than 0:037 μm2. The focus can be substantially smaller than the used probe. We use a comparison between the emission and excitation intensity to show the light is concentrated to a spot below the resolution of our oil-immersion objective. The results are in good agreement with an optimal concentration of linearly or elliptically polarized light.